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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2018	Mar. 05, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Trading Symbol	CARA
Entity Registrant Name	Cara Therapeutics, Inc.
Entity Central Index Key	0001346830
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Shell Company	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		39,547,558
Entity Public Float			$ 561,082,035

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 15,081	$ 9,388
Marketable securities	146,302	83,181
Income tax receivable	664	731
Other receivables	926	123
Prepaid expenses	4,805	1,635
Restricted cash, current	361
Total current assets	168,139	95,058
Marketable securities, non-current	21,396
Property and equipment, net	880	1,177
Restricted cash	408	769
Total assets	190,823	97,004
Current liabilities:
Accounts payable and accrued expenses	13,622	8,506
Current portion of deferred revenue	26,825
Total current liabilities	40,447	8,506
Deferred revenue, non-current	15,184
Deferred lease obligation	1,562	1,718
Commitments and contingencies (Note 17)
Stockholders’ equity:
Preferred stock; $0.001 par value; 5,000,000 shares authorized at December 31, 2018 and December 31, 2017; zero shares issued and outstanding at December 31, 2018 and December 31, 2017
Common stock; $0.001 par value; 100,000,000 shares authorized at December 31, 2018 and December 31, 2017; 39,547,558 shares and 32,662,255 shares issued and outstanding at December 31, 2018 and December 31, 2017, respectively	39	33
Additional paid-in capital	428,059	307,158
Accumulated deficit	(294,354)	(220,341)
Accumulated other comprehensive loss	(114)	(70)
Total stockholders’ equity	133,630	86,780
Total liabilities and stockholders’ equity	$ 190,823	$ 97,004

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	39,547,558	32,662,255
Common stock, shares outstanding	39,547,558	32,662,255

Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
Statements of Comprehensive Loss - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Total revenue	$ 13,469	$ 911	$ 86
Operating expenses:
Research and development	75,531	48,524	49,253
General and administrative	15,320	11,872	9,233
Total operating expenses	90,851	60,396	58,486
Operating loss	(77,382)	(59,485)	(58,400)
Other income	2,980	1,156	652
Loss before benefit from income taxes	(74,402)	(58,329)	(57,748)
Benefit from income taxes	389	204	468
Net loss	$ (74,013)	$ (58,125)	$ (57,280)
Net loss per share:
Basic and Diluted	$ (2.06)	$ (1.86)	$ (2.10)
Weighted average shares:
Basic and Diluted	35,892,786	31,202,842	27,279,008
Other comprehensive income (loss), net of tax of $0:
Change in unrealized gains (losses) on available for sale marketable securities	$ (44)	$ (73)	$ 38
Total comprehensive loss	(74,057)	(58,198)	(57,242)
License and Milestone Fees [Member]
Revenue:
Total revenue	13,436	530
Collaborative Revenue [Member]
Revenue:
Total revenue		313
Clinical Compound Revenue [Member]
Revenue:
Total revenue	$ 33	$ 68	$ 86

Statements of Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Other comprehensive income (loss), tax	$ 0	$ 0	$ 0

Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Follow-on Public Offering [Member]	Common Stock [Member]	Common Stock [Member] Follow-on Public Offering [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] Follow-on Public Offering [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance, Value at Dec. 31, 2015	$ 105,044		$ 27		$ 209,943		$ (104,891)	$ (35)
Balance, Shares at Dec. 31, 2015			27,254,863
Stock-based compensation expense	2,800				2,800
Shares issued upon exercise of stock options, Value	123				123
Shares issued upon exercise of stock options, Shares			42,000
Net loss	(57,280)						(57,280)
Other comprehensive income (loss)	38							38
Balance, Value at Dec. 31, 2016	50,725		$ 27		212,866		(162,171)	3
Balance, Shares at Dec. 31, 2016			27,296,863
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Value		$ 86,224		$ 5		$ 86,219
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares				5,117,500
Stock-based compensation expense	5,793				5,793
Modification of equity awards	537				537
Shares issued upon exercise of stock options, Value	1,699		$ 1		1,698
Shares issued upon exercise of stock options, Shares			247,892
Cumulative effect adjustment upon adoption of ASU 2016-09					45		(45)
Net loss	(58,125)						(58,125)
Other comprehensive income (loss)	(73)							(73)
Balance, Value at Dec. 31, 2017	86,780		$ 33		307,158		(220,341)	(70)
Balance, Shares at Dec. 31, 2017			32,662,255
Sale of common stock under license agreement, Value	14,556		$ 1		14,555
Sale of common stock under license agreement, Shares			1,174,827
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Value		$ 92,063		$ 5		$ 92,058
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares				5,175,000
Stock-based compensation expense	7,785				7,785
Modification of equity awards	616				616
Shares issued upon vesting of performance-based restricted stock units	1,693				1,693
Shares issued upon vesting of performance-based restricted stock units, Shares			83,791
Shares issued upon exercise of stock options, Value	4,194				4,194
Shares issued upon exercise of stock options, Shares			451,685
Net loss	(74,013)						(74,013)
Other comprehensive income (loss)	(44)							(44)
Balance, Value at Dec. 31, 2018	$ 133,630		$ 39		$ 428,059		$ (294,354)	$ (114)
Balance, Shares at Dec. 31, 2018			39,547,558

Statements of Stockholders' Equity (Parenthetical) - Follow-on Public Offering [Member] - Common Stock [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Sale of common stock, per share	$ 19.00	$ 18.00
Underwriting discounts and commissions and offering expenses	$ 6,262	$ 5,891

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (74,013)	$ (58,125)	$ (57,280)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	9,478	5,793	2,800
Modification of equity awards	616	537
Depreciation & amortization	370	495	1,465
Amortization/accretion of available-for-sale marketable securities	(1,820)	(582)	(218)
Realized gain on sale of available-for-sale marketable securities	5	(5)	(23)
Realized gain on sale of property and equipment		(41)
Deferred rent costs	(156)	148	(114)
Deferred revenue	42,009
Changes in operating assets and liabilities:
Income tax receivable	67	121	(468)
Other receivables	(803)	(36)	(7)
Prepaid expenses	(3,170)	(105)	199
Accounts payable and accrued expenses	5,116	(3,027)	6,265
Net cash used in operating activities	(22,301)	(54,827)	(47,381)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	175,300	82,156	80,380
Proceeds from sale of available-for-sale marketable securities	79,808	8,755	34,003
Purchase of available-for-sale marketable securities	(337,854)	(127,394)	(68,648)
Purchases of property and equipment	(73)	(58)	(717)
Proceeds from sale of property and equipment		41
Net cash (used in) provided by investing activities	(82,819)	(36,500)	45,018
Financing activities
Proceeds from sale of common stock in a follow-on public offering, net of issuance costs	92,063	86,224
Proceeds from the sale of common stock under license agreement	14,556
Proceeds from the exercise of stock options	4,194	1,699	123
Net cash provided by financing activities	110,813	87,923	123
Net increase (decrease) in cash, cash equivalents and restricted cash	5,693	(3,404)	(2,240)
Cash, cash equivalents and restricted cash at beginning of period	10,157	13,561	15,801
Cash, cash equivalents and restricted cash at end of period	$ 15,850	$ 10,157	13,561
Noncash financing activities
Tenant improvements paid by landlord			$ 1,094

Business

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Business

1. Business

Cara Therapeutics, Inc., or the Company, is a clinical-stage biopharmaceutical corporation formed on July 2, 2004. The Company is focused on developing and commercializing new chemical entities with a primary focus on pruritus as well as pain by selectively targeting peripheral kappa opioid receptors. The Company’s primary activities to date have been organizing and staffing the Company, developing its product candidates, including conducting preclinical studies and clinical trials of CR845/difelikefalin-based product candidates and raising capital.

As of December 31, 2018, the Company has raised aggregate net proceeds of $383,200 from several rounds of equity financing, including its initial public offering, or IPO, which closed in February 2014 and three follow-on public offerings of common stock, which closed in July 2018, April 2017 and August 2015, respectively, and the issuance of convertible preferred stock and debt prior to the IPO. The Company had also received $88,900 under its license agreements for CR845/difelikefalin, primarily with Vifor Fresenius Medical Care Renal Pharma Ltd., or VFMCRP, Maruishi Pharmaceutical Co. Ltd., or Maruishi, and Chong Kun Dang Pharmaceutical Corp., or CKDP, and an earlier product candidate for which development efforts ceased in 2007. Additionally, in May 2018, the Company received net proceeds of $14,556 from the issuance and sale of 1,174,827 shares of the Company’s common stock to Vifor (International) Ltd., or Vifor, in connection with the Company’s license agreement with VFMCRP (see Note 11, Collaboration and Licensing Agreements).

As of December 31, 2018, the Company had unrestricted cash and cash equivalents and marketable securities of $182,779 and an accumulated deficit of $294,354. The Company has incurred substantial net losses and negative cash flows from operating activities in nearly every fiscal period since inception and expects this trend to continue for the foreseeable future. The Company recognized a net loss of $74,013 and had net cash used in operating activities of $22,301 for the year ended December 31, 2018.

The Company is subject to risks common to other life science companies including, but not limited to, uncertainty of product development and commercialization, lack of marketing and sales history, development by its competitors of new technological innovations, dependence on key personnel, market acceptance of products, product liability, protection of proprietary technology, ability to raise additional financing, and compliance with Food and Drug Administration, or FDA, and other government regulations. If the Company does not successfully commercialize any of its product candidates, it will be unable to generate recurring product revenue or achieve profitability.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

Certain amounts in the prior years’ financial statements have been reclassified to conform to the current-year presentation due to the adoption of certain accounting standards (see Note 2, Other Accounting Pronouncements Recently Adopted: ASU 2016-18, Statement of Cash Flows (Topic 230), Restricted Cash).

Use of Estimates

The preparation of financial statements in conformity with generally-accepted accounting principles in the United States or GAAP, requires the Company to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities, as of the date of the financial statements as well as the reported amounts of revenues and expenses during the reporting period. Actual results could differ materially from the Company’s estimates and assumptions. Estimates include the fair value of marketable securities that are classified as level 2 of the fair value hierarchy, useful lives of fixed assets, the periods over which certain revenues will be recognized, including licensing and collaborative revenue recognized from non-refundable up-front and milestone payments, the determination of prepaid research and development, or R&D, clinical costs and accrued research projects, the amount of non-cash compensation costs related to share-based payments to employees and non-employees and the periods over which those costs are expensed and the likelihood of realization of deferred tax assets.

Concentrations of Credit Risk

Financial instruments, which potentially subject the Company to significant concentrations of credit risk consist primarily of cash equivalents and marketable securities. The Company invests its cash reserves in money market funds or high-quality marketable securities in accordance with its investment policy. The stated objectives of its investment policy are to preserve capital, provide liquidity consistent with forecasted cash flow requirements, maintain appropriate diversification and generate returns relative to these investment objectives and prevailing market conditions. The Company’s investment policy includes guidelines on acceptable investment securities, limits interest-bearing security investments to certain types of debt and money market instruments issued by the U.S. government and institutions with investment grade credit ratings and places restrictions on maturities and concentration by asset class and issuer. The Company's cash and cash equivalents and marketable securities are held by three major financial institutions. In accordance with the Company's policies, the Company monitors exposure with its counterparties. The Company also maintains deposits in federally insured financial institutions in excess of federally insured limits. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held.

Cash and Cash Equivalents

Cash and cash equivalents include cash on hand, demand deposits, deposits with banks and highly liquid money market funds with holdings of cash and other investments with original maturities of three months or less.

Marketable Securities

The Company deems certain of its investments to be marketable securities if the investment, or in the case of money market funds, the securities underlying the money market fund, meet the definition of a debt security in Accounting Standards Codification, or ASC, section 320-10-20. The Company considers its marketable securities to be available-for-sale and, accordingly, these investments are recorded at fair value with unrealized gains and losses recorded in Accumulated other comprehensive income (loss), or AOCI, as a separate component of stockholders’ equity. Available-for-sale marketable securities are reported as Marketable securities, current and Marketable Securities, noncurrent in the Balance Sheets. Other income includes interest and dividends, realized gains and losses on sales of securities and other-than-temporary impairment, or OTTI, declines in the fair value of securities, if any. The cost of securities sold is based on the specific identification method.

The Company reviews its available-for-sale marketable securities for OTTI declines in fair value below its cost basis each quarter and whenever events or changes in circumstances indicate that the cost basis of an asset may not be recoverable. This evaluation is based on a number of factors, including the length of time and the extent to which the fair value has been below its cost basis and adverse conditions related specifically to the security, including any changes to the credit rating of the security, and the intent to sell, or whether the Company will more likely than not be required to sell, the security before recovery of its amortized cost basis. The Company’s assessment of whether a security is other-than-temporarily impaired could change in the future due to new developments or changes in assumptions related to any particular security.

If a decline in the fair value of an available-for-sale marketable debt security in the Company's investment portfolio is deemed to be other-than-temporary, the Company writes down the security to its current fair value. If the Company intends to sell the security or it is more likely than not that the Company will be forced to sell the security before recovery of the amortized cost of the security, the loss is recognized in net income. Otherwise, the loss is separated into a portion representing a credit loss, which is recorded in net income, and the remainder is recorded in Other comprehensive income, or OCI, net of taxes. See Note 3, Available-for-Sale Marketable Securities, and Note 10, Fair Value Measurements.

Fair Value of Financial Instruments

The Company applies fair value accounting for all financial assets and liabilities that are recognized or disclosed at fair value in the financial statements on a recurring basis. The Company defines fair value as the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. When determining the fair value measurements for assets and liabilities which are required to be recorded at fair value, the Company considers the principal or most advantageous market in which it would transact and the market-based risk measurements or assumptions that market participants would use in pricing the asset or liability, such as risks inherent in valuation techniques, transfer restrictions and credit risks.

The Company’s financial instruments consist of cash, cash equivalents, available-for-sale marketable securities, prepaid expenses, restricted cash, accounts payable and accrued liabilities. The fair values of cash and cash equivalents, restricted cash, accounts payable and accrued liabilities approximate their carrying values due to the short-term nature of these financial instruments. Marketable securities are reported at their fair values, based upon pricing of securities with the same or similar investment characteristics as provided by pricing services, as described below.

Current accounting guidance defines fair value, establishes a framework for measuring fair value in accordance with ASC section 820, and requires certain disclosures about fair value measurements. The valuation techniques included in the guidance are based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect the Company’s assumptions about the inputs that market participants would use in pricing the asset or liability and are developed based on the best information available in the circumstances.

The Company classifies its investments in a fair value hierarchy that is intended to increase consistency and comparability in fair value measurements and related disclosures. The fair value hierarchy is divided into three levels based on the source of inputs as follows:

•

Level 1 – Observable inputs – quoted prices in active markets for identical assets and liabilities.

•

Level 2 – Observable inputs other than the quoted prices in active markets for identical assets and liabilities – such as quoted prices for similar instruments, quoted prices for identical or similar instruments in inactive markets, or other inputs that are observable or can be corroborated by observable market data.

•

Level 3 – Unobservable inputs – includes amounts derived from valuation models where one or more significant inputs are unobservable and require the Company to develop relevant assumptions.

The Company records transfers between levels in the hierarchy by assuming that the transfer occurred at the end of the quarter or year-to-date period.

Valuation Techniques - Level 2 Inputs

The Company estimates the fair values of its financial instruments categorized as level 2 in the fair value hierarchy, including U.S. Treasury securities, U.S. government agency obligations, corporate bonds, municipal bonds, commercial paper and money market funds with similar underlying investments by taking into consideration valuations obtained from third-party pricing services. The pricing services use industry standard valuation models, including both income- and market-based approaches, for which all significant inputs are observable, either directly or indirectly, to estimate fair value. These inputs include reported trades of and broker/dealer quotes on the same or similar securities, benchmark yields, issuer credit spreads, benchmark securities, and other observable inputs. The Company obtains a single price for each financial instrument and does not adjust the prices obtained from the pricing service.

The Company validates the prices provided by its third-party pricing services by reviewing their pricing methods, obtaining market values from other pricing sources and comparing them to the share prices presented by the third-party pricing services. After completing its validation procedures, the Company did not adjust or override any fair value measurements provided by its pricing services as of December 31, 2018 or December 31, 2017.

Property and Equipment

Property and equipment (consisting of computer, office and laboratory equipment, furniture and fixtures and leasehold improvements) are stated at cost, net of accumulated depreciation and amortization of leasehold improvements. Depreciation and amortization are calculated using the straight-line method over the estimated useful lives of the respective assets. Leasehold improvements are amortized over the lesser of their useful lives or the life of the lease.

Asset Category		Useful Lives
Computer and office equipment		5 years
Short-term laboratory equipment		2 years
Furniture and fixtures		7 years
Leasehold improvements		lesser of useful life of asset or life of lease (Stamford - 7 years)

ASC 360, Property, Plant and Equipment, addresses the financial accounting and reporting for impairment or disposal of long-lived assets. The Company reviews the recorded values of property and equipment for impairment whenever events or changes in business circumstances indicate that the carrying amount of an asset or group of assets may not be fully recoverable.

Revenue Recognition

On January 1, 2018, the Company adopted Accounting Standards Update, or ASU, 2014-09, Revenue from Contracts with Customers (Topic 606), or ASC 606, as amended by ASU 2016-08, 2016-10, 2016-12 and 2016-20 using the full retrospective method. Under ASC 606, the Company recognizes revenue in an amount that reflects the consideration to which it expects to be entitled in exchange for the transfer of promised goods or services to customers. To determine revenue recognition for contracts with customers that are within the scope of ASC 606, the Company performs the following steps: (1) identifies the contract with the customer, (2) identifies the performance obligations in the contract, (3) determines the transaction price, (4) allocates the transaction price to the performance obligations in the contract, and (5) recognizes revenue when (or as) the entity satisfies a performance obligation. The Company has concluded that upon adoption of ASC 606, as amended, there was no impact on its results of operations, financial position or cash flows for any period presented from its only two revenue-related contracts, which were in effect at that time: the CKDP Agreement or the Maruishi Agreement (see Note 11, Collaboration and Licensing Agreements and Note 12, Revenue Recognition).

The Company has entered into agreements to license its intellectual property, or IP, related to CR845/difelikefalin to develop, manufacture and/or commercialize drug products. These agreements typically contain multiple performance obligations, including licenses of IP and R&D services. Payments to the Company under these agreements may include nonrefundable license fees, payments for research activities, payments based upon the achievement of certain milestones and royalties on any resulting net product sales.

The Company identifies agreements as contracts that create enforceable rights and obligations when the agreement is approved by the parties, identifies the rights of the parties and the payment terms, has commercial substance and it is probable that the Company will collect the consideration to which it will be entitled in exchange for the goods and services that will be transferred to the customer. The counterparty is considered to be a customer when it has contracted with the Company to obtain goods and services that are the output of the Company’s ordinary activities (i.e., development of pharmaceutical products) in exchange for consideration.

A performance obligation is a promise to transfer distinct goods or services to a customer. Performance obligations that are both capable of being distinct and distinct within the context of the contract are considered to be separate performance obligations. Performance obligations are capable of being distinct if the counterparty is able to benefit from the good or service on its own or together with other resources that are readily available to it. Performance obligations are distinct within the context of the contract when each performance obligation is separately identifiable from each other; i.e., the Company is not using the goods or services as inputs to produce or deliver the combined output or outputs specified by the customer; one or more of the goods or services does not significantly modify or customize one of the other goods or services in the contract; and goods or services are not highly interdependent or not highly interrelated. Performance obligations that are not distinct are accounted for as a single performance obligation over the period that goods or services are transferred to the customer. The determination of whether performance obligations in a contract are distinct may require significant judgment.

