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Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Mar. 08, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
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 Common Stock, Shares Outstanding		32,690,236
Entity Public Float			$ 405,378,264

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 9,388	$ 12,092
Marketable securities	83,181	46,184
Income tax receivable	731	852
Other receivables	123	87
Prepaid expenses	1,635	1,530
Restricted cash, current		700
Total current assets	95,058	61,445
Property and equipment, net	1,177	1,614
Restricted cash	769	769
Total assets	97,004	63,828
Current liabilities:
Accounts payable and accrued expenses	8,506	11,533
Total current liabilities	8,506	11,533
Deferred lease obligation	1,718	1,570
Commitments and contingencies (Note 16)
Stockholders’ equity:
Preferred stock; $0.001 par value; 5,000,000 shares authorized at December 31, 2017 and December 31, 2016; zero shares issued and outstanding at December 31, 2017 and December 31, 2016
Common stock; $0.001 par value; 100,000,000 shares authorized at December 31, 2017 and December 31, 2016; 32,662,255 shares and 27,296,863 shares issued and outstanding at December 31, 2017 and December 31, 2016, respectively	33	27
Additional paid-in capital	307,158	212,866
Accumulated deficit	(220,341)	(162,171)
Accumulated other comprehensive (loss) income	(70)	3
Total stockholders’ equity	86,780	50,725
Total liabilities and stockholders’ equity	$ 97,004	$ 63,828

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
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	32,662,255	27,296,863
Common stock, shares outstanding	32,662,255	27,296,863

Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
Statements of Comprehensive Loss - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue:
License and milestone fees	$ 530		$ 1,710
Collaborative revenue	313		2,093
Clinical compound revenue	68	$ 86
Total revenue	911	86	3,803
Operating expenses:
Research and development	48,524	49,253	21,221
General and administrative	11,872	9,233	7,770
Total operating expenses	60,396	58,486	28,991
Operating loss	(59,485)	(58,400)	(25,188)
Other income	1,156	652	101
Loss before benefit from income taxes	(58,329)	(57,748)	(25,087)
Benefit from income taxes	204	468	397
Net loss	$ (58,125)	$ (57,280)	$ (24,690)
Net loss per share:
Basic and Diluted	$ (1.86)	$ (2.10)	$ (1.00)
Weighted average shares:
Basic and Diluted	31,202,842	27,279,008	24,620,372
Other comprehensive income (loss), net of tax of $0:
Change in unrealized gains (losses) on available for sale marketable securities	$ (73)	$ 38	$ (35)
Total comprehensive loss	$ (58,198)	$ (57,242)	$ (24,725)

Statements of Comprehensive Loss (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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, 2014	$ 51,662		$ 23		$ 131,840		$ (80,201)
Balance, Shares at Dec. 31, 2014			22,802,039
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Value		$ 75,231		$ 4		$ 75,227
Sale of common stock, net of underwriting discounts and commissions and offering expenses, Shares				4,327,956
Stock-based compensation expense	2,514				2,514
Shares issued upon exercise of stock options, Value	362				362
Shares issued upon exercise of stock options, Shares			124,868
Net loss	(24,690)						(24,690)
Other comprehensive income (loss)	(35)							$ (35)
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
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
Cumulative effect adjustment upon adoption of ASU 2016-09					$ 45		$ (45)

Statements of Stockholders' Equity (Parenthetical) - Follow-on Public Offering [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2015
Sale of common stock, per share	$ 18.00	$ 18.60
Underwriting discounts and commissions and offering expenses	$ 5,891	$ 5,269

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
Statements of Cash Flows - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities
Net loss	$ (58,125)	$ (57,280)	$ (24,690)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	5,793	2,800	2,514
Modification of equity awards	537
Depreciation & amortization	495	1,465	839
Amortization/accretion of available-for-sale marketable securities	(582)	(218)	(18)
Realized gain on sale of available-for-sale marketable securities	(5)	(23)
Realized gain on sale of property and equipment	(41)
Deferred rent costs	148	(114)	(289)
Loss on write-off of fixed assets			2
Changes in operating assets and liabilities:
Income tax receivable	121	(468)	(184)
Other receivables	(36)	(7)	(80)
Prepaid expenses	(105)	199	(1,442)
Accounts payable and accrued expenses	(3,027)	6,265	3,322
Deferred revenue			(1,452)
Net cash used in operating activities	(54,827)	(47,381)	(21,478)
Investing activities
Proceeds from maturities of available-for-sale marketable securities	82,156	80,380
Proceeds from sale of available-for-sale marketable securities	8,755	34,003
Purchase of available-for-sale marketable securities	(127,394)	(68,648)	(91,657)
Change in restricted cash	700	(769)
Purchases of property and equipment	(58)	(717)	(20)
Proceeds from sale of property and equipment	41
Net cash (used in) provided by investing activities	(35,800)	44,249	(91,677)
Financing activities
Proceeds from follow-on offering, net of issuance costs	86,224		75,231
Proceeds from the exercise of stock options	1,699	123	362
Net cash provided by financing activities	87,923	123	75,593
Net cash decrease for the period	(2,704)	(3,009)	(37,562)
Cash and cash equivalents at beginning of period	12,092	15,101	52,663
Cash and cash equivalents at end of period	$ 9,388	12,092	$ 15,101
Noncash financing activities
Tenant improvements paid by landlord		$ 1,094