The transaction price is the amount of consideration that the Company expects to be entitled to in exchange for transferring promised goods or services to the customer based on the contract terms at inception of a contract. There is a constraint on inclusion of variable consideration related to licenses of IP, such as milestone payments or sales-based royalty payments, in the transaction price if there is uncertainty at inception of the contract as to whether such consideration will be recognized in the future because it is probable that there will be a significant reversal of revenue in the future when the uncertainty is resolved. The determination of whether or not it is probable that a significant reversal of revenue will occur in the future depends on the likelihood and magnitude of the reversal. Factors that could increase the likelihood or magnitude of a reversal of revenue include (a) the susceptibility of the amount of consideration to factors outside the entity’s influence, such as the outcome of clinical trials, the timing of initiation of clinical trials by the counterparty and the approval of drug product candidates by regulatory agencies, (b) situations in which the uncertainty is not expected to be resolved for a long period of time, and (c) level of the Company’s experience in the field. When it becomes probable that events will occur, for which variable consideration was constrained at inception of the contract, the Company allocates the related consideration to the separate performance obligations in the same manner as described below.

At inception of a contract, the Company allocates the transaction price to the distinct performance obligations based upon their relative standalone selling prices. Standalone selling price is the price at which an entity would sell a promised good or service separately to a customer. The best evidence of standalone selling price is an observable price of a good or service when sold separately by an entity in similar circumstances to similar customers. Since the Company typically does not have such evidence, it estimates standalone selling price so that the amount that is allocated to each performance obligation equals the amount that the Company expects to receive for transferring goods or services. The methods that the Company uses to make such estimates include (1) the adjusted market assessment approach, under which the Company forecasts and analyzes CR845/difelikefalin in the appropriate market, the phase of clinical development as well as considering recent similar license arrangements within the same phase of clinical development, therapeutic area, type of agreement, etc. and (2) the expected cost of satisfying the performance obligations plus a margin, or the expected cost plus a margin approach.

The Company recognizes revenue when, or as, it satisfies a performance obligation by transferring a promised good or service to a customer and the customer obtains control of the good or service. Revenue related to the grant of a license that is a distinct performance obligation and that is deemed to be functional IP is recognized at the point in time that the Company has the right to payment for the license, the customer has legal title to the license and can direct the use of the license (for example, to grant sublicenses), the customer has the significant risks and rewards of ownership of the license and the customer has accepted the asset (license) by signing the license agreement.

Recognition of revenue related to R&D services that are a distinct performance obligation or that are combined with granting of a license as a single performance obligation is deferred at inception of a contract and is recognized as those services are performed based on the costs incurred as a percentage of the estimated total costs to be incurred to complete the performance obligation.

Milestone payments are considered to be variable consideration and are not included in the transaction price at inception of the contract if it is uncertain that the milestone will be achieved. Rather, when it becomes probable that the milestone will be achieved and, therefore, there will not be a significant reversal of revenue in future periods, the respective amount to be earned is included in the transaction price, allocated to the distinct performance obligations based on their relative standalone selling price and recognized as revenue, as described above. Sales milestones and sales-based royalty payments related to a license of IP are recognized as revenue when the respective sales occur.

Research and Development Expenses

Research and development, or R&D, costs are charged to expense as incurred. Costs incurred under agreements with third parties are charged to expense as incurred in accordance with the specific contractual performance terms of such agreements. R&D expenses include, among other costs, compensation and other personnel-related costs, including consultant costs, and costs to conduct clinical trials using Clinical Research Organizations, or CRO’s, which include upfront, milestone and monthly expenses as well as reimbursement for pass through costs. The amount of clinical trial expense recognized in any period varies depending on the duration and progress of each clinical trial, including the required level of patient enrollment, the rate at which patients actually enroll in and drop-out of the clinical trial, and the number of sites involved in the trial as well as the activities to be performed by the sites each period. R&D costs also include costs to manufacture product candidates and clinical supplies, laboratory supplies costs, facility-related costs and stock-based compensation for R&D personnel. Non-refundable R&D advance payments are deferred and capitalized as prepaid R&D expense. The capitalized amounts are expensed as the related goods are delivered or services are performed. As of December 31, 2018 and 2017, the Company recorded $4,377 and $1,287 as prepaid R&D expense, respectively.

Income Taxes

The Company accounts for income taxes using the asset and liability method. Under this method, deferred tax assets and liabilities are determined based on differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. Deferred income tax assets are reduced, as necessary, by a valuation allowance when management determines it is more likely than not that some or all of the tax benefits will not be realized.

The Company applies the provisions of ASC 740, Income Taxes, which prescribes a comprehensive model for how a company should recognize, measure, present and disclose in its financial statements uncertain tax positions that it has taken or expects to take on a tax return. There were no material uncertain tax positions taken as of December 31, 2018 and December 31, 2017. The Company does not have any interest or penalties accrued related to tax positions as it does not have any unrecognized tax benefits. In the event the Company determines that accrual of interest or penalties are necessary in the future, the amount will be presented as a component of interest expense.

Stock-Based Compensation

The Company grants stock options to employees, non-employee members of the Company’s Board of Directors and non-employee consultants as compensation for services performed. Employee and non-employee members of the Board of Directors’ awards of stock-based compensation are accounted for in accordance with ASC 718, Compensation - Stock Compensation or ASC 718. ASC 718 requires all share-based payments to employees and non-employee directors, including grants of stock options, to be recognized based on their grant date fair values. The grant date fair value of stock options is estimated using the Black-Scholes option valuation model.

Using this model, fair value is calculated based on assumptions with respect to (i) the fair value or market price of the Company’s common stock on the grant date; (ii) expected volatility of the Company’s common stock price, (iii) the periods of time over which employees and members of the Company’s Board of Directors are expected to hold their options prior to exercise (expected term), (iv) expected dividend yield on the Company’s common stock, and (v) risk-free interest rates.

The Company’s common stock has been traded on a public exchange only since January 31, 2014. Since that time, exercises of stock options have been limited due to various factors, including fluctuations in the Company’s stock price to below the exercise prices of awards and blackout periods during which exercises are not allowed, among others. Therefore, the Company believes that as of December 31, 2018, it does not have sufficient company-specific information available to determine the expected term based on its historical data. As a result, the expected term of stock options granted to employees and members of the Company’s Board of Directors is determined using the average of the vesting period and term (6.25 years), an accepted method for the Company’s option grants under the SEC’s Staff Accounting Bulletin No. 110, Share-Based Payment.

Similarly, because the Company does not have sufficient company-specific information available to calculate the volatility of its common stock during the periods of the expected term of stock option grants (as noted above), expected volatility is based on an analysis of guideline companies, in accordance with ASC 718.

The expected dividend yield is zero as the Company has never paid dividends and does not currently anticipate paying any in the foreseeable future. Risk-free interest rates are based on quoted U.S. Treasury rates for securities with maturities approximating the option’s expected term.

On the grant date of each stock option award prior to January 1, 2017, the Company applied a forfeiture rate in order to accrue share-based compensation expense based on an estimate of the number of stock options that are expected to vest. Estimated forfeiture rates were based upon historical data of awards that were cancelled prior to vesting. The Company adjusted the total amount of compensation cost recognized for each award, in the period in which each award vested, to reflect the actual forfeitures related to that award. To the extent that the actual forfeiture rate for an award was lower than the estimated forfeiture rate, additional compensation expense was recorded in the period that the award vested. Changes in the Company’s estimated forfeiture rate resulted in changes in the rate at which compensation cost for an award was recognized over its vesting period. As of January 1, 2017, the Company adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting. On the date of adoption of ASU 2016-09, the Company began to account for forfeitures of unvested stock options as they occur rather than estimate forfeiture rates that were applied to unvested stock option awards, as under the previous accounting guidance (see Note 13, Stock-Based Compensation).

The Company accounts for options granted to non-employee consultants under ASC 505-50, Equity-Based Payments to Non-Employees. As such, the Company estimates the fair value of each option to non-employees using the Black-Scholes model, with the expected term of stock options granted to non-employees initially equal to the options’ maximum contractual life of ten years, at issuance. On each subsequent reporting date until performance is complete, the Company revalues all outstanding options granted to non-employee consultants during the vesting period of each tranche. Under ASC 505-50, upon re-measurement of each award, income or expense is recognized during its vesting term. As of January 1, 2019, the Company will adopt ASU 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Non-employee Share-Based Payment Accounting, which expands the scope of ASC 718 to include share-based payment transactions for acquiring goods and services from non-employees. As a result, the fair value of all outstanding unvested stock options that had been granted to non-employees as of January 1, 2019 will be remeasured under ASC 718 (see Note 2, Recent Accounting Pronouncements Not Yet Adopted). For all share-based payments granted to employees and non-employees, compensation cost relating to awards with service-based graded vesting schedules is recognized using the straight-line method over the requisite service period.

Income (Loss) Per Share

The Company computes basic net income (loss) per share by dividing net income (loss) by the weighted average number of shares of common stock outstanding. Diluted net income per share includes the potential dilutive effect of common stock equivalents as if such securities were exercised during the period, when the effect is dilutive. Common stock equivalents may include outstanding stock options, which are included under the treasury stock method when dilutive. For each of the years ended December 31, 2018, 2017 and 2016, the Company excluded the effects of potentially dilutive shares that were outstanding during those respective periods from the denominator as their inclusion would be anti-dilutive due to the Company’s net losses during those periods.

Segment Reporting

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. The Company views its operations and manages its business as one operating segment, which includes all activities related to the discovery and development of novel therapeutics to treat serious medical conditions, including pruritus and pain.

Leases

The Company recognizes rent expense for operating leases on a straight-line basis over the term of the lease, beginning on the date the Company takes possession of the property. Rent expense includes the base amounts stated in the lease agreement as well as the effect of reduced or free rent and rent escalations. At lease inception, the Company determines the lease term by assuming the exercise of those renewal options that are reasonably assured because of the significant economic penalty that exists for not exercising those options. The exercise of renewal options is at the Company’s sole discretion. The expected lease term is one of the factors used to determine whether a lease is classified as operating or capital and is used to calculate the straight-line rent expense. The difference between the cash paid to the landlord and the amount recognized as rent expense on a straight-line basis is included in deferred rent and classified within long-term liabilities. Lease incentives made by landlords to or on behalf of the Company for leasehold improvements are recorded as deferred rent and classified as long-term liabilities. Deferred rent related to landlord incentives is amortized using the straight-line method over the lease term as an offset to rent expense. Penalties paid to landlords to terminate a lease before the contractual end date of the lease are recognized on an undiscounted basis in the Statements of Comprehensive Loss. On January 1, 2019, the Company will adopt ASU No. 2016-02, Leases (Topic 842), which amends the previous guidance for accounting and disclosure of leases (ASC 840) for both lessees and lessors. The primary effect of adoption will be the requirement to record a right-of-use asset and a corresponding lease obligation for the Stamford operating lease (see Note 2, Recent Accounting Pronouncements Not Yet Adopted).

Litigation Reserves

From time to time, the Company may become subject to arbitration, litigation or claims arising in the ordinary course of its business. Accruals are recorded when it is probable that a liability has been incurred and the amount of the liability can be reasonably estimated. The Company reviews these reserves at least quarterly and adjusts these reserves to reflect current law, progress of each case, opinions and views of legal counsel and other advisers, the Company’s experience in similar matters and intended response to the litigation. The Company expenses amounts for administering or litigating claims as incurred. Accruals for legal proceedings, if any, are included in Accounts payable and accrued expenses in the Balance Sheets.

Other Accounting Pronouncements Recently Adopted

In August 2018, the SEC adopted amendments to certain disclosure requirements in Securities Act Release No. 33-10532, Disclosure Update and Simplification. These amendments eliminate, modify, or integrate into other SEC requirements certain disclosure rules. Among the amendments is the requirement to present an analysis of changes in stockholders’ equity in the interim financial statements included in quarterly reports on Form 10-Q. The analysis, which can be presented as a footnote or separate statement, is required for the current and comparative quarter and year-to-date interim periods. The amendments are effective for all filings made on or after November 5, 2018. In light of the anticipated timing of effectiveness of the amendments and expected proximity of effectiveness to the filing date for most filers’ quarterly reports, the SEC’s Division of Corporate Finance issued a Compliance and Disclosure Interpretation related to Exchange Act Forms, or CDI – Question 105.09, that provides transition guidance related to this disclosure requirement. CDI – Question 105.09 states that the SEC would not object if the filer’s first presentation of the changes in stockholders’ equity is included in its Form 10-Q for the quarter that begins after the effective date of the amendments. As such, the Company adopted these SEC amendments on November 5, 2018 and will present the analysis of changes in stockholders’ equity in its interim financial statements in the Company’s Quarterly Report on Form 10-Q for the quarter ending March 31, 2019. The adoption of these SEC amendments will not have a material effect on the Company’s financial position, results of operations, cash flows or stockholders’ equity.

As of January 1, 2018, the Company adopted ASU No. 2017-09, Compensation – Stock Compensation (Topic 718) - Scope of Modification Accounting, or ASU 2017-09, which clarifies that a change to the terms or conditions of a share-based payment award should be accounted for as a modification only if the fair value, vesting conditions or classification (as equity or liability) of the award changes as a result of the change in terms or conditions. Modification of a share-based payment award may result in the Company recognizing additional compensation expense. The Company does not expect to frequently modify, the fair value, vesting conditions or classification of its share-based payment awards. The Company does not expect this guidance to have a material effect on its financial position, results of operations or cash flows. However, if and when modifications occur, their effect could be material to the Company’s financial position, results of operations or cash flows.

As of January 1, 2018, the Company adopted ASU No. 2017-01, Business Combinations (Topic 805), Clarifying the Definition of a Business, or ASU 2017-01, that clarifies the definition of a business to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. ASU 2017-01 requires an entity to evaluate if substantially all of the fair value of the gross assets acquired or disposed of is concentrated in a single identifiable asset or a group of similar identifiable assets; if so, the set of transferred assets and activities is not a business. ASU 2017-01 also requires a business to include at least an input and one substantive process that together significantly contribute to the ability to create output and removes the evaluation of whether a market participant could replace missing elements. The adoption of ASU 2017-01 did not have a material effect on the Company’s financial position, results of operations or cash flows.

As of January 1, 2018, the Company adopted ASU No. 2016-18, Statement of Cash Flows (Topic 230), Restricted Cash (a consensus of the Emerging Issues Task Force), or ASU 2016-18, which changes the presentation of the cash flow statement to include amounts generally described as restricted cash or restricted cash equivalents, together with cash and cash equivalents, when reconciling the beginning-of-period and end-of-period amounts shown on the statement of cash flows. ASU 2016-18 also requires additional disclosures concerning the nature of the restrictions on cash and cash equivalents and a reconciliation between amounts of cash, cash equivalents and restricted cash on the balance sheet and statement of cash flows for each period presented. Upon adoption, ASU 2016-18 was applied retrospectively to all periods presented. The Company historically presented changes in restricted cash as an investing activity in the statement of cash flows. Upon adoption of ASU 2016-18, such changes are reflected in the beginning and ending balances of cash, cash equivalents and restricted cash for all periods presented (see Note 7, Restricted Cash).

Recent Accounting Pronouncements Not Yet Adopted

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606, or ASU 2018-18, which clarifies the interaction between Topic 808 and Topic 606 by (1) clarifying that certain transactions between collaborative arrangement participants should be accounted for under Topic 606; (2) adding unit-of-account guidance in Topic 808 to align with the guidance in Topic 606; and (3) clarifying presentation guidance for transactions with a collaborative arrangement participant that are not accounted for under Topic 606. ASU 2018-18 is effective for public business entities for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. Early adoption is permitted, including adoption in any interim period. The Company has determined that ASU 2018-18 will not have any effect on its financial position, results of operations or cash flows since all three of its collaboration and licensing agreements are accounted for under Topic 606 (see Note 11, Collaboration and Licensing Agreements and Note 12, Revenue Recognition).

In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement, or ASU 2018-13, which modifies the disclosure requirements on fair value measurements in Topic 820 to remove the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, the policy for timing of transfers between levels, and the valuation processes for Level 3 fair value measurements. ASU 2018-13 also amends Topic 820 to clarify that the measurement uncertainty disclosure is to communicate information about the uncertainty in measurement as of the reporting date. ASU 2018-13 also requires additional disclosure for changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period as well as the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. ASU 2018-13 is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The amendments on changes in unrealized gains and losses, the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty should be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption. All other amendments should be applied retrospectively to all periods presented upon their effective date. Early adoption is permitted upon issuance of ASU 2018-13. The Company will adopt ASU 2018-13, as applicable, on January 1, 2020. The Company does not expect that the adoption of ASU 2018-13 will have a material effect on its results of operations, financial position, cash flows or footnote disclosures.

In June 2018, the FASB issued ASU No. 2018-07, Compensation—Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting, or ASU 2018-07, which expands the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. ASU 2018-07 applies to all share-based payment transactions in which a grantor acquires goods or services to be used or consumed in a grantor’s own operations by issuing share-based payment awards. Accordingly, under ASU 2018-07, the fair value of stock options granted to nonemployees will be measured only on the grant date, the amount of which will be recognized as compensation expense over the nonemployee’s service (vesting) period in the same period(s) and in the same manner as if the Company had paid cash for the goods or services instead of paying with or using share-based payment awards. On an award-by-award basis, the Company may elect to use the contractual term as the expected term when estimating the fair value of a nonemployee award to satisfy the measurement objective. Prior guidance under Subtopic 505-50 required the fair value of nonemployee stock options to be marked to market at each reporting period during the service period, which resulted in volatility of compensation expense during that period. ASU 2018-07 is effective for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. Early adoption is permitted, but no earlier than an entity’s adoption date of Topic 606. The Company will adopt ASU 2018-07 on January 1, 2019 on a modified retrospective basis and will remeasure, on that date, the fair value of all outstanding unvested stock options that had been granted to nonemployees. The Company expects that the adoption of ASU 2018-07 will not have a material effect on its results of operations, financial position or cash flows because grants of stock options to nonemployees have been insignificant.

In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments—Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments, or ASU 2016-13, which replaces the incurred loss impairment methodology in current GAAP, that delays recognition of a credit loss until it is probable that such loss has been incurred, with a methodology that reflects expected credit losses and requires consideration of a broader range of reasonable and supportable information to inform credit loss estimates. ASU 2016-13 modifies the other-than-temporary impairment model for available-for-sale debt securities by requiring (1) estimating expected credit losses only when the fair value is below the amortized cost of the asset; (2) recording a credit loss without regard to the length of time a security has been in an unrealized loss position; (3) limiting the measurement of the credit loss to the difference between the security’s amortized cost basis and its fair value and (4) presenting credit losses as an allowance rather than as a write-down, which will allow the Company to record reversals of credit losses in current period net income, a practice that is currently prohibited. ASU 2016-13 will be effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. As such, the Company expects to adopt ASU 2016-13 on January 1, 2020 and is currently evaluating the effect it will have on its results of operations, financial position and cash flows.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), or ASU 2016-02, which amends the current guidance for the accounting and disclosure of leases (ASC 840) for both lessees and lessors. The Company has completed its review of existing contracts and has identified one material contract that contains a lease. The primary effect of adoption will be the requirement to record a right-of-use asset and a corresponding lease obligation for the Stamford operating lease (see Note 17, Commitments and Contingencies). ASU 2016-02 is effective for interim and annual periods beginning after December 31, 2018 but may be adopted earlier. ASU 2016-02 requires modified retrospective adoption. However, the FASB issued ASU 2018-11, Leases (Topic 842): Targeted Improvements, or ASU 2018-11, which allows entities to elect an optional transition method by continuing to apply the guidance in ASC 840, including its disclosure requirements, in the comparative periods presented in the year that they adopt the new leases guidance in ASC 842. Entities that elect this optional transition method would record the cumulative effect of adoption on the effective date rather than at the beginning of the earliest comparative period presented. The Company will adopt ASU 2016-02 and ASU 2018-11 on January 1, 2019 using the optional transition method from ASU 2018-11. The Company expects that the adoption of ASU 2016-02 or ASU 2018-11 will not have a material impact on its Statements of Comprehensive Loss or its Statements of Cash Flows, but it expects that the lease liability recorded on the Balance Sheet beginning on January 1, 2019 will be between $5,000 and $6,000.