Business	12 Months Ended
Business	Dec. 31, 2017
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 designed to alleviate pruritus and pain by selectively targeting 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, 2017, the Company has raised aggregate net proceeds of approximately $291,100 from several rounds of equity financing, including its initial public offering, or IPO, which closed in February 2014 and two follow-on public offerings of common stock, which closed in April 2017 and August 2015, respectively, and the issuance of convertible preferred stock and debt prior to the IPO. In addition, the Company received approximately $33,500 under its license agreements for CR845/difelikefalin, primarily with 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 (see Note 11, Collaborations). On April 5, 2017, the Company completed its second follow-on public offering, raising aggregate proceeds of approximately $86,224, net of underwriting discounts and commissions and offering expenses paid by the Company. The offering was conducted pursuant to a shelf registration statement on Form S-3, which was filed on March 13, 2017 and declared effective by the Securities and Exchange Commission, or the SEC, on March 24, 2017 (see Note 9, Stockholders’ Equity). As of December 31, 2017, the Company had unrestricted cash and cash equivalents and marketable securities of $92,569 and an accumulated deficit of $220,341. 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 $58,125 and had net cash used in operating activities of $54,827 for the year ended December 31, 2017. 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, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

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. Significant 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 two 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 and has a maturity at the time of purchase of more than three months. 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 generally 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 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, Marketable Securities, and Note 10, Fair Value Measurement.

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, 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 on the Company’s Balance Sheets 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, 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, 2016 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
Laboratory equipment	8 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)

Long-Lived Assets

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 long-lived assets 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

In general, the Company recognizes revenue when all of the following criteria are met: persuasive evidence of an arrangement exists; the Company’s price to the customer is fixed or determinable; collectability is reasonably assured and delivery has occurred or services have been rendered.

The Company has entered into license agreements to develop, manufacture and commercialize drug products. The terms of these agreements typically contain multiple elements, including licenses 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. There are no performance, cancellation, termination or refund provisions in any of the arrangements that contain material financial consequences to the Company.

The Company records revenue related to these agreements in accordance with ASC 605-25, Revenue Recognition Multiple-Element Arrangements. In order to account for these agreements, the Company identifies the deliverables included within an arrangement and evaluates which deliverables represent separate units of accounting based on whether certain criteria are met, including whether the delivered element has standalone value to the counterparty. The consideration received is then allocated among the separate units of accounting based on each unit’s relative selling price.

The identification of individual elements in a multiple-element arrangement and the estimation of the selling price of each element involves significant judgment, including evaluation as to whether each delivered element has standalone value. The Company has determined that its license deliverables represent separate units of accounting because the counterparty has the right to sublicense and manufacture in its territory, as defined. The Company has determined that its R&D services deliverables, as applicable, represent separate units of accounting because similar services are sold separately by other vendors.

The Company determines the estimated selling price for deliverables within each agreement using vendor specific objective evidence, or VSOE, of selling price, if available, or third-party evidence, or TPE, of selling price if VSOE is not available, or the Company’s best estimate of selling price, if neither VSOE nor TPE is available. Determining the best estimate of selling price for a deliverable requires significant judgment. Because the Company does not have VSOE or third-party evidence of selling price to determine the estimated selling price of a license to its proprietary technology, it typically uses its best estimate of a selling price to estimate the selling prices for licenses to its proprietary technology. In making these estimates, the Company considers market conditions and entity-specific factors, including those contemplated in negotiating the agreements, as well as internally developed estimates that include assumptions related to the market opportunity, estimated development costs, probability of success and the time needed to commercialize a product candidate pursuant to the license. In validating its best estimate of selling price, the Company evaluates whether changes in the key assumptions used to determine its best estimate of selling price will have a significant effect on the allocation of arrangement consideration between deliverables. The Company recognizes consideration allocated to an individual element when all other revenue recognition criteria are met for that element.

Arrangement consideration allocated to license deliverables that represent separate units of accounting is recognized as revenue at the outset of the agreement assuming the general criteria for revenue recognition noted above have been met. Arrangement consideration allocated to license deliverables which do not represent separate units of accounting is deferred. Arrangement consideration allocated to R&D services which represent separate units of accounting is recognized as the services are performed, assuming the general criteria for revenue recognition noted above have been met.

The Company’s license agreements include contingent milestone payments related to specified clinical development milestones and regulatory milestones. Development milestones are payable when a product candidate initiates or advances into different clinical trial phases. Regulatory milestones are payable upon submission for marketing approval with the FDA or other countries’ regulatory authorities or on receipt of actual marketing approvals for the compound or for additional indications. At the inception of each agreement that includes milestone payments, the Company evaluates whether each such payment is a milestone payment as defined by ASC 605-28, Revenue Recognition – Milestone Method, because achievement requires performance by the Company and, at inception of the arrangement, there is substantive uncertainty that the event will be achieved, or whether the payment is a contingent payment, because achievement requires performance by the counterparty.

If the payment meets the criteria of a milestone payment, the Company evaluates whether such milestone is considered to be substantive. This evaluation includes an assessment of whether (a) the consideration is commensurate with either (1) the entity’s performance to achieve the milestone, or (2) the enhancement of the value of the delivered item(s) as a result of a specific outcome resulting from the entity’s performance to achieve the milestone, (b) the consideration relates solely to past performance and (c) the consideration is reasonable relative to all of the deliverables and payment terms within the arrangement. The Company evaluates factors such as the scientific, regulatory, commercial and other risks that must be overcome to achieve the respective milestone and the level of effort and investment required to achieve the respective milestone in making this assessment.

The Company recognizes substantive milestone payments as revenue upon the achievement of the milestone, assuming all other revenue recognition criteria are met. If any milestone payment is considered not to be a substantive milestone or is considered to be a contingent payment, the Company initially defers the milestone payment, allocates it to the deliverables based on relative selling price in the same proportion as at inception of the agreement, immediately recognizes revenue to the extent of any delivered elements and recognizes the portion attributable to any undelivered elements over the remaining term of its performance obligations. If no such performance obligations exist, milestones that are considered not to be substantive or are considered to be contingent payments are generally recognized as revenue upon achievement, assuming all other revenue recognition criteria are met.

Royalty revenue is recognized when earned. To date, no royalties have been earned or were otherwise due to the Company.

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, salaries 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 and facility-related costs. 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, 2017 and 2016, the Company recorded $1,287 and $1,256 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. The financial statements reflect expected future tax consequences of such positions presuming the taxing authorities possess full knowledge of the position and all relevant facts. There were no material uncertain tax positions taken as of December 31, 2017 and December 31, 2016. 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 in the Statements of Comprehensive Loss 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, 2017, 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. A higher volatility input to the Black-Scholes option valuation model increases the resulting compensation expense, while a shorter expected term would result in a lower compensation expense.

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 Accounting Standards Update, or ASU, 2016-09, Improvements to Employee Share-Based Payment Accounting, and accounts for forfeitures as they occur from that date (see Accounting Pronouncements Recently Adopted, below).

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 such option 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. Compensation cost relating to awards with service-based graded vesting schedules is recognized using the straight-line method.

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, 2017, 2016 and 2015, 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.

Litigation Reserves

The Company may become involved in the future in various lawsuits, claims, investigations and proceedings that arise in the ordinary course of 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.

Accounting Pronouncements Recently Adopted

As of January 1, 2017, the Company adopted ASU No. 2016-09, Improvements to Employee Share-Based Payment Accounting, or ASU 2016-09, which amends ASC Topic 718, Compensation – Stock Compensation. ASU 2016-09 simplifies several aspects of the accounting for share-based payment transactions, including the accounting for forfeitures, income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. Certain of the amendments were applied using a modified retrospective transition method by means of a cumulative-effect adjustment to equity as of January 1, 2017, while other amendments were applied retrospectively, prospectively or using either a prospective or a retrospective transition method. Upon adoption, the Company began to account for forfeitures as they occur rather than estimate forfeiture rates for stock option awards. As a result, the Company recorded a cumulative-effect adjustment to stockholders’ equity of $45 on the date of initial adoption for all stock option awards that were unvested as of that date. In periods subsequent to adoption, a higher expense will be recognized earlier during the respective vesting periods of stock-based awards that are not forfeited. The Company expects that the income tax amendments within ASU 2016-09 will have no impact on its results of operations or cash flows because it is in a net operating loss position with a full valuation allowance against its deferred tax assets.