Available-for-Sale Marketable Securities

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Available-for-Sale Marketable Securities

3. Available-for-Sale Marketable Securities

As of December 31, 2018, and 2017, the Company’s available-for-sale marketable securities consisted of debt securities issued by U.S. government-sponsored entities and by investment grade institutions. As of December 31, 2018, the Company’s available-for-sale marketable securities also included debt securities issued by the U.S. Treasury and municipal bonds. As of December 31, 2017, the Company’s available-for-sale securities also included a money market fund.

The following tables summarize the Company's available-for-sale marketable securities by major type of security as of December 31, 2018, and 2017:

As of December 31, 2018

			Gross Unrealized					Estimated Fair
Type of Security	Amortized Cost		Gains		Losses			Value
U.S. Treasury securities	$	19,540	$	—	$	(1	)	$	19,539
U.S. government agency obligations		17,860		—		(1	)		17,859
Corporate bonds		75,999		5		(94	)		75,910
Commercial paper		50,413		—		(23	)		50,390
Municipal bonds		4,000		—		—			4,000
Total available-for-sale marketable securities	$	167,812	$	5	$	(119	)	$	167,698

As of December 31, 2017

			Gross Unrealized					Estimated Fair
Type of Security	Amortized Cost		Gains		Losses			Value
Money market funds	$	39,988	$	—	$	(37	)	$	39,951
U.S. government agency obligations		7,799		—		(5	)		7,794
Corporate bonds		15,919		—		(12	)		15,907
Commercial paper		19,545		—		(16	)		19,529
Total available-for-sale marketable securities	$	83,251	$	—	$	(70	)	$	83,181

All available-for-sale marketable securities are classified as Marketable securities, current or Marketable Securities, non-current depending on the contractual maturity date of the individual available-for-sale security.

The Company classifies its marketable debt securities based on their contractual maturity dates. As of December 31, 2018, the Company’s marketable debt securities mature at various dates through November 2020. The amortized cost and fair values of marketable debt securities by contractual maturity were as follows. The table does not include money market funds that are classified as available-for sale marketable securities as of December 31, 2017.

	As of December 31,
	2018				2017
Contractual maturity	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Less than one year	$	146,363	$	146,302	$	43,263	$	43,230
One year to two years		21,449		21,396		—		—
Total	$	167,812	$	167,698	$	43,263	$	43,230

During the years ended December 31, 2018 and 2017, the Company sold shares of its investments in available-for-sale marketable securities with total fair values of $79,808 and $8,755, respectively. The cost of the available-for-sale marketable securities that were sold was determined by specific identification. The sales of the investments in available-for-sale marketable securities during each year resulted in realized (losses) gains, totaling $(5) and $5, respectively.

The following tables show the fair value of the Company's available-for-sale marketable securities that have unrealized losses and that are deemed to be only temporarily impaired, aggregated by investment category and length of time that the individual investments have been in a continuous unrealized loss position.

	Less than 12 Months					12 Months or Greater				Total
			Gross					Gross				Gross
			Unrealized					Unrealized				Unrealized
As of December 31, 2018	Fair Value		Losses			Fair Value		Losses		Fair Value		Losses
U.S. Treasury securities	$	16,392	$	(1	)	$	—	$	—	$	16,392	$	(1	)
U.S. government agency obligations		5,596		(1	)		—		—		5,596		(1	)
Corporate bonds		71,322		(94	)		—		—		71,322		(94	)
Commercial paper		39,445		(23	)		—		—		39,445		(23	)
Total	$	132,755	$	(119	)	$	—	$	—	$	132,755	$	(119	)

	Less than 12 Months					12 Months or Greater				Total
			Gross					Gross				Gross
			Unrealized					Unrealized				Unrealized
As of December 31, 2017	Fair Value		Losses			Fair Value		Losses		Fair Value		Losses
Money market funds	$	39,951	$	(37	)	$	—	$	—	$	39,951	$	(37	)
U.S. government agency obligations		7,794		(5	)		—		—		7,794		(5	)
Corporate bonds		15,907		(12	)		—		—		15,907		(12	)
Commercial paper		19,031		(16	)		—		—		19,031		(16	)
Total	$	82,683	$	(70	)	$	—	$	—	$	82,683	$	(70	)

As of December 31, 2018, and 2017, the Company held a total of 69 out of 84 positions and 30 out of 31 positions, respectively, that were in an unrealized loss position, none of which had been in an unrealized loss position for 12 months or greater. Based on the Company’s review of these securities, the Company believes that the cost basis of its available-for-sale marketable securities is recoverable and that, therefore, it had no other-than-temporary impairments on these securities as of December 31, 2018, or 2017. The Company does not intend to sell these debt securities before maturity and the Company believes it is not more likely than not that it will be required to sell these securities before the recovery of their amortized cost basis, which may be maturity.

Accumulated Other Comprehensive Income (Loss)

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Accumulated Other Comprehensive Income (Loss)

4. Accumulated Other Comprehensive Income (Loss)

The following table summarizes the changes in accumulated other comprehensive income (loss), or AOCI, net of tax, from unrealized gains (losses) on available-for-sale marketable securities, the Company's only component of AOCI, for the years ended December 31, 2018, 2017 and 2016.

	Total Accumulated Other Comprehensive Income (Loss)
Balance, December 31, 2015	$	(35	)
Other comprehensive income before reclassifications		61
Amount reclassified from accumulated other comprehensive income		(23	)
Net current period other comprehensive income		38
Balance, December 31, 2016		3
Other comprehensive loss before reclassifications		(68	)
Amount reclassified from accumulated other comprehensive income		(5	)
Net current period other comprehensive loss		(73	)
Balance, December 31, 2017		(70	)
Other comprehensive loss before reclassifications		(49	)
Amount reclassified from accumulated other comprehensive loss		5
Net current period other comprehensive loss		(44	)
Balance, December 31, 2018	$	(114	)

The reclassifications out of AOCI and into net loss were as follows:

	Year Ended December 31,							Affected Line Item in the Statements of
Component of AOCI	2018			2017		2016		Comprehensive Loss
Unrealized gains (losses) on available-for- sale marketable securities
Realized (losses) gains on sale of securities	$	(5	)	$	5	$	23	Other income
		—			—		—	Income tax benefit
	$	(5	)	$	5	$	23

The amounts reclassified out of AOCI into net loss were determined by specific identification.

Prepaid Expenses

12 Months Ended

Dec. 31, 2018

Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]

Prepaid Expenses

5. Prepaid Expenses

As of December 31, 2018, the amount of prepaid expenses was $4,805, consisting of $4,377 of prepaid R&D clinical costs, $245 of prepaid insurance and $183 of other costs. As of December 31, 2017, the amount of prepaid expenses was $1,635, consisting of $1,287 of prepaid R&D clinical costs, $124 of prepaid insurance and $224 of other costs.

Property and Equipment, Net

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

6. Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2018		2017
Computer and office equipment	$	211	$	158
Laboratory equipment		628		628
Furniture and fixtures		47		27
Leasehold improvements		1,128		1,128
	$	2,014	$	1,941
Less accumulated depreciation and amortization		1,134		764
Property and equipment, net	$	880	$	1,177

Depreciation and amortization expense included in R&D expense and General and administrative expense was $370, $495 and $1,465 for the years ended December 31, 2018, 2017 and 2016, respectively.

In connection with the Company’s relocation of its operating facility from Shelton, Connecticut to Stamford, Connecticut, the Company accelerated the amortization of the Shelton leasehold improvements during the period from December 2015 (signing of the Stamford lease) to May 2016 (the date that the Shelton facility was vacated) (see Note 17, Commitments and Contingencies). In addition, during the years ended December 31, 2017 and 2016, the Company wrote-off $7,816 and $397, respectively, of fully-depreciated Shelton property and equipment, including leasehold improvements, that was not re-located to the Stamford headquarters. During the year ended December 31, 2017, the Company sold fully-depreciated Shelton property and equipment for net proceeds of $41.

Restricted Cash

12 Months Ended

Dec. 31, 2018

Receivables [Abstract]

Restricted Cash

7. Restricted Cash

The Company is required to maintain a stand-by letter of credit as a security deposit under its lease for its office space in Stamford, Connecticut (refer to Note 17, Commitments and Contingencies). The fair value of the letter of credit approximates its contract value. The Company’s bank requires the Company to maintain a restricted cash balance to serve as collateral for the letter of credit issued to the landlord by the bank. As of December 31, 2018, the restricted cash balance for the Stamford lease was invested in a commercial money market account.

The letter of credit balance for the Stamford lease is required to remain at $769 through May 2019 and may, upon request from the Company, thereafter be reduced to $408 through the end of the lease term in November 2023. The reduction in the balance of the letter of credit for the Stamford lease is contingent upon the Company not being in default of any provisions of that lease prior to request for the reduction. As of December 31, 2018, the Company had $361 of restricted cash related to the Shelton lease in current assets and $408 in long-term assets. As of December 31, 2017, the Company had $769 of restricted cash related to the Stamford lease in long-term assets.

The following table provides a reconciliation of cash, cash equivalents and restricted cash reported within the Balance Sheets that sum to the total of the same such amounts shown in the Statements of Cash Flows.

	December 31,
	2018		2017
Cash and cash equivalents	$	15,081	$	9,388
Restricted cash, current assets		361		—
Restricted cash, long-term assets		408		769
Total cash, cash equivalents, and restricted cash shown in the Statements of Cash Flows	$	15,850	$	10,157

Accounts Payable and Accrued Expenses

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Accounts Payable and Accrued Expenses

8. Accounts Payable and Accrued Expenses

Accounts payable and accrued expenses consist of the following:

	December 31,
	2018		2017
Accounts payable	$	4,371	$	3,829
Accrued research projects		6,079		2,356
Accrued professional fees		802		384
Accrued compensation and benefits		2,370		1,864
Accrued other		—		73
	$	13,622	$	8,506

Stockholders' Equity

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Stockholders' Equity

9. Stockholders’ Equity

The Company’s Board of Directors has authorized 100,000,000 shares of the Company’s common stock, par value $0.001 per share, and 5,000,000 shares of undesignated preferred stock, par value $0.001 per share, that may be issued from time to time by the Board of Directors of the Company in one or more series. As of December 31, 2018, there were 39,547,558 shares of common stock and no shares of preferred stock issued and outstanding.

Each share of common stock entitles the holder to one vote on all matters submitted to a vote of the Company’s stockholders. Common stockholders are entitled to dividends when and if declared by the Board of Directors, subject to the preferential rights of the holders of preferred stock, if any.

On March 30, 2017, the Company entered into an underwriting agreement with Piper Jaffray & Co. and Stifel, Nicolaus & Company, Incorporated, as representatives of the several underwriters named therein, relating to the issuance and sale by the Company of 5,117,500 shares of its common stock, including 667,500 shares of common stock the underwriters had the option to purchase, at a public offering price of $18.00 per share, or the 2017 Offering. The 2017 Offering was made pursuant to the Company’s Registration Statement on Form S-3 (File No. 333-216657), filed with the SEC on March 13, 2017 and declared effective on March 24, 2017, and a related prospectus supplement dated March 30, 2017, which was filed with the SEC on March 31, 2017.

On April 5, 2017, the Company closed the 2017 Offering, including the full exercise of the underwriters’ option to purchase 667,500 additional shares of common stock. The Company received net proceeds of $86,224, after deducting $5,891 relating to underwriting discounts and commissions and offering expenses.

On May 17, 2018, the Company issued 1,174,827 shares of its common stock to Vifor in connection with the license agreement entered into with VFMCRP (refer to Note 11, Collaboration and Licensing Agreements).

On July 18, 2018, the Company entered into an underwriting agreement with Jefferies LLC and Merrill Lynch, Pierce, Fenner & Smith Incorporated, as representatives of the several underwriters named therein, relating to the issuance and sale by the Company of up to 5,175,000 shares of its common stock, including 675,000 shares of common stock the underwriters had the option to purchase, at a public offering price of $19.00 per share. This offering was made pursuant to the Company’s Registration Statement on Form S-3 (File No. 333-216657), filed with the SEC on March 13, 2017 and declared effective on March 24, 2017, and a related prospectus dated March 24, 2017 and prospectus supplement dated July 18, 2018, which was filed with the SEC on July 20, 2018.

On July 23, 2018, the Company closed the offering, including the full exercise of the underwriters’ option to purchase 675,000 additional shares of common stock. The Company received net proceeds of $92,063, after deducting $6,262 relating to underwriting discounts and commissions and offering expenses.

In December 2018, as a result of the achievement of a clinical performance target, restricted stock units of various executive officers vested and were converted into 83,791 shares of the Company’s common stock (see Note 13, Stock-Based Compensation).

Fair Value Measurements

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurements

10. Fair Value Measurements

The following tables summarize the Company’s financial assets measured at fair value on a recurring basis as of December 31, 2018 and 2017 and by level within the fair value hierarchy:

Fair value measurement as of December 31, 2018:

			Quoted prices in		Significant other		Significant
Financial assets			active markets for		observable		unobservable
			identical assets		inputs		inputs
Type of Instrument	Total		(Level 1)		(Level 2)		(Level 3)
Cash and cash equivalents:
Money market funds and checking accounts	$	15,081	$	15,081	$	—	$	—
Available-for-sale marketable securities:
U.S. Treasury securities		19,539		—		19,539		—
U.S. government agency obligations		17,859		—		17,859		—
Corporate bonds		75,910		—		75,910		—
Commercial paper		50,390		—		50,390		—
Municipal bonds		4,000		—		4,000		—
Restricted cash:
Commercial money market account		769		769		—		—
Total financial assets	$	183,548	$	15,850	$	167,698	$	—

Fair value measurement as of December 31, 2017:

			Quoted prices in		Significant other		Significant
Financial assets			active markets for		observable		unobservable
			identical assets		inputs		inputs
Type of Instrument	Total		(Level 1)		(Level 2)		(Level 3)
Cash and cash equivalents:
Money market fund and checking accounts	$	9,388	$	9,388	$	—	$	—
Available-for-sale marketable securities:
Money market fund		39,951		—		39,951		—
U.S. government agency obligations		7,794		—		7,794		—
Corporate bonds		15,907		—		15,907		—
Commercial paper		19,529		—		19,529		—
Restricted cash:
Commercial money market account		769		769		—		—
Total financial assets	$	93,338	$	10,157	$	83,181	$	—

There were no purchases, sales or maturities of Level 3 financial assets and no unrealized gains or losses related to Level 3 available-for-sale marketable securities for the years ended December 31, 2018, 2017 and 2016. There were no transfers of financial assets between Levels 1, 2, or 3 classifications during the years ended December 31, 2018, 2017 and 2016.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Collaboration and Licensing Agreements

12 Months Ended

Dec. 31, 2018

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Collaboration and Licensing Agreements

11. Collaboration and Licensing Agreements

Vifor Fresenius Medical Care Renal Pharma Ltd.

On May 17, 2018, the Company entered into a license agreement, or the VFMCRP Agreement, with VFMCRP under which the Company granted VFMCRP an exclusive, royalty-bearing license, or the VFMCRP License, to seek regulatory approval to commercialize, import, export, use, distribute, offer for sale, promote, sell and otherwise commercialize CR845/difelikefalin injection, or the Licensed Product, for all therapeutic uses to prevent, inhibit or treat itch associated with pruritus in hemodialysis and peritoneal-dialysis patients, or the Field, worldwide (excluding the United States, Japan and South Korea), or the Territory. VFMCRP cannot perform development activities on their own unless specifically allocated to VFMCRP by the Joint Development Committee, or JDC, and Joint Steering Committee, or JSC. The Company’s membership on the JSC or JDC is at its sole discretion and is not its obligation.

The Company is responsible, at its own cost, to undertake clinical and non-clinical development, or the R&D services. The Company is also responsible to provide all content and subject matter expertise required for registration with the European Medicines Agency, or EMA, in the European Union, or the EU, that will be needed by VFMCRP for such registration, including participation in regulatory meetings, as needed. If third-party costs incurred by the Company with respect to its clinical development for the EMA registration exceed $20,000, such excess costs will be shared equally by the Company and VFMCRP. VFMCRP will contribute, at its own cost, its clinical development expertise as reasonably useful for such development activities, such as preparing the clinical results that the Company presents to it in a format acceptable to the EMA to obtain marketing approval in the EU.

The Company has identified two performance obligations under ASC 606: (1) granting of the VFMCRP License and (2) the R&D services. The Company has determined that these two performance obligations are not capable of being distinct (i.e., do not have standalone value for VFMCRP) because VFMCRP cannot benefit (derive potential cash flows) from either one on its own or together with other resources that are readily available to it since VFMCRP is relying on the Company’s expertise in investigating chronic kidney disease-associated pruritus, or CKD-aP, and its know-how obtained from multiple years of pre-clinical and clinical development, and years of interactions with the FDA which other companies or CROs would not have. The VFMCRP License does not provide benefit to VFMCRP until and unless the Company conducts the pivotal clinical trials and other supportive trials in CKD-aP to gather sufficient clinical data for VFMCRP to obtain marketing approval in the Territory. Furthermore, VFMCRP does not have the right to perform development activities on its own unless specifically allocated by the JDC or JSC.

The two identified performance obligations are also not distinct within the context of the contract, (i.e., are not separately identifiable from each other) because of the nature of the promise within the context of the contract. The nature of the promise is to transfer a combined deliverable to VFMCRP based on the agreement (to support the ability of VFMCRP to commercialize the Licensed Product) and the Company determined that the VFMCRP License and the R&D services are inputs rather than a transfer of each of these goods and services individually. In addition, the two identified performance obligations are highly interrelated and interdependent because satisfaction of both performance obligations is required for VFMCRP to derive benefit from the VFMCRP Agreement for commercialization of the Licensed Product in the Territory. Therefore, the two performance obligations are not distinct from each other and are accounted for as a single performance obligation.

Upon entry into the VFMCRP Agreement, VFMCRP made a non-refundable, non-creditable $50,000 upfront payment to the Company and Vifor purchased 1,174,827 shares of the Company’s common stock, or the Vifor Shares, for $20,000 at a price of $17.024 per share, which represents a premium over a pre-determined average closing price of the Company’s common stock. The purchase of the Company’s common stock was governed by a separate stock purchase agreement. The excess of the stock purchase price over the cost of the Vifor Shares at the closing price of the Company’s common stock on the purchase date of $5,444 was added to the upfront payment for accounting purposes.

The Company is eligible to receive from VFMCRP regulatory and commercial milestone payments in the aggregate of up to $470,000, consisting of up to $30,000 in regulatory milestones and up to $440,000 in tiered commercial milestones, all of which are sales-related. The Company is also eligible to receive tiered double-digit royalty payments based on annual net sales, as defined in the VFMCRP Agreement, of CR845/difelikefalin injection in the Licensed Territories. The Company retains full commercialization rights for CR845/difelikefalin injection for the treatment of CKD-aP in the United States except in the dialysis clinics of Fresenius Medical Care North America, or FMCNA, where VFMCRP and the Company will promote CR845/difelikefalin injection under a profit-sharing arrangement (subject to the terms and conditions of the VFMCRP Agreement) based on net FMCNA clinic sales recorded by the Company.

At inception of the VFMCRP Agreement, there was significant uncertainty as to whether marketing approval would be obtained in the Territory for the Licensed Product. Therefore, at that time, there was a significant probability that any potential revenue from sales of the Licensed Product that would be included in the transaction price would be reversed when the uncertainty is resolved. Consequently, any sales royalties and sales milestones are constrained from the transaction price at inception of the VFMCRP Agreement and will be recognized as revenue if, and when, such sales transactions occur in the future.

At inception of the VFMCRP Agreement, the transaction price of $55,444 was allocated entirely to the one combined performance obligation, as described above, and was initially recorded as deferred revenue. License and milestone revenue will be recognized proportionately as the R&D services are conducted (i.e., prior to submission of an NDA).

The license also requires VFMCRP to promote and take orders in the U.S. for sale by the Company to FMC U.S. Dialysis Clinics and allows VFMCRP to grant sub-licenses, which, in certain cases, requires the Company’s prior written consent. The Company retains the rights to import, distribute, promote, sell and otherwise commercialize the Licensed Product outside of the Field and outside of the Territory.

The Company retains the rights to make and have made the Licensed Product in the Territory for commercial sale by VFMCRP in the Field in or outside the Territory and for supply of Licensed Product to VFMCRP under the terms of a supply agreement, or the Supply Agreement. The supply price will be the Company’s cost of goods sold, as calculated under U.S. GAAP, plus an agreed upon margin. The Supply Agreement will co-terminate with the VFMCRP Agreement. In regards to a supply agreement, the VFMCRP Agreement only includes a requirement for the Company to negotiate in good faith with VFMCRP. After the execution of the VFMCRP Agreement, a separate agreement to supply them with the Licensed Product would be entered into, although the Company has no obligation to execute a supply agreement. In the event that the parties fail to enter into a Supply Agreement or if the Company fails to provide Licensed Product on a timely basis, VFMCRP has the right to manufacture or have manufactured the Licensed Product in and outside the Territory.

The Supply Agreement will be accounted for as a customer option that is not a material right because the selling price of the Licensed Product under the Supply Agreement is the Company’s cost of goods sold plus an agreed upon margin, which is commensurate with the “cost of goods sold plus” model that the Company would charge other parties under similar agreements (the standalone selling price) and not at a discount. Therefore, the sale of clinical compound to VFMCRP is not a performance obligation under the VFMCRP Agreement but rather the Supply Agreement is a separate agreement from the VFMCRP Agreement. The only performance obligation under the Supply Agreement is the delivery of the Licensed Product to VFMCRP for commercialization. Revenue from the sale of the Licensed Product to VFMCRP will be recognized as Clinical Supply revenue in the Company’s Statements of Comprehensive Loss as sales of the Licensed Product occur. As of December 31, 2018, no supply agreement has been entered into between the Company and VFMCRP.

The VFMCRP Agreement terminates upon the expiration of all royalty terms with respect to the Licensed Products, which expire on a Product-by-Product and country-by-country basis, at the latest of (a) the expiration of all patent rights licensed to VFMCRP covering such Licensed Product; (b) the expiration of all regulatory and data exclusivity applicable to such Licensed Product in such country and (c) the tenth anniversary of the first commercial sale of such Product in such country.

The VFMCRP Agreement may be terminated earlier by either party for material breach that is not cured within 60 days, bankruptcy by either party and by both parties upon mutual written consent. The Company may terminate the VFMCRP Agreement if VFMCRP challenges the validity of any licensed patent rights, except if such patent challenge results from the Company’s action against VFMCRP for infringement of any licensed patent in the Territory. In addition, upon the earlier of (1) the acceptance for filing of an NDA covering Licensed Product filed with the FDA (after completion of the Phase 3 program) or (2) the third anniversary of the Effective Date, the VFMCRP Agreement may be terminated by VFMCRP in its entirety or with respect to any countries within the Territory upon written notice to the Company. Such termination will be effective twelve months following the date of such notice.

If the VFMCRP Agreement terminates early for any reason stated above, VFMCRP’s licenses will terminate, VFMCRP’s rights to use the Company’s confidential information and the Company’s know-how will revert to the Company and VFMCRP will assign and transfer to the Company all right, title and interest in all regulatory applications (IND’s and NDA’s), regulatory approval applications and regulatory approvals in the Territory covering Licensed Product.

Maruishi Pharmaceutical Co., Ltd.

In April 2013, the Company entered into a license agreement with Maruishi, or the Maruishi Agreement, under which the Company granted Maruishi an exclusive license to develop, manufacture, and commercialize drug products containing CR845/difelikefalin for acute pain and/or uremic pruritus in Japan. Maruishi has the right to grant sub-licenses in Japan, which entitles the Company to receive sub-license fees, net of prior payments made by Maruishi to the Company. Under the Maruishi Agreement, the Company and Maruishi are required to use commercially reasonable efforts, at their own expense, to develop, obtain regulatory approval for and commercialize CR845/difelikefalin in the United States and Japan, respectively. In addition, the Company provided Maruishi specific clinical development services for CR845/difelikefalin used in Maruishi’s field of use.

Under the Maruishi Agreement, the Company identified two performance obligations in accordance with ASC 606: (1) the license; and (2) the R&D services specific to the uremic pruritus field of use (specified as Phase 1 and proof-of-concept clinical trials), both of which were determined to have standalone value. The Company determined that these performance obligations had standalone value due to the fact that Maruishi obtained the right to develop the compound on its own and the Company was specifically contracted to perform specific R&D services as noted above. The Company believes that these early stage R&D services performed by the Company did not require any specific expertise or know-how, but rather could have been completed by outside third parties, therefore providing standalone value to Maruishi.

In March 2017, Maruishi entered into a sub-license agreement with Kissei Pharmaceutical Co. Ltd., or Kissei, for the development and sales/marketing of CR845/difelikefalin (called MR13A9 by Maruishi) for the treatment of uremic pruritus in dialysis patients in Japan. Consequently, for the year ended December 31, 2017, the Company recognized revenue of $843 related to the sub-license fee. The Company allocated the amount of the sub-license fee to each of the two identified performance obligations in the same proportion as the upfront license fee that the Company received at inception of the Maruishi Agreement. Accordingly, $530 was recognized as license and milestone fees revenue and $313 was recognized as collaborative revenue.

Under the terms of the Maruishi Agreement, the Company is eligible to receive milestone payments upon the achievement of defined clinical and regulatory events as well as tiered, low double-digit royalties with respect to any sales of the licensed product sold in Japan by Maruishi, if any, and share in any sub-license fees.

During the years ended December 31, 2018, 2017 and 2016, the Company recognized clinical compound revenue of $33, $68 and $86, respectively, from the sale of clinical compound to Maruishi.

The Company incurred R&D expense related to the Maruishi Agreement of $30, $61 and $78 (all related to the cost of clinical compound sold to Maruishi) during the years ended December 31, 2018, 2017 and 2016, respectively.

Chong Kun Dang Pharmaceutical Corporation

In April 2012, the Company entered into a license agreement, or the CKDP Agreement, with Chong Kun Dang Pharmaceutical Corporation, or CKDP, in South Korea, under which the Company granted CKDP an exclusive license to develop, manufacture and commercialize drug products containing CR845/difelikefalin in South Korea. The Company and CKDP are each required to use commercially reasonable efforts, at their respective expense, to develop, obtain regulatory approval for and commercialize CR845/difelikefalin in the United States and South Korea, respectively. The Company identified the granting of the license as its only performance obligation under the CKDP Agreement.

Under the terms of the CKDP Agreement, the Company is eligible to receive milestone payments upon the achievement of defined clinical and regulatory events as well as tiered royalties, with percentages ranging from the high single digits to the high teens, based on net sales of products containing CR845/difelikefalin in South Korea, if any, and share in any sub-license fees.

Revenue Recognition

12 Months Ended

Dec. 31, 2018

Revenue Recognition [Abstract]

Revenue Recognition

12. Revenue Recognition

The Company currently recognizes revenue in accordance with ASC 606, as amended, for the VFMCRP, Maruishi and CKDP agreements (see Note 11, Collaboration and Licensing Agreements). Under each of these agreements, the Company has recognized revenue from upfront payments and, under the Maruishi Agreement and the CKDP Agreement, from clinical development milestone payments. The Company has also recognized revenue from a sub-license payment earned under the Maruishi Agreement. Under the Maruishi Agreement and the CKDP Agreement, the Company may earn additional future milestone payments upon the achievement of defined clinical events, and under the VFMCRP Agreement, the Maruishi Agreement and the CKDP Agreement upon the achievement of defined regulatory events and, under the VFMCRP Agreement and the Maruishi Agreement, from sales milestones. The Company may also recognize revenue in the future from royalties on net sales under all three agreements. In addition, the Company has recognized revenue upon the delivery of clinical compound to Maruishi in accordance with separate supply agreements.

Contract balances

As of December 31, 2018, the Company had deferred revenue, current of $26,825 and deferred revenue, non-current of $15,184 related to the performance obligations from the VFMCRP Agreement and had no balances of receivables or other assets related to the VFMCRP Agreement. There were no balances of receivables, other assets or deferred revenue relating to the Maruishi and CKDP agreements as of December 31, 2018. As of December 31, 2017, the Company had no balances of receivables, other assets or deferred revenue related to the Maruishi and CKDP Agreements.

Performance obligations

Under the VFMCRP Agreement, the Company’s performance obligations of granting a license to allow VFMCRP to commercialize CR845/difelikefalin injection worldwide, except in the United States, Japan and South Korea, which occurred at inception of the contract in May 2018, and performing R&D services by the Company to obtain sufficient clinical data which will be shared with VFMCRP to allow them to receive regulatory approval to sell CR845/difelikefalin in the licensed territory, are not distinct, and are accounted for as a single performance obligation during the period that the R&D services are rendered (see Note 11, Collaboration and Licensing Agreements).

The Company’s distinct performance obligations under the Maruishi Agreement include transfer of the license to the Company’s IP, which allowed Maruishi to develop and commercialize CR845/difelikefalin, for acute pain and uremic pruritus indications in Japan, which occurred at inception of the contract in 2013, and performance of R&D services, which occurred from 2013 to 2015, as those services were rendered. The Company agreed to conduct limited work on an oral tablet formulation of CR845/difelikefalin and to conduct Phase 1 and proof-of-concept Phase 2 clinical trials of an intravenous formulation of CR845/difelikefalin to be used to treat patients with uremic pruritus. The Company agreed to transfer the data and information from such development to Maruishi for its efforts to obtain regulatory approval in Japan. These activities are referred to as R&D services.

The Company’s only performance obligation under the supply agreement with Maruishi is to deliver clinical compound to Maruishi in accordance with the receipt of purchase orders. If and when the Company enters into a supply agreement with VFMCRP, the Company’s only performance obligation under this supply agreement would be to deliver CR845/difelikefalin injection to VFMCRP in accordance with the receipt of purchase orders.

Under the CKDP Agreement, the Company’s only performance obligation is to transfer the license to the Company’s IP related to CR845/difelikefalin, which occurred at inception of the contract in 2012.

Upon execution of the VFMCRP Agreement, the Maruishi Agreement and the CKDP Agreement, the Company received a single fixed payment from each counterparty in exchange for granting the respective licenses and performing its other obligations. In addition, each of the counterparties made an equity investment in the Company’s common stock.

Transaction price allocated to the remaining performance obligations

At inception of the VFMCRP Agreement, the entire transaction price of $55,444 was allocated to the one combined performance obligation, as described above. As of December 31, 2018, $13,436 of that amount was recognized as license and milestone fees revenue based on the percentage of R&D services that had been completed. As of December 31, 2018, there were no remaining performance obligations under either the Maruishi Agreement or the CKDP Agreement, although the Company is eligible to receive milestone payments and sales royalties in the future.

Significant judgments

In applying ASC 606, as amended, to its three contracts, the Company made the following judgments that significantly affect the timing and amount of revenue recognition:

Determination of the number of distinct performance obligations in a contract

The VFMCRP Agreement contains one combined performance obligation, which includes the Company’s two performance obligations to grant a license to VFMCRP and conduct R&D services. Both of those performance obligations are inputs to the promise, within the context of the contract, to transfer a combined output for which VFMCRP has contracted (the ability of VFMCRP to commercialize the Licensed Product) (see Note 11, Collaboration and Licensing Agreements, for further discussion).

The Maruishi Agreement contains two distinct performance obligations: the granting of the license and the promise to deliver defined R&D services. Under the Maruishi Agreement, the license and the R&D services represent distinct goods or services from each other because Maruishi is able to benefit from the license on its own or together with other resources that are readily available to it (i.e., capable of being distinct). Maruishi’s ability to benefit from the license without the R&D services is indicated by its ability to conduct clinical trials of CR845/difelikefalin on its own and by the provision in the Maruishi Agreement whereby if the Company suspends or discontinues its development activity, the Company will provide information regarding its development efforts up to that point so that Maruishi may continue development and commercialization of the product in Japan. Therefore, the R&D services do not significantly affect Maruishi’s ability to use and benefit from the license.

In addition, the Company’s promise in the Maruishi contract to transfer the license is separately identifiable from the promise to provide defined R&D services (i.e., distinct within the context of the contract) because the Company is not using the goods or services as inputs to produce or deliver the combined output or outputs specified by the customer. The combined output specified by Maruishi is its right to conduct development activities related to CR845/difelikefalin in Japan, which could result in regulatory approval in Japan. That right is derived from the Company’s grant of the license. Maruishi is conducting clinical trials on its own and does not require the R&D services provided by the Company. Furthermore, the R&D services do not significantly modify or customize the license and vice versa. Finally, the license and R&D services are not highly interdependent or highly interrelated because the Company is able to fulfill its promise to transfer the initial license independently from its promise to subsequently provide the R&D services, which Maruishi can obtain on its own.

The only performance obligation in the CKDP Agreement is the granting of the license.

Determination of the transaction price, including whether any variable consideration is included at inception of the contract

The transaction price is the amount of consideration that the Company expects to be entitled to in exchange for transferring promised goods or services to the customer. The transaction price must be determined at inception of a contract and may include amounts of variable consideration. However, there is a constraint on inclusion of variable consideration, such as milestone payments or sales-based royalty payments, in the transaction price related to licenses of IP, if there is uncertainty at inception of the contract as to whether such consideration will be recognized in the future (see Note 2, Significant Accounting Policies: Revenue Recognition).

The decision as to whether or not it is probable that a significant reversal of revenue will occur in the future, depends on the likelihood and magnitude of the reversal and is highly susceptible to factors outside the entity’s influence (for example, the Company cannot determine the outcome of clinical trials; the Company cannot determine if or when they or the counterparty will initiate or complete clinical trials; and the Company’s ability to obtain regulatory approval is difficult). In addition, the uncertainty is not expected to be resolved for a long period of time (in the order of years) and finally, the Company has limited experience in the field.

Therefore, at inception of the VFMCRP Agreement, the Maruishi Agreement and the CKDP Agreement, milestones and sales-based royalty payments were not included in the transaction price based on the factors noted above.

Under the VFMCRP Agreement, the single combined performance obligation will be satisfied as the R&D services are rendered and the transaction price, including the upfront payment of $50,000 and the premium on the common stock purchased by VFMCRP of $5,444, will be recognized as revenue as the R&D services are performed based on the costs incurred as a percentage of the estimated total costs to be incurred to complete the performance obligation. The remaining potential consideration was considered to be variable consideration and was constrained at inception of the contract, including regulatory and sales milestones and sales royalties (see Note 11, Collaboration and Licensing Agreements).

All performance obligations under the Maruishi Agreement and the CKDP Agreement were satisfied by the end of 2015. In the future, any milestone event will be recognized in accordance with Note 2, Significant Accounting Polices: Revenue Recognition, as milestone and license fee revenue and collaboration revenue based upon the relative standalone selling prices of the two performance obligations at inception of the Maruishi Agreement, and as milestone and license fee revenue under the CKDP Agreement.

Under the Maruishi Agreement, the transaction price includes only the non-refundable and non-creditable upfront license fee of $15,337, including the premium of $337 from the sale of Company stock to Maruishi, that was paid to the Company at inception of the contract. The remaining potential consideration was considered to be variable consideration and was constrained at inception of the contract, including an aggregate of up to $10,500, which the Company is eligible to receive upon achievement of clinical development and regulatory milestones, a one-time sales milestone of one billion Yen when a certain sales level is attained; a mid-double-digit percentage of all non-royalty payments received by Maruishi from its sub-licensees, if any; and tiered royalties based on net sales of products containing CR845/difelikefalin in Japan, if any, with minimum royalty rates in the low double digits and maximum royalty rates in the low twenties.

Under the CKDP Agreement, the transaction price includes only the non-refundable and non-creditable upfront license fee of $646, including the premium of $83 from the sale of Company stock to CKDP, that was paid to the Company at inception of the contract. The remaining consideration was considered to be variable consideration and was constrained at inception of the contract, including an aggregate of up to $3,750, which the Company is eligible to earn upon achievement of clinical development and regulatory milestones. The Company is also eligible to receive a mid-double-digit percentage of all non-royalty payments received by CKDP from its sub-licensees, if any, and tiered royalties ranging from the high single digits to the high teens based on net sales of products containing CR845/difelikefalin in South Korea, if any.

Determination of the estimate of the standalone selling price of performance obligations

In order to recognize revenue under ASC 606, as amended, for contracts for which more than one distinct performance obligation has been identified, the Company must allocate the transaction price to the performance obligations based upon their standalone selling prices. The best evidence of standalone selling price is an observable price of a good or service when sold separately by an entity in similar circumstances to similar customers. If such evidence is not available, standalone selling price should be estimated so that the amount that is allocated to each performance obligation equals the amount that the entity expects to receive for transferring goods or services. The Company has identified more than one performance obligation only in the Maruishi Agreement. Since evidence based on observable prices is not available for the performance obligations under the Maruishi Agreement, the Company considered market conditions and entity-specific factors, including those contemplated in negotiating the agreements, as well as certain internally developed estimates.

At inception of the Maruishi Agreement, the Company determined the estimate of standalone selling price for the license performance obligation by using the adjusted market assessment approach. Under this method, the Company forecasted and analyzed CR845/difelikefalin in the Japanese market, the phase of clinical development as well as considered recent similar license arrangements within the same phase of clinical development, therapeutic area, type of agreement, etc. To estimate the standalone selling price of the R&D services, the Company forecasted its expected costs of satisfying that performance obligation and added a margin for that service.

Determination of the method of allocation of the transaction price to the distinct performance obligations

At inception of the Maruishi Agreement, the Company allocated the transaction price of $15,337 between the two performance obligations based on their relative standalone selling prices, determined as described above. The Company determined that the license and the R&D services had estimated standalone selling prices of $10,200 and $6,200, respectively. The resulting percentage allocations were applied to the $15,337 of total transaction price, which resulted in $9,637 being allocated to the license performance obligation, which was recognized immediately as license revenue, while $5,700 was allocated to the R&D services performance obligation. The amount allocated to the R&D services performance obligation was initially recorded as deferred revenue and was recognized as collaborative revenue as the R&D services were provided through July 2015.

Since both the VFMCRP Agreement and the CKDP Agreement each contain only one distinct performance obligation, at the inception of each of those agreements, the entire transaction price was allocated to the respective performance obligation.

Determination of the timing of revenue recognition for contracts

Revenue should be recognized when, or as, an entity satisfies a performance obligation by transferring a promised good or service to a customer; i.e., when the customer obtains control of the good or service. The licenses granted to both Maruishi and CKDP are being accounted for as distinct performance obligations. As discussed below, both licenses relate to functional IP for which revenue is recognized at a point in time – in the case of these two license agreements, the point in time is at inception of the contract because the customer obtained control of the license at that point.

The licenses grant Maruishi and CKDP the right to use the Company’s IP relating to CR845/difelikefalin as it existed at the point in time that the licenses were granted. That IP has significant standalone functionality as it provides the customer with the ability to perform a function or task, such as to manufacture CR845/difelikefalin and conduct clinical trials, and is considered to be functional IP.

During the license periods, the Company is continuing to develop and advance CR845/difelikefalin by conducting clinical trials. Those development efforts are for its own benefit and do not substantively change the significant standalone functionality of the licensed IP granted to Maruishi or CKDP. Therefore, the Company’s ongoing development efforts do not significantly affect the IP’s utility to which Maruishi or CKDP have rights. Furthermore, if the Company abandons its development efforts, Maruishi or CKDP may still continue to develop CR845/difelikefalin in their respective countries.

The R&D services performance obligation under the Maruishi Agreement represents a separate performance obligation. The R&D services were provided to Maruishi by the Company from inception of the agreement in 2013 through the third quarter of 2015, at which time the Company had fulfilled its promise related to the R&D services. Revenue related to the R&D services performance obligation was recognized as services were performed based on the costs incurred as a percentage of the estimated total costs to be incurred to complete the performance obligation.

Similarly, under the VFMCRP Agreement, revenue related to the single distinct performance obligation, which includes both granting of the license and performance of the R&D services, will be recognized as the R&D services are performed, based on the costs incurred as a percentage of the estimated total costs to be incurred to complete the performance obligation. The Company expects that the remaining amount of the transaction price that was allocated to the combined performance obligation of $42,009 at December 31, 2018 will be recognized by 2020, as the R&D services are performed, subject to certain development and regulatory uncertainties.

Determination of consideration as variable consideration, including factors related to inclusion in the transaction price at inception of the contract and timing of recognition as revenue.

The VFMCRP Agreement, the Maruishi Agreement and the CKDP Agreement contain potential payments related to achievement of defined milestone events and royalties upon net sales of future products, which are considered to be variable consideration because of the uncertainty of occurrence of any of those events specified in those agreements at inception of the agreements. Therefore, those potential payments were not included in the transaction price at the inception of the agreements.

Revenue related to achievement of milestone events is recognized when the Company has determined that it is probable that a milestone event will be achieved and there will not be a significant reversal of revenue in future periods. Upon probability of achievement of a milestone event, the most likely amount of variable consideration is included in the transaction price. Subsequent changes to the transaction price, after contract initiation, are allocated to the performance obligations in the contract on the same basis as at contract inception. Revenue for variable consideration is recognized in the same manner (point in time or over time) as for the performance obligations to which the payment amounts were allocated.

The Maruishi Agreement and the CKDP Agreement specify that certain development milestones will be achieved at pre-specified defined phases of a clinical trial (such as initiation or completion or other pre-specified time during a clinical trial as specified in the agreements).

During the years ended December 31, 2018, 2017 and 2016, no milestone events were probable of occurrence or achieved.

Sublicense payments

VFMCRP’s, Maruishi’s and CKDP’s right to grant sub-licenses is explicitly stated in their respective license agreements. The amount of any potential sub-license fees to be received by the Company, which is based on a formula, is considered to be variable consideration and is constrained from inclusion in the transaction price at inception of the contract since at that time it was probable that there would be a reversal of such revenue in the future because the Company did not know if a sublicense would be granted in the future.

In March 2017, Maruishi entered into a sub-license agreement to the Maruishi Agreement with Kissei in Japan for development and sales/marketing of CR845/difelikefalin for the treatment of uremic pruritus in dialysis patients in Japan. The Company first learned that the terms of the sub-license agreement had been finalized less than a month before the sub-licensee publicly announced the agreement. At that time, the Company determined that the sub-license fee would not be constrained from inclusion in the transaction price. Consequently, the Company included the amount of the sub-license fee in the transaction price and recognized revenue of $843 in the same manner as described above for milestone payments.

Sales-based Royalty Payments

The VFMCRP Agreement, CKDP Agreement and Maruishi Agreement each allow the Company to earn sales-based royalty payments in exchange for a license of intellectual property. In that case, the Company will recognize revenue for a sales-based royalty only when (or as) the later of the following events occurs:

The subsequent sale or usage occurs.

The performance obligation to which some or all of the sales-based royalty has been allocated has been satisfied (or partially satisfied).

Since the sale (item a, above) occurs after the license was delivered (item b, above), the sales-based royalty exception, to exclude such royalty payments from the transaction price, applies to the overall revenue stream. Therefore, sales-based royalty payments are recognized as revenue when the customer’s sales occur. To date, no royalties have been earned or were otherwise due to the Company.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2018

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

13. Stock-Based Compensation

2014 Equity Incentive Plan

The Company’s 2014 Equity Incentive Plan, or the 2014 Plan, is administered by the Company’s Board of Directors or a duly authorized committee thereof, referred to as the Plan administrator. The 2014 Plan provides for the grant of incentive stock options, non-statutory stock options, restricted stock awards, restricted stock unit awards, stock appreciation rights, performance stock awards and other forms of equity compensation, collectively referred to as Stock Awards. Additionally, the 2014 Plan provides for the grant of performance cash awards. Incentive stock options may be granted only to employees. All other awards may be granted to employees, including officers, non-employee directors, and consultants. No incentive stock options may be granted under the 2014 Plan after the tenth anniversary of the effective date of the 2014 Plan. Stock Awards granted under the 2014 Plan vest at the rate specified by the Plan administrator. Initial grants of Stock Awards made to employees and non-employee consultants generally vest as to 25% on the first anniversary of the date of grant and the balance ratably over the next 36 months and subsequent grants vest monthly over a period of four years from the grant date. Beginning in 2018, stock options initially granted to members of the Company’s Board of Directors vest over a period of three years in equal installments from the date of the grant, subject to the option holder’s continued service as a Director through such date. Subsequent grants to Directors that are made automatically at Annual Meetings of Stockholders vest fully on the first anniversary of the date of grant. The Plan administrator determines the term of Stock Awards granted under the 2014 Plan up to a maximum of ten years.

The aggregate number of shares of the Company’s common stock reserved for issuance under the 2014 Plan has automatically increased on January 1 of each year, beginning on January 1, 2015 and will continue to increase on January 1 of each year through and including January 1, 2024, by 3% of the total number of shares of the Company’s capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s Board of Directors. On January 1, 2019, the aggregate number of shares of common stock that may be issued pursuant to Stock Awards under the 2014 Plan automatically increased from 4,900,481 to 6,086,907. The maximum number of shares that may be issued pursuant to the exercise of incentive stock options under the 2014 Plan is 30,000,000 shares.

Restricted Stock Units

In September 2018, the Company granted a total of 83,791 restricted stock units to executive officers under the 2014 Plan with a grant date fair value of $20.21 per share. Vesting of the restricted stock units was contingent on the achievement of certain performance targets through the first quarter of 2019, subject to the recipient’s continuous service through the vesting events. At the date of grant, the Company concluded that the probability of achievement of the performance targets could not be determined until they were achieved, and accordingly, the Company would recognize compensation expense associated with these awards when, and to the extent, the restricted stock units vested in accordance with achievement of the performance targets. As of December 31, 2018, all of the performance targets had been achieved and, consequently, all of the restricted stock units had vested. As a result, $1,693 of stock compensation expense relating to the vesting of restricted stock units was recognized in the Statement of Comprehensive Loss for the year ended December 31, 2018, consisting of $1,217 relating to G&A stock compensation expense and $476 relating to R&D stock compensation expense. In addition, all of the 83,791 restricted stock units were converted to outstanding shares of the Company’s common stock as of December 31, 2018.

2004 Stock Incentive Plan

The Company’s 2004 Stock Incentive Plan, or the 2004 Plan, as amended, was adopted by the Company’s Board of Directors and stockholders. Under the 2004 Plan, the Company has granted stock options to selected officers, employees and consultants of the Company. The Company’s Board of Directors administers the 2004 Plan. Options granted under the 2004 Plan have a maximum term of ten years. Options issued generally vest 25% on the first anniversary date of grant and the balance ratably over the next 36 months. Following the effectiveness of the 2014 Plan in January 2014, no additional options or restricted share awards were granted under the 2004 Plan. As of September 30, 2014, the 2004 Plan expired and no further grants of stock options or restricted stock are allowed.

The Company accounts for stock options granted to employees and non-employee members of the Board of Directors in accordance with ASC 718, Compensation – Stock Compensation. The Company also occasionally grants stock options to non-employee consultants. Such grants are accounted for pursuant to ASC 505-50, Equity-Based Payments to Non-Employees (refer to Note 2, Summary of Significant Accounting Policies - Stock-Based Compensation).

A summary of the Company’s stock option activity related to employees, non-employee members of the Board of Directors and non-employee consultants as of and for the year ended December 31, 2018 is as follows:

				Weighted-		Aggregate
	Number of			Average		Intrinsic
	Options			Exercise Price		Value
Outstanding at December 31, 2017		3,492,141		$	11.75
Granted		1,197,500			16.15
Exercised		(451,685	)		9.29
Expired		(60,111	)		10.79
Forfeited		(173,423	)		12.12
Outstanding at December 31, 2018		4,004,422		$	13.34	$	6,627
Weighted average remaining contractual life as of December 31, 2018 (in years)		7.92
Options exercisable at December 31, 2018		1,974,979		$	11.70	$	4,985
Weighted average remaining contractual life as of December 31, 2018 (in years)		7.00
Options vested and expected to vest as of December 31, 2018		4,004,422		$	13.34	$	6,627
Weighted average remaining contractual life as of December 31, 2018 (in years)		7.92

The total fair value of options vested during the years ended December 31, 2018, 2017 and 2016 was $9,023, $5,303 and $3,589, respectively. The intrinsic value of options exercised during the years ended December 31, 2018, 2017 and 2016 was $3,893, $2,285 and $126, respectively.

During the years ended December 31, 2018, 2017 and 2016, the Company granted 1,197,500, 1,328,500 and 1,078,000 stock options, respectively, to employees, non-employee members of the Board of Directors or non-employee consultants. The fair values of the stock options granted to those groups were estimated using the Black-Scholes option valuation model with the following ranges of assumptions (see Note 2, Summary of Significant Accounting Policies - Stock-Based Compensation):

	Year Ended December 31,
	2018		2017	2016
Risk-free interest rate	2.51% - 3.09%		1.85% - 2.57%	1.19% - 1.93%
Expected volatility	82.6% - 92.8%		75.3% - 84.5%	67.8% - 77.8%
Expected dividend yield	0%		0%	0%
Expected life of employee and Board of Directors' options (in years)	6.25		6.25	6.25
Expected life of non-employee options (in years)		—	10	10

The weighted average grant date fair value of options granted to employees, non-employee members of the Board of Directors for their Board service and non-employee consultants during the years ended December 31, 2018, 2017 and 2016 was $11.99, $11.46 and $4.28, respectively.

At the end of each fiscal quarter during the years ended December 31, 2018, 2017 and 2016, the Company used the Black-Scholes option valuation model with the following ranges of assumptions to re-measure the fair value of all outstanding options that had been granted to non-employee consultants during the vesting period of each tranche in accordance with ASC 505-50:

	Year Ended December 31,
	2018	2017	2016
Risk-free interest rate	1.82% - 3.02%	1.28% - 2.39%	1.35% - 2.38%
Expected volatility	58.2% - 101.0%	74.6% - 87.3%	70.8% - 75.5%
Expected dividend yield	0%	0%	0%
Expected life of non-employee options (in years)	0.25 - 8.94	0.62 - 9.94	7.08 - 9.60

Under ASC 505-50, upon re-measurement of each award, income or expense is recognized during its vesting term (see Note 2, Accounting Pronouncements Not Yet Adopted for the adoption of ASU 2018-07 on January 1, 2019).

The weighted average fair value of outstanding options that had been granted to nonemployee consultants, as re-measured during the vesting period of each tranche in accordance with ASC 505-50 during the years ended December 31, 2018, 2017 and 2016 was $8.74, $10.16 and $4.81, respectively.

On January 1, 2017, the Company adopted ASU 2016-09, Improvements to Employee Share-Based Payment Accounting (see Note 2, Basis of Presentation - Recently Adopted Accounting Pronouncements). On the date of adoption of ASU 2016-09, the Company began to account for forfeitures of unvested stock options as they occur rather than estimate forfeiture rates that were applied to unvested stock option awards, as under the previous accounting guidance. Accordingly, on the date of adoption, the Company recorded a cumulative-effect adjustment to stockholders’ equity of $45 for all stock option awards that were unvested as of that date.

During the years ended December 31, 2018, 2017 and 2016, the Company recognized compensation expense relating to stock options (excluding compensation expense related to the vesting of restricted stock units of $476 in R&D and $1,217 in G&A in 2018), as follows:

	Year Ended December 31,
	2018		2017		2016
Research and development	$	3,919	$	2,433	$	1,301
General and administrative		4,482		3,897		1,499
Total stock option expense	$	8,401	$	6,330	$	2,800

Included in the table above are the following amounts of compensation expense recognized with regard to stock options that were granted to non-employee consultants, including the effect of re-measurement of the fair values of those options, as described above:

	Year Ended December 31,
	2018		2017		2016
Research and development	$	195	$	170	$	(79	)
General and administrative		192		200		(20	)
Total stock option expense	$	387	$	370	$	(99	)

In October 2018, the Company modified the terms of its former Chief Medical Officer’s outstanding Stock Awards to accelerate 50% of the unvested shares underlying his outstanding stock options immediately as of the modification date, and specify that the remainder of the unvested shares will vest monthly through the date of termination of his continuous service to the Company as a Consultant. As of the modification date, the Company entered into a consulting agreement with the former Chief Medical Officer under which he will provide continuous service to the Company as a Consultant by providing transition services and other services upon request by the Company. Pursuant to the terms of the separation and consulting agreement, such Stock Awards will continue to vest under their original vesting conditions as long as he provides continuous service to the Company (including as a consultant). The term of his consulting agreement is through July 22, 2019, if not terminated earlier per the terms of the consulting agreement or extended by the Company.

In August 2017, the Company modified the terms of its former Chief Financial Officer’s outstanding Stock Awards to (1) accelerate 50% of the unvested shares underlying his outstanding Stock Awards immediately as of the modification date and specify that the remainder will vest monthly through the date of termination of his continuous service to the Company; and (2) extend the period during which his outstanding Stock Awards may be exercised through the six-month anniversary of the date of termination of his continuous service to the Company. As of the modification date, the Company entered into a consulting agreement with the former Chief Financial Officer under which he provided continuous service to the Company by assisting with the transition of his role to the Company’s Chief Financial Officer. Pursuant to the terms of the 2014 Plan and his outstanding Stock Awards, such Stock Awards continued to vest under their original vesting conditions as long as he provided continuous service to the Company (including as a consultant). The term of his consulting agreement ended on February 15, 2018.

The Company determined that the acceleration of vesting for Stock Awards in 2018 and 2017 that would have vested based on their original vesting terms through the term of the consulting services were Type 1 modifications pursuant to ASC 718, Compensation – Stock Compensation, because those Stock Awards would have vested whether or not the vesting of those Stock Awards had been accelerated. However, acceleration of vesting for the remaining Stock Awards was a Type 3 modification pursuant to ASC 718 because absent the modification terms, those Stock Awards would have been forfeited as of the last day that the former Chief Medical Officer and Chief Financial Officer provided continuous service as a consultant.

During the year ended December 31, 2018, with respect to these modifications for the former Chief Medical Officer, the Company recognized $520 of compensation expense, including expense based on marking to market the fair value of the modified Stock Awards in accordance with ASC 505-50, which is included in Research and development expense in the total compensation expense table above.

During the years ended December 31, 2018 and 2017, with respect to these modifications for the former Chief Financial Officer, the Company recognized $96 and $537 of compensation expense, respectively, including expense based on marking to market the fair value of the modified Stock Awards in accordance with ASC 505-50, which is included in General and administrative expense in the total compensation expense table above.

As of December 31, 2018, the total compensation expense relating to unvested options granted to employees, non-employee members of the Board of Directors and non-employee consultants that had not yet been recognized was $20,474, which is expected to be realized over a weighted average period of 2.84 years. The Company will issue shares upon exercise of options from common stock reserved.

The Company does not expect to realize any tax benefits from its stock option activity or the recognition of stock-based compensation expense because the Company currently has net operating losses and has a full valuation allowance against its deferred tax assets. Accordingly, no amounts related to excess tax benefits have been reported in cash flows from operations or cash flows from financing activities for the years ended December 31, 2018, 2017 and 2016.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

14. Income Taxes

The Company’s benefit from income taxes is as follows:

	December 31,
	2018			2017			2016
Current:
Federal	$	—		$	—		$	—
State		(389	)		(204	)		(468	)
		(389	)		(204	)		(468	)
Deferred:
Federal		—			—			—
State		—			—			—
		—			—			—
Benefit from income taxes	$	(389	)	$	(204	)	$	(468	)

The Company’s tax benefits relate to state R&D tax credits exchanged for cash. The State of Connecticut provides companies with the opportunity to exchange certain R&D credit carryforwards for cash in exchange for foregoing the carryforward of the R&D credit. The program provides for such exchange of the R&D credits at a rate of 65% of the annual R&D credit, as defined.

A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations is as follows:

	December 31,
	2018			2017			2016
Income taxes using U.S. federal statutory rate		21.00	%		34.00	%		34.00	%
State income taxes, net of federal benefit		6.82	%		5.33	%		5.44	%
Tax Cuts and Jobs Act		0.00	%		-44.43	%		0.00	%
Impact of R&D tax credit on effective tax rate		3.48	%		3.25	%		3.24	%
Stock option shortfalls and cancellations		-0.43	%		0.21	%		-0.07	%
Permanent items and other		-0.15	%		-0.56	%		-0.64	%
Change in valuation allowance		-31.76	%		2.55	%		-41.17	%
Provision to return		0.03	%		0.00	%		0.00	%
Non-taxable revenue		1.54	%		0.00	%		0.00	%
		0.53	%		0.35	%		0.80	%

Significant components of the Company’s deferred tax assets and liabilities are as follows:

	December 31,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards	$	73,578		$	54,831
Federal and state tax credits		11,108			8,401
Deferred revenue		1,111			—
Stock-based compensation expense		3,605			2,382
Other		420			582
		89,822			66,196
Deferred tax liabilities:
Accelerated depreciation		(7	)		(23	)
Valuation allowance		(89,815	)		(66,173	)
Net deferred tax asset	$	—		$	—

A 100% valuation allowance has been recorded on the deferred tax asset as of December 31, 2018 and 2017 because management believes it is more likely than not that the asset will not be realized. The change in the valuation allowance during 2018 and 2017 was $23,642 and $618, respectively.

In 2017, the Company recorded a cumulative-effect adjustment for the tax benefit of approximately $840 related to the exercise of non-qualified stock options and the disqualified disposition of incentive stock options. As a result of the adoption of ASU 2016-09 on January 1, 2017, the tax benefit related to the exercise of stock options was recognized as a deferred tax asset with a corresponding cumulative adjustment to retained earnings, that is offset by a valuation allowance against retained earnings.

The Company applies the provisions of ASC 740, Income Taxes, which prescribes a comprehensive model for how a company should recognize, measure, present, and disclose in its financial statements uncertain tax positions that the Company has taken or expects to take on a tax return. As of December 31, 2018 and 2017, the Company had no unrecognized tax benefits or related interest and penalties accrued. In the event the Company determines that accrual of interest or penalties are necessary in the future, the amount will be presented as a component of income tax expense.

The Company files income tax returns in the United States and the State of Connecticut. All tax years since the date of the Company’s incorporation remain open to examination by the major taxing jurisdictions (state and federal) to which the Company is subject, as carry-forward attributes generated in years past may still be adjusted upon examination by the Internal Revenue Service, or IRS, or other authorities if they have or will be used in a future period. The Company is not currently under examination by the IRS, or any other jurisdictions, for any tax year.

At December 31, 2018, the Company had federal and state net operating loss carryforwards of approximately $274,764 and $267,973, respectively. The federal and state tax loss carryforwards will begin to expire in 2026 and 2027, respectively, unless previously utilized. The federal net operating losses arising in 2018 and forward have an unlimited carryforward period, however will only offset 80% of taxable income in a carryforward year. The federal losses may also be subject to limitation pursuant to Internal Revenue Code section 382. The Company also had federal and state R&D tax credit carryforwards of approximately $9,925 and $1,236, respectively. The federal credits will begin expiring in 2025 unless previously utilized. The Connecticut credit carryforwards have no expiration period. Because of the net operating loss and research credit carryforwards, tax years 2006 through 2018 remain open to U.S. federal and state tax examinations.

On December 22, 2017, the United States enacted the Tax Cuts and Jobs Act (the “Act”). The Act, which is also commonly referred to as “U.S. tax reform”, significantly changes U.S. corporate income tax laws by, among other provisions, reducing the maximum U.S. corporate income tax rate from 35% to 21% starting in 2018. During the year ended December 31, 2017, the Company reduced deferred tax assets by $25,913, offset by a corresponding reduction to its valuation allowance, as a result of the re-measurement of deferred tax assets and liabilities from its 34% effective rate under existing law to the new lower statutory rate of 21%. As of December 31, 2018 and 2017, the Company did not have any foreign subsidiaries and the international aspects of the Act were not applicable.

On December, 22, 2017, SAB 118 was issued due to the complexities involved in accounting for the recently enacted Tax Act. SAB 118 requires the Company to include in its financial statements a reasonable estimate of the impact of the Tax Act on earnings to the extent such estimate has been determined. Accordingly, the U.S. provision for income tax for 2017 was based on the reasonable estimate guidance provided by SAB 118. The Company has finalized its accounting for the Act as of December 31, 2018, which resulted in insignificant adjustments.

Net Loss Per Share

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Net Loss Per Share

15. Net Loss per Share

The Company computes net loss per share in accordance with ASC 260-10, Earnings per Share (see Note 2, Significant Accounting Policies – Income (Loss) per Share).

The denominators used in the net loss per share computations are as follows:

	Year Ended December 31,
	2018		2017		2016
Basic:
Weighted average shares outstanding		35,892,786		31,202,842		27,279,008
Diluted:
Weighted average shares outstanding - Basic		35,892,786		31,202,842		27,279,008
Common stock options *		—		—		—
Denominator for diluted net loss per share		35,892,786		31,202,842		27,279,008

*	No amounts were considered as their effects would be anti-dilutive.

Basic and diluted net loss per share are computed as follows:

	Year Ended December 31,
	2018			2017			2016
Net loss	$	(74,013	)	$	(58,125	)	$	(57,280	)
Weighted-average common shares outstanding:
Basic and Diluted		35,892,786			31,202,842			27,279,008
Net loss per share:
Basic and Diluted	$	(2.06	)	$	(1.86	)	$	(2.10	)

As of December 31, 2018, 2017 and 2016, 4,004,422, 3,492,141 and 2,548,408 stock options, respectively, were outstanding, which could potentially dilute basic earnings per share in the future, but were not included in the computation of diluted net loss per share because to do so would have been anti-dilutive.

Employee Benefit Plan

12 Months Ended

Dec. 31, 2018

Postemployment Benefits [Abstract]

Employee Benefit Plan

16. Employee Benefit Plan

In February 2006, the Company adopted a defined contribution retirement plan that complies with Section 401(k) of the Internal Revenue Code. All employees over the age of 21 are eligible to participate in the plan at the beginning of the calendar quarter after three consecutive months of service. Employees are able to defer a portion of their pay into the plan on the first day of the quarter on or after the day all age and service requirements have been met. All eligible employees receive an employer contribution equal to 3% of their salary up to the annual IRS limit. During the years ended December 31, 2018, 2017 and 2016, employer contributions to the plan were $198, $174 and $118, respectively.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

17. Commitments and Contingencies

Contractual obligations and commitments as of December 31, 2018, comprising future minimum lease payments under the Company’s Stamford lease, were as follows:

	Payment Due for the Year Ending December 31,
	2019		2020		2021		2022		2023		Total
Stamford operating lease	$	1,215	$	1,240	$	1,264	$	1,288	$	1,164	$	6,171

Stamford Operating Lease

In December 2015, the Company entered into a lease agreement, or the Stamford Lease, for office space in Stamford, Connecticut, or the Premises, for the purpose of relocating its headquarters. The initial term of the Stamford Lease commenced in May 2016, or the Commencement Date, and ends in November 2023. The Stamford Lease requires monthly lease payments, including rent escalations and rent holidays, during the initial lease term. The Company began to make rental payments from the Commencement Date. The Company records monthly rent expense on a straight-line basis from March 2016, upon taking possession of the Premises, through November 2023. As of December 31, 2018 and 2017, the balance of deferred lease obligation, representing the difference between cash rent paid and straight-line rent expense, was $864 and $876, respectively. The Stamford Lease is renewable for one five-year term.

As of the Commencement Date, the Stamford Lease landlord had made tenant improvements of $1,094 to the leased premises. Such amount was included in Property and equipment, net and in Deferred lease obligation. The portion of Deferred lease obligation that is related to tenant improvements is being amortized as a reduction to rent expense over the same term as rent expense. As of December 31, 2018 and 2017, the balance of Deferred lease obligation related to tenant improvements was $698 and $842, respectively.

Total rent expense under the Stamford Lease was $974, $935 and $797 for the years ended December 31, 2018, 2017 and 2016, respectively.

In connection with the signing of the Stamford Lease, the Company entered into a standby letter of credit agreement for $769, which serves as a security deposit for the Premises. The standby letter of credit is automatically renewed annually through November 2023. This standby letter of credit is secured with restricted cash in a money market account (refer to Note 7, Restricted Cash).

Shelton Operating Lease

In May 2016, the Company relocated its headquarters to Stamford, Connecticut and vacated its former operating facility in Shelton, Connecticut, which the Company continued to lease under an operating lease, or the Shelton Lease. The Shelton Lease terminated in November 2017.

The Shelton Lease, as amended, required monthly lease payments through its term. The Company recorded monthly rent expense associated with the Shelton Lease on a straight-line basis from inception of the lease in October 2007 through May 2016, when the facility was vacated. In accordance with the accounting guidance in ASC 420-10-25-13 regarding exit or disposal cost obligations, as of May 2016, the Company recorded rent expense, within R&D expense and General and administrative expense, and accrued a liability of $1,312, which represented the fair value of costs that continued to be incurred during the remaining term of the Shelton Lease without economic benefit to the Company.

Total rent expense under the Shelton Lease was $1,127 for the year ended December 31, 2016.

In conjunction with the signing of the Shelton Lease, the Company entered into a standby letter of credit agreement, which expired on December 13, 2017, as a security deposit for the premises. The balance of the letter of credit was $700, which was secured with restricted cash.

The Company accelerated the amortization of the Shelton leasehold improvements from the date of signing of the Stamford lease in December 2015 through the date that the Company vacated the Shelton facility in May 2016. Additional amortization expense as a result of such acceleration amounted to $899 (additional net loss per share of $0.03) for the year ended December 31, 2016.

Legal Matters

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Legal Matters

18. Legal Matters

From time to time, the Company may become subject to arbitration, litigation or claims arising in the ordinary course of its business. The Company is not currently a party to any arbitration or legal proceeding that, if determined adversely to the Company, would have a material adverse effect on its business, operating results or financial condition. The results of any future claims or proceedings cannot be predicted with certainty, and regardless of the outcome, litigation can have an adverse impact on the Company because of defense and settlement costs, diversion of management resources and other factors.

Quarterly Results of Operations (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Quarterly Results of Operations (Unaudited)

19. Quarterly Results of Operations (Unaudited)

The following tables contain selected financial data for each quarter of the years ended December 31, 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair presentation of the information for each quarter of the years ended December 31, 2018 and 2017. The operating results for any period are not necessarily indicative of results for any future periods.

	Year Ended December 31, 2018
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
Revenues	$	—		$	2,874		$	5,062		$	5,533
Net loss - Basic and Diluted		(16,767	)		(17,194	)		(19,400	)		(20,652	)
Loss per share - Basic and Diluted	$	(0.51	)	$	(0.52	)	$	(0.51	)	$	(0.52	)

	Year Ended December 31, 2017
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
Revenues	$	911		$	—		$	—		$	—
Net loss - Basic and Diluted		(22,204	)		(9,300	)		(12,444	)		(14,177	)
Loss per share - Basic and Diluted	$	(0.81	)	$	(0.29	)	$	(0.38	)	$	(0.43	)	(a)

(a)

The difference between the sum of net loss per share, basic and diluted, as calculated on a quarterly basis for 2017 ($1.91), and net loss per share, basic and diluted, for the year ended December 31, 2017 ($1.86) is due to the denominator used for the year ended December 31, 2017, which weights shares outstanding on a cumulative basis and reflects the issuance of 5.1 million shares of the Company’s common stock during the year ended December 31, 2017 (see Note 9, Stockholders’ Equity).

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Use of Estimates

Concentrations of Credit Risk

Cash and Cash Equivalents

Marketable Securities

Fair Value of Financial Instruments

•

Level 1 – Observable inputs – quoted prices in active markets for identical assets and liabilities.

•

Level 3 – Unobservable inputs – includes amounts derived from valuation models where one or more significant inputs are unobservable and require the Company to develop relevant assumptions.

The Company records transfers between levels in the hierarchy by assuming that the transfer occurred at the end of the quarter or year-to-date period.

Valuation Techniques - Level 2 Inputs

Property and Equipment

Asset Category		Useful Lives
Computer and office equipment		5 years
Short-term laboratory equipment		2 years
Furniture and fixtures		7 years
Leasehold improvements		lesser of useful life of asset or life of lease (Stamford - 7 years)

Revenue Recognition

Research and Development Expenses

Income Taxes

Stock-Based Compensation

Income (Loss) Per Share

Segment Reporting

Leases

Litigation Reserves

Recent Accounting Pronouncements

Other Accounting Pronouncements Recently Adopted

Recent Accounting Pronouncements Not Yet Adopted

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Useful Lives of Property and Equipment

Asset Category		Useful Lives
Computer and office equipment		5 years
Short-term laboratory equipment		2 years
Furniture and fixtures		7 years
Leasehold improvements		lesser of useful life of asset or life of lease (Stamford - 7 years)

Available-for-Sale Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2018

Investments Debt And Equity Securities [Abstract]

Summary of Available-for-Sale Marketable Securities by Major Type of Security

The following tables summarize the Company's available-for-sale marketable securities by major type of security as of December 31, 2018, and 2017:

As of December 31, 2018

			Gross Unrealized					Estimated Fair
Type of Security	Amortized Cost		Gains		Losses			Value
U.S. Treasury securities	$	19,540	$	—	$	(1	)	$	19,539
U.S. government agency obligations		17,860		—		(1	)		17,859
Corporate bonds		75,999		5		(94	)		75,910
Commercial paper		50,413		—		(23	)		50,390
Municipal bonds		4,000		—		—			4,000
Total available-for-sale marketable securities	$	167,812	$	5	$	(119	)	$	167,698

As of December 31, 2017

			Gross Unrealized					Estimated Fair
Type of Security	Amortized Cost		Gains		Losses			Value
Money market funds	$	39,988	$	—	$	(37	)	$	39,951
U.S. government agency obligations		7,799		—		(5	)		7,794
Corporate bonds		15,919		—		(12	)		15,907
Commercial paper		19,545		—		(16	)		19,529
Total available-for-sale marketable securities	$	83,251	$	—	$	(70	)	$	83,181

Schedule of Amortized Cost and Fair Values of Marketable Debt Securities by Contractual Maturity

	As of December 31,
	2018				2017
Contractual maturity	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Less than one year	$	146,363	$	146,302	$	43,263	$	43,230
One year to two years		21,449		21,396		—		—
Total	$	167,812	$	167,698	$	43,263	$	43,230

Schedule of Fair Values and Continuous Unrealized Loss Positions of Available-for-Sale Marketable Securities

	Less than 12 Months					12 Months or Greater				Total
			Gross					Gross				Gross
			Unrealized					Unrealized				Unrealized
As of December 31, 2018	Fair Value		Losses			Fair Value		Losses		Fair Value		Losses
U.S. Treasury securities	$	16,392	$	(1	)	$	—	$	—	$	16,392	$	(1	)
U.S. government agency obligations		5,596		(1	)		—		—		5,596		(1	)
Corporate bonds		71,322		(94	)		—		—		71,322		(94	)
Commercial paper		39,445		(23	)		—		—		39,445		(23	)
Total	$	132,755	$	(119	)	$	—	$	—	$	132,755	$	(119	)

	Less than 12 Months					12 Months or Greater				Total
			Gross					Gross				Gross
			Unrealized					Unrealized				Unrealized
As of December 31, 2017	Fair Value		Losses			Fair Value		Losses		Fair Value		Losses
Money market funds	$	39,951	$	(37	)	$	—	$	—	$	39,951	$	(37	)
U.S. government agency obligations		7,794		(5	)		—		—		7,794		(5	)
Corporate bonds		15,907		(12	)		—		—		15,907		(12	)
Commercial paper		19,031		(16	)		—		—		19,031		(16	)
Total	$	82,683	$	(70	)	$	—	$	—	$	82,683	$	(70	)

Accumulated Other Comprehensive Income (Loss) (Tables)

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Summary of Changes in Accumulated Other Comprehensive Income (Loss), Net of Tax, from Unrealized Gains (Losses) on Available-for-Sale Marketable Securities

	Total Accumulated Other Comprehensive Income (Loss)
Balance, December 31, 2015	$	(35	)
Other comprehensive income before reclassifications		61
Amount reclassified from accumulated other comprehensive income		(23	)
Net current period other comprehensive income		38
Balance, December 31, 2016		3
Other comprehensive loss before reclassifications		(68	)
Amount reclassified from accumulated other comprehensive income		(5	)
Net current period other comprehensive loss		(73	)
Balance, December 31, 2017		(70	)
Other comprehensive loss before reclassifications		(49	)
Amount reclassified from accumulated other comprehensive loss		5
Net current period other comprehensive loss		(44	)
Balance, December 31, 2018	$	(114	)

Schedule of Reclassification Out of Accumulated Other Comprehensive Income

The reclassifications out of AOCI and into net loss were as follows:

	Year Ended December 31,							Affected Line Item in the Statements of
Component of AOCI	2018			2017		2016		Comprehensive Loss
Unrealized gains (losses) on available-for- sale marketable securities
Realized (losses) gains on sale of securities	$	(5	)	$	5	$	23	Other income
		—			—		—	Income tax benefit
	$	(5	)	$	5	$	23

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2018

Property Plant And Equipment [Abstract]

Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2018		2017
Computer and office equipment	$	211	$	158
Laboratory equipment		628		628
Furniture and fixtures		47		27
Leasehold improvements		1,128		1,128
	$	2,014	$	1,941
Less accumulated depreciation and amortization		1,134		764
Property and equipment, net	$	880	$	1,177

Restricted Cash (Tables)

12 Months Ended

Dec. 31, 2018

Receivables [Abstract]

Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash

	December 31,
	2018		2017
Cash and cash equivalents	$	15,081	$	9,388
Restricted cash, current assets		361		—
Restricted cash, long-term assets		408		769
Total cash, cash equivalents, and restricted cash shown in the Statements of Cash Flows	$	15,850	$	10,157

Accounts Payable and Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2018

Payables And Accruals [Abstract]

Schedule of Accounts Payable and Accrued Expenses

Accounts payable and accrued expenses consist of the following:

	December 31,
	2018		2017
Accounts payable	$	4,371	$	3,829
Accrued research projects		6,079		2,356
Accrued professional fees		802		384
Accrued compensation and benefits		2,370		1,864
Accrued other		—		73
	$	13,622	$	8,506

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Summary of Financial Assets Measured at Fair Value on Recurring Basis

The following tables summarize the Company’s financial assets measured at fair value on a recurring basis as of December 31, 2018 and 2017 and by level within the fair value hierarchy:

Fair value measurement as of December 31, 2018:

			Quoted prices in		Significant other		Significant
Financial assets			active markets for		observable		unobservable
			identical assets		inputs		inputs
Type of Instrument	Total		(Level 1)		(Level 2)		(Level 3)
Cash and cash equivalents:
Money market funds and checking accounts	$	15,081	$	15,081	$	—	$	—
Available-for-sale marketable securities:
U.S. Treasury securities		19,539		—		19,539		—
U.S. government agency obligations		17,859		—		17,859		—
Corporate bonds		75,910		—		75,910		—
Commercial paper		50,390		—		50,390		—
Municipal bonds		4,000		—		4,000		—
Restricted cash:
Commercial money market account		769		769		—		—
Total financial assets	$	183,548	$	15,850	$	167,698	$	—

Fair value measurement as of December 31, 2017:

			Quoted prices in		Significant other		Significant
Financial assets			active markets for		observable		unobservable
			identical assets		inputs		inputs
Type of Instrument	Total		(Level 1)		(Level 2)		(Level 3)
Cash and cash equivalents:
Money market fund and checking accounts	$	9,388	$	9,388	$	—	$	—
Available-for-sale marketable securities:
Money market fund		39,951		—		39,951		—
U.S. government agency obligations		7,794		—		7,794		—
Corporate bonds		15,907		—		15,907		—
Commercial paper		19,529		—		19,529		—
Restricted cash:
Commercial money market account		769		769		—		—
Total financial assets	$	93,338	$	10,157	$	83,181	$	—

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Summary of Stock Option Activity

				Weighted-		Aggregate
	Number of			Average		Intrinsic
	Options			Exercise Price		Value
Outstanding at December 31, 2017		3,492,141		$	11.75
Granted		1,197,500			16.15
Exercised		(451,685	)		9.29
Expired		(60,111	)		10.79
Forfeited		(173,423	)		12.12
Outstanding at December 31, 2018		4,004,422		$	13.34	$	6,627
Weighted average remaining contractual life as of December 31, 2018 (in years)		7.92
Options exercisable at December 31, 2018		1,974,979		$	11.70	$	4,985
Weighted average remaining contractual life as of December 31, 2018 (in years)		7.00
Options vested and expected to vest as of December 31, 2018		4,004,422		$	13.34	$	6,627
Weighted average remaining contractual life as of December 31, 2018 (in years)		7.92

Summary of Assumptions Used in Black-Scholes Option Pricing Model

The fair values of the stock options granted to those groups were estimated using the Black-Scholes option valuation model with the following ranges of assumptions (see Note 2, Summary of Significant Accounting Policies - Stock-Based Compensation):

	Year Ended December 31,
	2018		2017	2016
Risk-free interest rate	2.51% - 3.09%		1.85% - 2.57%	1.19% - 1.93%
Expected volatility	82.6% - 92.8%		75.3% - 84.5%	67.8% - 77.8%
Expected dividend yield	0%		0%	0%
Expected life of employee and Board of Directors' options (in years)	6.25		6.25	6.25
Expected life of non-employee options (in years)		—	10	10

Summary of Compensation Expense Relating to Stock Options

	Year Ended December 31,
	2018		2017		2016
Research and development	$	3,919	$	2,433	$	1,301
General and administrative		4,482		3,897		1,499
Total stock option expense	$	8,401	$	6,330	$	2,800

Non-employee Consultants [Member]

Summary of Compensation Expense Relating to Stock Options

	Year Ended December 31,
	2018		2017		2016
Research and development	$	195	$	170	$	(79	)
General and administrative		192		200		(20	)
Total stock option expense	$	387	$	370	$	(99	)

Re-Measurement [Member]

Summary of Assumptions Used in Black-Scholes Option Pricing Model

	Year Ended December 31,
	2018	2017	2016
Risk-free interest rate	1.82% - 3.02%	1.28% - 2.39%	1.35% - 2.38%
Expected volatility	58.2% - 101.0%	74.6% - 87.3%	70.8% - 75.5%
Expected dividend yield	0%	0%	0%
Expected life of non-employee options (in years)	0.25 - 8.94	0.62 - 9.94	7.08 - 9.60

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Components of Income Taxes (Benefit)

The Company’s benefit from income taxes is as follows:

	December 31,
	2018			2017			2016
Current:
Federal	$	—		$	—		$	—
State		(389	)		(204	)		(468	)
		(389	)		(204	)		(468	)
Deferred:
Federal		—			—			—
State		—			—			—
		—			—			—
Benefit from income taxes	$	(389	)	$	(204	)	$	(468	)

Schedule of Reconciliation of Income Taxes Computed Using U.S. Federal Statutory Rate to that Reflected in Operations

A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations is as follows:

	December 31,
	2018			2017			2016
Income taxes using U.S. federal statutory rate		21.00	%		34.00	%		34.00	%
State income taxes, net of federal benefit		6.82	%		5.33	%		5.44	%
Tax Cuts and Jobs Act		0.00	%		-44.43	%		0.00	%
Impact of R&D tax credit on effective tax rate		3.48	%		3.25	%		3.24	%
Stock option shortfalls and cancellations		-0.43	%		0.21	%		-0.07	%
Permanent items and other		-0.15	%		-0.56	%		-0.64	%
Change in valuation allowance		-31.76	%		2.55	%		-41.17	%
Provision to return		0.03	%		0.00	%		0.00	%
Non-taxable revenue		1.54	%		0.00	%		0.00	%
		0.53	%		0.35	%		0.80	%

Schedule of Significant Components of Company's Deferred Tax Assets and Liabilities

Significant components of the Company’s deferred tax assets and liabilities are as follows:

	December 31,
	2018			2017
Deferred tax assets:
Net operating loss carryforwards	$	73,578		$	54,831
Federal and state tax credits		11,108			8,401
Deferred revenue		1,111			—
Stock-based compensation expense		3,605			2,382
Other		420			582
		89,822			66,196
Deferred tax liabilities:
Accelerated depreciation		(7	)		(23	)
Valuation allowance		(89,815	)		(66,173	)
Net deferred tax asset	$	—		$	—

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Computation of Denominators Used in Net Loss per Share

The denominators used in the net loss per share computations are as follows:

	Year Ended December 31,
	2018		2017		2016
Basic:
Weighted average shares outstanding		35,892,786		31,202,842		27,279,008
Diluted:
Weighted average shares outstanding - Basic		35,892,786		31,202,842		27,279,008
Common stock options *		—		—		—
Denominator for diluted net loss per share		35,892,786		31,202,842		27,279,008

*	No amounts were considered as their effects would be anti-dilutive.

Computation of Basic and Diluted Net Loss per Share

Basic and diluted net loss per share are computed as follows:

	Year Ended December 31,
	2018			2017			2016
Net loss	$	(74,013	)	$	(58,125	)	$	(57,280	)
Weighted-average common shares outstanding:
Basic and Diluted		35,892,786			31,202,842			27,279,008
Net loss per share:
Basic and Diluted	$	(2.06	)	$	(1.86	)	$	(2.10	)

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments And Contingencies Disclosure [Abstract]

Schedule of Contractual Obligations and Commitments Comprising of Future Minimum Lease Payments

Contractual obligations and commitments as of December 31, 2018, comprising future minimum lease payments under the Company’s Stamford lease, were as follows:

	Payment Due for the Year Ending December 31,
	2019		2020		2021		2022		2023		Total
Stamford operating lease	$	1,215	$	1,240	$	1,264	$	1,288	$	1,164	$	6,171

Quarterly Results of Operations (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information Disclosure [Abstract]

Schedule of Selected Financial Data for Each Quarter

	Year Ended December 31, 2018
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
Revenues	$	—		$	2,874		$	5,062		$	5,533
Net loss - Basic and Diluted		(16,767	)		(17,194	)		(19,400	)		(20,652	)
Loss per share - Basic and Diluted	$	(0.51	)	$	(0.52	)	$	(0.51	)	$	(0.52	)

	Year Ended December 31, 2017
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
Revenues	$	911		$	—		$	—		$	—
Net loss - Basic and Diluted		(22,204	)		(9,300	)		(12,444	)		(14,177	)
Loss per share - Basic and Diluted	$	(0.81	)	$	(0.29	)	$	(0.38	)	$	(0.43	)	(a)

(a)

Business - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Jul. 18, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nature Of Business [Line Items]
Date of formation										Jul. 02, 2004
Proceeds from equity and debt financing		$ 383,200								$ 383,200
Upfront and milestone payments received in connection with license agreements		88,900								88,900
Proceeds from sale of common stock in connection with license agreement										14,556
Unrestricted cash and cash equivalents and marketable securities		182,779								182,779
Accumulated deficit		294,354				$ 220,341				294,354	$ 220,341
Net loss		$ 20,652	$ 19,400	$ 17,194	$ 16,767	$ 14,177	$ 12,444	$ 9,300	$ 22,204	74,013	58,125	$ 57,280
Net cash used in operating activities										(22,301)	$ (54,827)	$ (47,381)
Common Stock [Member]
Nature Of Business [Line Items]
Common stock, shares issued	5,175,000
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member]
Nature Of Business [Line Items]
Proceeds from sale of common stock in connection with license agreement										$ 14,556
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] \| Common Stock [Member]
Nature Of Business [Line Items]
Common stock, shares issued										1,174,827

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) Financial_Institution Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016	Jan. 01, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Prepaid R&D expense	$ 4,377	$ 1,287
Expected dividend yield	0.00%	0.00%	0.00%
Number of operating segments \| Segment	1
Maximum [Member] \| ASU 2016-02 [Member] \| Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating lease, liability				$ 6,000
Minimum [Member] \| ASU 2016-02 [Member] \| Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating lease, liability				$ 5,000
Employee Options [Member]
Summary Of Significant Accounting Policies [Line Items]
Expected life of options (in years)	6 years 3 months
Nonemployee Options [Member]
Summary Of Significant Accounting Policies [Line Items]
Expected life of options (in years)		10 years	10 years
Nonemployee Options [Member] \| Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Expected life of options (in years)	10 years
Cash And Cash Equivalents And Marketable Securities [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of financial institutions \| Financial_Institution	3

Summary of Significant Accounting Policies - Summary of Useful Lives of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2018
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	5 years
Short-term Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	2 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	7 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, description	lesser of useful life of asset or life of lease (Stamford - 7 years)
Leasehold Improvements [Member] \| Stamford [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	7 years

Available-for-Sale Marketable Securities - Summary of Available-for-Sale Marketable Securities by Major Type of Security (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 167,812	$ 83,251
Gross Unrealized Gains	5
Gross Unrealized Losses	(119)	(70)
Estimated Fair Value	167,698	83,181
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,540
Gross Unrealized Losses	(1)
Estimated Fair Value	19,539
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,860	7,799
Gross Unrealized Losses	(1)	(5)
Estimated Fair Value	17,859	7,794
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	75,999	15,919
Gross Unrealized Gains	5
Gross Unrealized Losses	(94)	(12)
Estimated Fair Value	75,910	15,907
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	50,413	19,545
Gross Unrealized Losses	(23)	(16)
Estimated Fair Value	50,390	19,529
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,000
Estimated Fair Value	$ 4,000
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		39,988
Gross Unrealized Losses		(37)
Estimated Fair Value		$ 39,951

Available-for-Sale Marketable Securities - Schedule of Amortized Cost and Fair Values of Marketable Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Amortized Cost of marketable debt securities, contractual maturity, Less than one year	$ 146,363	$ 43,263
Amortized Cost of marketable debt securities, contractual maturity, One year to two years	21,449
Total Amortized Cost of marketable debt securities, contractual maturity	167,812	43,263
Fair value of marketable debt securities, contractual maturity, Less than one year	146,302	43,230
Fair value of marketable debt securities, contractual maturity,One year to two years	21,396
Total fair Value of marketable debt securities, contractual maturity	$ 167,698	$ 43,230

Available-for-Sale Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 USD ($) Position	Dec. 31, 2017 USD ($) Position	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale of available-for-sale marketable securities	$ 79,808,000	$ 8,755,000	$ 34,003,000
Realized (losses) gains	$ (5,000)	$ 5,000	$ 23,000
Number of available-for-sale marketable securities in unrealized loss positions \| Position	69	30
Total number of positions \| Position	84	31
Other-than-temporary impairments on securities	$ 0	$ 0
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale of available-for-sale marketable securities	79,808,000	8,755,000
Realized (losses) gains	$ (5,000)	$ 5,000

Available-for-Sale Marketable Securities - Schedule of Fair Values and Continuous Unrealized Loss Positions of Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	$ 132,755	$ 82,683
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(119)	(70)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	132,755	82,683
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(119)	(70)
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	16,392
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(1)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	16,392
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(1)
U.S. Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	5,596	7,794
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(1)	(5)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	5,596	7,794
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(1)	(5)
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	71,322	15,907
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(94)	(12)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	71,322	15,907
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(94)	(12)
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	39,445	19,031
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(23)	(16)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	39,445	19,031
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	$ (23)	(16)
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value		39,951
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses		(37)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value		39,951
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses		$ (37)

Accumulated Other Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Income (Loss), Net of Tax, from Unrealized Gains (Losses) on Available-for-Sale Marketable Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	$ 86,780	$ 50,725	$ 105,044
Other comprehensive income (loss) before reclassifications	(49)	(68)	61
Amount reclassified from accumulated other comprehensive loss (income)	5	(5)	(23)
Net current period other comprehensive income (loss)	(44)	(73)	38
Balance, Value	133,630	86,780	50,725
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	(70)	3	(35)
Net current period other comprehensive income (loss)	(44)	(73)	38
Balance, Value	$ (114)	$ (70)	$ 3

Accumulated Other Comprehensive Income (Loss) - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Other income	$ 2,980	$ 1,156	$ 652
Income tax benefit	(389)	(204)	(468)
Net of tax	(5)	5	23
Accumulated Net Unrealized Investment Gain (Loss) Attributable to Parent [Member] \| Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Other income	(5)	5	23
Income tax benefit	$ 0	$ 0	$ 0

Prepaid Expenses - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 4,805	$ 1,635
Prepaid R&D clinical costs	4,377	1,287
Prepaid insurance	245	124
Other prepaid costs	$ 183	$ 224

Property and Equipment, Net - Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,014	$ 1,941
Less accumulated depreciation and amortization	1,134	764
Property and equipment, net	880	1,177
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	211	158
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	628	628
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	47	27
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,128	$ 1,128

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 370	$ 495	$ 1,465
Net proceeds from sale of property and equipment		41
Shelton Operating Lease [Member]
Property, Plant and Equipment [Line Items]
Wrote-off of fully-depreciated Shelton property and equipment		7,816	$ 397
Net proceeds from sale of property and equipment		$ 41

Restricted Cash - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash in current assets	$ 361
Restricted cash balance	408	$ 769
Stamford Lease [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Letter of credit, remaining amount	769
Letter of credit, through the end of lease term	$ 408
Letter of credit, remaining amount date	2019-05
Operating lease, expiration date	Nov. 30, 2023
Restricted cash balance		$ 769
Shelton Lease [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash in current assets	$ 361
Restricted cash balance	$ 408

Restricted Cash - Schedule of Reconciliation of Cash, Cash Equivalents and Restricted Cash (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash And Cash Equivalents [Abstract]
Cash and cash equivalents	$ 15,081	$ 9,388
Restricted cash, current assets	361
Restricted cash, long-term assets	408	769
Total cash, cash equivalents, and restricted cash shown in the Statements of Cash Flows	$ 15,850	$ 10,157	$ 13,561	$ 15,801

Accounts Payable and Accrued Expenses - Schedule of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accounts payable	$ 4,371	$ 3,829
Accrued research projects	6,079	2,356
Accrued professional fees	802	384
Accrued compensation and benefits	2,370	1,864
Accrued other		73
Total	$ 13,622	$ 8,506

Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands						1 Months Ended	12 Months Ended
	Jul. 23, 2018	Jul. 18, 2018	May 17, 2018	Apr. 05, 2017	Mar. 30, 2017	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Subsidiary, Sale of Stock [Line Items]
Common stock, shares authorized						100,000,000	100,000,000	100,000,000
Common stock, par value						$ 0.001	$ 0.001	$ 0.001
Preferred stock, shares authorized						5,000,000	5,000,000	5,000,000
Preferred stock, par value						$ 0.001	$ 0.001	$ 0.001
Common stock, shares outstanding						39,547,558	39,547,558	32,662,255
Preferred stock, shares issued						0	0	0
Preferred stock, shares outstanding						0	0	0
Conversion of shares						83,791
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued			1,174,827
Public offering price per share			$ 17.024
Common Stock [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued		5,175,000
Conversion of shares							83,791
Common Stock [Member] \| Follow-on Public Offering [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued					5,117,500		5,175,000	5,117,500
Public offering price per share		$ 19.00			$ 18.00	$ 19.00	$ 19.00	$ 18.00
Proceeds from issuance of common stock net of underwriting discounts and offering costs	$ 92,063			$ 86,224
Underwriting discounts, commission and offering expenses	$ 6,262			$ 5,891
Common Stock [Member] \| Underwriter's Over-Allotment [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued		675,000
Common Stock [Member] \| Underwriter's Over-Allotment [Member] \| Maximum [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued	675,000			667,500	667,500

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financial assets
Available-for-sale marketable securities	$ 167,698	$ 83,181
Recurring [Member]
Financial assets
Total financial assets	183,548	93,338
U.S. Government Agency Obligations [Member]
Financial assets
Available-for-sale marketable securities	17,859	7,794
U.S. Government Agency Obligations [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	17,859	7,794
Corporate Bonds [Member]
Financial assets
Available-for-sale marketable securities	75,910	15,907
Corporate Bonds [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	75,910	15,907
Municipal Bonds [Member]
Financial assets
Available-for-sale marketable securities	4,000
Municipal Bonds [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	4,000
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Recurring [Member]
Financial assets
Total financial assets	15,850	10,157
Significant Other Observable Inputs (Level 2) [Member] \| Recurring [Member]
Financial assets
Total financial assets	167,698	83,181
Significant Other Observable Inputs (Level 2) [Member] \| U.S. Government Agency Obligations [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	17,859	7,794
Significant Other Observable Inputs (Level 2) [Member] \| Corporate Bonds [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	75,910	15,907
Significant Other Observable Inputs (Level 2) [Member] \| Municipal Bonds [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	4,000
Money Market Funds [Member] \| Recurring [Member]
Financial assets
Cash and cash equivalents	15,081	9,388
Available-for-sale marketable securities		39,951
Restricted cash	769	769
Money Market Funds [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Recurring [Member]
Financial assets
Cash and cash equivalents	15,081	9,388
Restricted cash	769	769
Money Market Funds [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities		39,951
U.S. Treasury Securities [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	19,539
U.S. Treasury Securities [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	19,539
Commercial Paper [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	50,390	19,529
Commercial Paper [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	$ 50,390	$ 19,529

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Purchases of financial assets	$ 0	$ 0	$ 0
Sales of financial assets	0	0	0
Maturities of financial assets	0	0	0
Transfer of financial asset into level 3 of fair value	0	0	0
Fair value assets level 2 to level 1 transfers	0	0	0
Fair value assets level 1 to level 2 transfers	0	0	0
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unrealized gains	0	0	0
Unrealized losses	$ 0	$ 0	$ 0

Collaboration and Licensing Agreements - Additional Information (Detail) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	May 17, 2018 USD ($) PerformanceObligation $ / shares shares	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) PerformanceObligation	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer			$ 13,469	$ 911	$ 86
License and Milestone Fees [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer			13,436	530
Collaborative Revenue [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer				313
Clinical Compound Revenue [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer			$ 33	68	86
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of performance obligations for revenue recognized \| PerformanceObligation			2
Non-refundable, non-creditable upfront payment	$ 50,000
Common stock, shares issued \| shares	1,174,827
Purchase of common stock value	$ 20,000
Purchase common stock per share amount \| $ / shares	$ 17.024
Closing prices of company common stock description	over a pre-determined average closing price of the Company’s common stock.
Premium from sale of stock	$ 5,444
Contract liabilities	$ 55,444
Number of combined performance obligations for revenue recognized \| PerformanceObligation	1
Number of days to terminate agreement			60 days
Termination notice effective period			12 months
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] \| Maximum [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Third-party cost for clinical development	$ 20,000
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] \| Maximum [Member] \| Regulatory and Commercial Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable	470,000
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] \| Maximum [Member] \| Regulatory Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable	30,000
Vifor Fresenius Medical Care Renal Pharma Ltd. [Member] \| Maximum [Member] \| Tiered Commercial Milestones [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable	$ 440,000
Maruishi Pharmaceutical Co., Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Number of performance obligations for revenue recognized \| PerformanceObligation			2
Cost of clinical compound related to R&D expense			$ 30	61	78
Maruishi Pharmaceutical Co., Ltd. [Member] \| Sub-License Fee [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer		$ 843		843
Maruishi Pharmaceutical Co., Ltd. [Member] \| License and Milestone Fees [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer				530
Maruishi Pharmaceutical Co., Ltd. [Member] \| Collaborative Revenue [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer				313
Maruishi Pharmaceutical Co., Ltd. [Member] \| Clinical Compound Revenue [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Revenue from contract with customer			$ 33	$ 68	$ 86

Revenue Recognition - Additional Information (Detail) ¥ in Billions		1 Months Ended			12 Months Ended
	May 17, 2018 USD ($) PerformanceObligation	Mar. 31, 2017 USD ($)	Apr. 30, 2013 USD ($) PerformanceObligation	Apr. 30, 2012 USD ($)	Dec. 31, 2018 USD ($) PerformanceObligation Milestone	Dec. 31, 2017 USD ($) Milestone	Dec. 31, 2016 USD ($) Milestone	Apr. 30, 2013 JPY (¥)
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Deferred revenue, current					$ 26,825,000
Deferred revenue, non current					15,184,000
Total revenue					13,469,000	$ 911,000	$ 86,000
Remaining performance obligations					$ 0
Number of milestone event probable of occurrence or achieved \| Milestone					0	0	0
Maruishi Pharmaceutical Co., Ltd. [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Number of performance obligations for revenue recognized \| PerformanceObligation					2
License and Milestone Fees [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue					$ 13,436,000	$ 530,000
License and Milestone Fees [Member] \| Maruishi Pharmaceutical Co., Ltd. [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue						530,000
License [Member] \| Maruishi Pharmaceutical Co., Ltd. [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue		$ 843,000				843,000
Royalty [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Revenue from contract with customer					0
VFMCRP Agreement [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Deferred revenue, current					26,825,000
Deferred revenue, non current					15,184,000
Receivables					0
Other assets					0
Deferred revenue	$ 55,444,000
Number of combined performance obligations for revenue recognized \| PerformanceObligation	1
Remaining performance obligations					$ 42,009,000
Number of performance obligations for revenue recognized \| PerformanceObligation	2
Upfront payment	$ 50,000,000
Premium from sale of stock	$ 5,444,000
Remaining performance obligation, expected timing of satisfaction, year					2020
VFMCRP Agreement [Member] \| License and Milestone Fees [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Total revenue					$ 13,436,000
Maruishi and CKDP Agreements [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Receivables					0	0
Other assets					0	0
Deferred revenue					$ 0	$ 0
Maruishi Agreement [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Number of performance obligations for revenue recognized \| PerformanceObligation			2		2
Premium from sale of stock			$ 337,000
Upfront non-refundable, non-creditable license fee and premium from sale of stock			15,337,000
Maruishi Agreement [Member] \| Clinical Development and Regulatory Milestones [Member] \| Maximum [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Remaining potential consideration			10,500,000
Maruishi Agreement [Member] \| One-time Sales Milestone [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Remaining potential consideration \| ¥								¥ 1
Maruishi Agreement [Member] \| License [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Estimated selling price			10,200,000
Upfront payments on obligations			9,637,000
Maruishi Agreement [Member] \| R&D Services [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Estimated selling price			6,200,000
Upfront payments on obligations			$ 5,700,000
CKDP Agreement [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Premium from sale of stock				$ 83,000
CKDP Agreement [Member] \| Clinical Development and Regulatory Milestones [Member] \| Maximum [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Remaining potential consideration				3,750,000
CKDP Agreement [Member] \| License [Member]
Revenue Recognition Multiple Deliverable Arrangements [Line Items]
Revenue from contract with customer				$ 646,000

Stock-Based Compensation - Additional Information (Detail) - USD ($)	1 Months Ended			9 Months Ended	12 Months Ended
	Oct. 31, 2018	Sep. 30, 2018	Aug. 31, 2017	Sep. 30, 2014	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2019	Jan. 01, 2017	Jan. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					$ 8,401,000	$ 6,330,000	$ 2,800,000
Common stock, shares outstanding					39,547,558	32,662,255
Fair value of options vested					$ 9,023,000	$ 5,303,000	3,589,000
Intrinsic value of options exercised					$ 3,893,000	$ 2,285,000	$ 126,000
Options granted					1,197,500	1,328,500	1,078,000
Fair value of options granted					$ 11.99	$ 11.46	$ 4.28
Compensation expense not yet recognized					$ 20,474,000
Weighted average period of compensation expense not yet recognized					2 years 10 months 2 days
Excess tax benefits from stock option activity or stock-based compensation expense recognized in cash flows from operations					$ 0	$ 0	$ 0
Excess tax benefits from stock option activity or stock-based compensation expense recognized in cash flows from financing activities					0	0	0
ASU 2016-09 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative-effect adjustment to stockholders' equity										$ 45,000
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					4,482,000	3,897,000	1,499,000
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					3,919,000	2,433,000	$ 1,301,000
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					$ 1,693,000
Common stock, shares outstanding					83,791
Restricted Stock Units [Member] \| General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					$ 1,217,000
Restricted Stock Units [Member] \| Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					476,000
Nonemployee Options [Member] \| Modified Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerate vesting percentage	50.00%		50.00%
Consulting agreement maturity date	Jul. 22, 2019
Modification period during which stock options may be exercised			6 months
Nonemployee Options [Member] \| General and Administrative [Member] \| Modified Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					96,000	$ 537,000
Nonemployee Options [Member] \| Research and Development [Member] \| Modified Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense					$ 520,000
Nonemployee Consultants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair values of outstanding options					$ 8.74	$ 10.16	$ 4.81
2014 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock options that may be granted after the tenth anniversary of the 2014 Plan					0
Annual increases in number of shares reserved for issuance as a percentage of shares of capital stock outstanding through January 1, 2024								3.00%
Share-based compensation arrangement by share-based payment award, number of shares authorized					4,900,481
Options granted					1,197,500
2014 Equity Incentive Plan [Member] \| Subsequent Event [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, number of shares authorized									6,086,907
2014 Equity Incentive Plan [Member] \| Share-Based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of vested shares on first anniversary of grant date					25.00%
2014 Equity Incentive Plan [Member] \| Share-based Compensation Award, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of awards granted					36 months
2014 Equity Incentive Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Term of awards granted					10 years
2014 Equity Incentive Plan [Member] \| Maximum [Member] \| Incentive Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, number of shares authorized								30,000,000
2014 Equity Incentive Plan [Member] \| Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of awards granted					4 years
2014 Equity Incentive Plan [Member] \| Board of Directors [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of awards granted					3 years
2014 Equity Incentive Plan [Member] \| Executive Officers [Member] \| Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock units, granted		83,791
Restricted stock units, grant date fair value		$ 20.21
2004 Stock Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares available for future issuance				0							0
2004 Stock Incentive Plan [Member] \| Share-Based Compensation Award, Tranche One [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of vested shares on first anniversary of grant date				25.00%
2004 Stock Incentive Plan [Member] \| Share-based Compensation Award, Tranche Two [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period of awards granted				36 months
2004 Stock Incentive Plan [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Term of awards granted				10 years

Stock-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Options, Granted	1,197,500	1,328,500	1,078,000
2014 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Options, Outstanding at December 31, 2017	3,492,141
Number of Options, Granted	1,197,500
Number of Options, Exercised	(451,685)
Number of Options, Expired	(60,111)
Number of Options, Forfeited	(173,423)
Number of Options, Outstanding at December 31, 2018	4,004,422	3,492,141
Number of Options, Outstanding, Weighted average remaining contractual life as of December 31, 2018	7 years 11 months 1 day
Number of Options, Options exercisable at December 31, 2018	1,974,979
Number of Options, Exercisable, Weighted average remaining contractual life as of December 31, 2018	7 years
Number of Options, Options vested and expected to vest at December 31, 2018	4,004,422
Number of Options, Vested and expected to vest, Weighted average remaining contractual life as of December 31, 2018	7 years 11 months 1 day
Weighted-Average Exercise Price, Outstanding at December 31, 2017	$ 11.75
Weighted-Average Exercise Price, Granted	16.15
Weighted-Average Exercise Price, Exercised	9.29
Weighted-Average Exercise Price, Expired	10.79
Weighted-Average Exercise Price, Forfeited	12.12
Weighted-Average Exercise Price, Outstanding at December 31, 2018	13.34	$ 11.75
Weighted-Average Exercise Price, Options exercisable at December 31, 2018	11.70
Weighted-Average Exercise Price, Options vested and expected to vest at December 31, 2018	$ 13.34
Aggregate Intrinsic Value, Outstanding at December 31, 2018	$ 6,627
Aggregate Intrinsic Value, Options exercisable at December 31, 2018	4,985
Aggregate Intrinsic Value, Options vested and expected to vest at December 31, 2018	$ 6,627

Stock-Based Compensation - Summary of Assumptions Used in Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	2.51%	1.85%	1.19%
Risk-free interest rate, maximum	3.09%	2.57%	1.93%
Expected volatility, minimum	82.60%	75.30%	67.80%
Expected volatility, maximum	92.80%	84.50%	77.80%
Expected dividend yield	0.00%	0.00%	0.00%
Employee and Board of Directors Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of options (in years)	6 years 3 months	6 years 3 months	6 years 3 months
Nonemployee Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of options (in years)		10 years	10 years

Stock-Based Compensation - Summary of Re-measurement of Assumptions Used in Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	2.51%	1.85%	1.19%
Risk-free interest rate, maximum	3.09%	2.57%	1.93%
Expected volatility, minimum	82.60%	75.30%	67.80%
Expected volatility, maximum	92.80%	84.50%	77.80%
Expected dividend yield	0.00%	0.00%	0.00%
Nonemployee Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of options (in years)		10 years	10 years
Nonemployee Options [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of options (in years)	10 years
Re-Measurement [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	1.82%	1.28%	1.35%
Risk-free interest rate, maximum	3.02%	2.39%	2.38%
Expected volatility, minimum	58.20%	74.60%	70.80%
Expected volatility, maximum	101.00%	87.30%	75.50%
Expected dividend yield	0.00%	0.00%	0.00%
Re-Measurement [Member] \| Nonemployee Options [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of options (in years)	3 months	7 months 13 days	7 years 29 days
Re-Measurement [Member] \| Nonemployee Options [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected life of options (in years)	8 years 11 months 8 days	9 years 11 months 8 days	9 years 7 months 6 days

Stock-Based Compensation - Summary of Compensation Expense Relating to Stock Options (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense	$ 8,401	$ 6,330	$ 2,800
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense	3,919	2,433	1,301
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense	$ 4,482	$ 3,897	$ 1,499

Stock-Based Compensation - Summary of Compensation Expense Recognized With Regard to Stock Options Granted to Non-employee Consultants Including Effect of Re-measurement of Fair Values of Options (Detail) - Non-employee Consultants [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense including effect of re-measurement of fair value	$ 387	$ 370	$ (99)
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense including effect of re-measurement of fair value	195	170	(79)
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense including effect of re-measurement of fair value	$ 192	$ 200	$ (20)

Income Taxes - Components of Income Taxes (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Federal	$ 0	$ 0	$ 0
State	(389)	(204)	(468)
Total Current	(389)	(204)	(468)
Deferred:
Federal	0	0	0
State	0	0	0
Total Deferred	0	0	0
Benefit from income taxes	$ (389)	$ (204)	$ (468)

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Percentage of annual research and development credit for cash	65.00%
Valuation allowance on the deferred tax asset	100.00%	100.00%
Change in valuation allowance	$ 23,642,000	$ 618,000
Cumulative-effect adjustment for tax benefit on exercise of non-qualified stock options and disqualified disposition of incentive stock options		840,000
Unrecognized tax benefits, related interest and penalties	$ 0	$ 0
Tax examinations, description	Tax years 2006 through 2017 remain open to U.S. federal and state tax examinations.
Open tax years	2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018
Income tax rate	21.00%	34.00%	34.00%
Reduction in deferred tax assets		$ 25,913,000
Maximum [Member]
Income Taxes [Line Items]
Income tax rate		35.00%
Federal [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 274,764,000
Net operating losses on taxable income, threshold percentage in carryforward year	80.00%
Federal [Member] \| R&D Tax Credit Carryforwards [Member]
Income Taxes [Line Items]
Tax credit carryforwards	$ 9,925,000
Federal [Member] \| Earliest Tax Year [Member]
Income Taxes [Line Items]
Tax loss carryforwards expiration year	2026
Federal [Member] \| Earliest Tax Year [Member] \| R&D Tax Credit Carryforwards [Member]
Income Taxes [Line Items]
Tax credit expiration year	2025
State and Local [Member]
Income Taxes [Line Items]
Net operating loss carryforwards	$ 267,973,000
State and Local [Member] \| R&D Tax Credit Carryforwards [Member]
Income Taxes [Line Items]
Tax credit carryforwards	$ 1,236,000
State and Local [Member] \| Earliest Tax Year [Member]
Income Taxes [Line Items]
Tax loss carryforwards expiration year	2027

Income Taxes - Schedule of Reconciliation of Income Taxes Computed Using U.S. Federal Statutory Rate to that Reflected in Operations (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income taxes using U.S. federal statutory rate	21.00%	34.00%	34.00%
State income taxes, net of federal benefit	6.82%	5.33%	5.44%
Tax Cuts and Jobs Act	0.00%	(44.43%)	0.00%
Impact of R&D tax credit on effective tax rate	3.48%	3.25%	3.24%
Stock option shortfalls and cancellations	(0.43%)	0.21%	(0.07%)
Permanent items and other	(0.15%)	(0.56%)	(0.64%)
Change in valuation allowance	(31.76%)	2.55%	(41.17%)
Provision to return	0.03%	0.00%	0.00%
Non-taxable revenue	1.54%	0.00%	0.00%
Total	0.53%	0.35%	0.80%

Income Taxes - Schedule of Significant Components of Company's Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 73,578	$ 54,831
Federal and state tax credits	11,108	8,401
Deferred revenue	1,111
Stock-based compensation expense	3,605	2,382
Other	420	582
Gross deferred tax asset	89,822	66,196
Deferred tax liabilities:
Accelerated depreciation	(7)	(23)
Valuation allowance	(89,815)	(66,173)
Net deferred tax asset	$ 0	$ 0

Net Loss per Share - Computation of Denominators Used in Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic:
Weighted average common shares outstanding	35,892,786	31,202,842	27,279,008
Diluted:
Weighted average shares outstanding - Basic	35,892,786	31,202,842	27,279,008
Common stock options	0	0	0
Denominator for diluted net loss per share	35,892,786	31,202,842	27,279,008

Net Loss per Share - Computation of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net loss	$ (20,652)	$ (19,400)	$ (17,194)	$ (16,767)	$ (14,177)	$ (12,444)	$ (9,300)	$ (22,204)	$ (74,013)	$ (58,125)	$ (57,280)
Weighted-average common shares outstanding:
Basic and Diluted									35,892,786	31,202,842	27,279,008
Net loss per share:
Basic and Diluted	$ (0.52)	$ (0.51)	$ (0.52)	$ (0.51)	$ (0.43)	$ (0.38)	$ (0.29)	$ (0.81)	$ (2.06)	$ (1.86)	$ (2.10)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss per Share - Additional Information (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Options [Member]
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Anti-dilutive common stock equivalents	4,004,422	3,492,141	2,548,408

Employee Benefit Plan - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2015	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Defined contribution, plan name		401(k)
Defined contribution retirement plan, description		All employees over the age of 21 are eligible to participate in the plan at the beginning of the calendar quarter after three consecutive months of service.
Employer contributions to the plan		$ 198	$ 174	$ 118
Defined contribution retirement plan, employer contribution percentage	3.00%
Minimum [Member]
Defined Contribution Plan Disclosure [Line Items]
Defined contribution retirement plan, employee eligible age to participate		21 years
Defined contribution retirement plan, employee eligible service period		3 months

Commitments and Contingencies - Schedule of Contractual Obligations and Commitments Comprising of Future Minimum Lease Payments (Detail) - Stamford Operating Lease [Member] $ in Thousands	Dec. 31, 2018 USD ($)
Operating Leased Assets [Line Items]
2019	$ 1,215
2020	1,240
2021	1,264
2022	1,288
2023	1,164
Total	$ 6,171

Commitments and Contingencies - Additional Information (Detail) - USD ($)		12 Months Ended
	May 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Commitments [Line Items]
Deferred lease obligation		$ 1,562,000	$ 1,718,000
Stamford Operating Lease [Member]
Other Commitments [Line Items]
Operating Lease, description		The Stamford Lease requires monthly lease payments, including rent escalations and rent holidays, during the initial lease term. The Company began to make rental payments from the Commencement Date. The Company records monthly rent expense on a straight-line basis from March 2016, upon taking possession of the Premises, through November 2023. As of December 31, 2018 and 2017, the balance of deferred lease obligation, representing the difference between cash rent paid and straight-line rent expense, was $864 and $876, respectively. The Stamford Lease is renewable for one five-year term.
Operating Lease, renewable term		5 years
Operating lease, expiration date		Nov. 30, 2023
Deferred lease obligation		$ 864,000	876,000
Tenant improvement expenses	$ 1,094,000
Lease, rent expense		974,000	935,000	$ 797,000
Stamford Operating Lease [Member] \| Standby Letter of Credit [Member]
Other Commitments [Line Items]
Letter of credit, amount		$ 769,000
Operating lease, renewable term description		Automatically renewed annually through November 2023.
Stamford Operating Lease [Member] \| Standby Letter of Credit [Member] \| Maximum [Member]
Other Commitments [Line Items]
Letter of credit, renewal date		Nov. 30, 2023
Stamford Operating Lease [Member] \| Leasehold Improvements [Member]
Other Commitments [Line Items]
Deferred lease obligation		$ 698,000	$ 842,000
Shelton Operating Lease [Member]
Other Commitments [Line Items]
Operating lease, expiration date		Nov. 30, 2017
Lease, rent expense				1,127,000
Accrued cease use date liability	$ 1,312,000
Additional amortization expense				$ 899,000
Additional loss per share				$ 0.03
Shelton Operating Lease [Member] \| Standby Letter of Credit [Member]
Other Commitments [Line Items]
Letter of credit, remaining amount		$ 700,000
Letter of credit, expiration date		Dec. 13, 2017

Quarterly Results of Operations (Unaudited) - Schedule of Selected Financial Data for Each Quarter (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 5,533	$ 5,062	$ 2,874					$ 911
Net loss - Basic and Diluted	$ (20,652)	$ (19,400)	$ (17,194)	$ (16,767)	$ (14,177)	$ (12,444)	$ (9,300)	$ (22,204)	$ (74,013)	$ (58,125)	$ (57,280)
Loss per share - Basic and Diluted	$ (0.52)	$ (0.51)	$ (0.52)	$ (0.51)	$ (0.43)	$ (0.38)	$ (0.29)	$ (0.81)	$ (2.06)	$ (1.86)	$ (2.10)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Quarterly Results of Operations (Unaudited) - Schedule of Selected Financial Data for Each Quarter (Parenthetical) (Detail) - $ / shares			3 Months Ended								12 Months Ended
	Jul. 18, 2018	Mar. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information [Line Items]
Sum of net loss per share calculated on quarterly basis												$ (1.91)
Loss per share - Basic and Diluted			$ (0.52)	$ (0.51)	$ (0.52)	$ (0.51)	$ (0.43)	$ (0.38)	$ (0.29)	$ (0.81)	$ (2.06)	$ (1.86)	$ (2.10)
Common Stock [Member]
Quarterly Financial Information [Line Items]
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares	5,175,000
Common Stock [Member] \| Follow-on Public Offering [Member]
Quarterly Financial Information [Line Items]
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares		5,117,500									5,175,000	5,117,500