Recent Accounting Pronouncements Not Yet Adopted

In May 2017, the Financial Accounting Standards Board, or FASB, issued 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. ASU 2017-09 is effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017. The Company generally has not modified, and does not expect to frequently modify, the fair value, vesting conditions or classification of its share-based payment awards. As a result, for reporting periods following the adoption of ASU 2017-09, the Company generally 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 (see Note 12, Stock-based Compensation).

In January 2017, the FASB issued 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. ASU 2017-01 will be applied prospectively and is effective for annual periods beginning after December 15, 2017 and interim periods within those annual periods. The Company does not expect that the adoption of ASU 2017-01 will have a material effect on its financial position, results of operations or cash flows since it has not and does not expect to acquire or dispose of assets for which the fair value is divided among diverse identifiable assets.

In December 2016, the FASB issued ASU No. 2016-20, Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers, or ASU 2016-20, that allows entities not to disclose variable consideration allocated to performance obligations related to either: (1) sales - or usage -based royalties on licenses of intellectual property or (2) variable consideration allocated entirely to a wholly unsatisfied performance obligation or to a wholly unsatisfied promise to transfer a distinct good or service that forms part of a single performance obligation when certain criteria are met. ASU 2016-20 also requires entities that use any of the new or previously existing optional exemptions to expand their qualitative disclosures. It also makes 12 additional technical corrections and improvements to the new revenue standard, ASU 2014-09. The amendments have the same effective date and transition requirements as ASU 2014-09. The Company does not expect the adoption of ASU 2016-20 to have a material effect on its financial position, results of operations or cash flows.

In November, 2016, the FASB issued 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. ASU 2016-18 will be applied retrospectively to all periods presented and is effective for public business entities for fiscal years beginning after December 15, 2017, and interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. The Company currently presents changes in restricted cash as an investing activity in the statement of cash flows. Upon adoption of ASU 2016-18, such changes will be reflected in the beginning and ending balances of cash, cash equivalents and restricted cash for all periods presented.

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. The Company is currently evaluating the effect that adoption of ASU 2016-13 will have on its results of operations, financial position and cash flows.

In May 2016, the FASB issued ASU No. 2016-12, Revenue from Contracts with Customers (Topic 606), Narrow-Scope Improvements and Practical Expedients, or ASU 2016-12, which amends guidance in the new revenue standard, ASU No. 2014-09 Revenue from Contracts with Customers (Topic 606), or ASU 2014-09, on collectability, noncash consideration, presentation of sales tax and transition. The amendments in ASU 2016-12 are effective for annual reporting periods beginning after December 15, 2017 (i.e., January 1, 2018), including interim periods within those reporting periods, which is the same as for ASU 2014-09, as amended by ASU No. 2015-14, Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date, or ASU 2015-14. See ASU 2014-09, below, for a discussion of the effect that adoption of ASU 2016-12 is expected to have on the Company’s financial statements.

In April 2016, the FASB issued ASU No. 2016-10, Revenue from Contracts with Customers (Topic 606), Identifying Performance Obligations and Licensing, or ASU 2016-10, which clarifies the principle for determining whether a good or service is “separately identifiable” from other promises in the contract and, therefore, should be accounted for as a separate performance obligation. In that regard, ASU 2016-10 requires that an entity determine whether its promise is to transfer individual goods or services to the customer, or a combined item (or items) to which the individual goods and services are inputs. In addition, ASU 2016-10 categorizes intellectual property, or IP, into two categories: “functional” and “symbolic.” Functional IP has significant standalone functionality. All other IP is considered symbolic IP. Revenue from licenses of functional IP is generally recognized at a point in time, while revenue from licenses of symbolic IP is recognized over time. ASU 2016-10 has the same effective date and transition requirements as ASU 2014-09, as amended by ASU 2015-14. See ASU 2014-09, below, for a discussion of the effect that adoption of ASU 2016-10 is expected to have on the Company’s financial statements.

In March 2016, the FASB issued ASU No. 2016-08, Revenue from Contracts with Customers (Topic 606), Principal versus Agent Considerations (Reporting Revenue Gross versus Net), or ASU 2016-08, which clarifies the implementation guidance on principal versus agent considerations contained in ASU 2014-09 by specifying that the determination as to whether an entity that is involved in providing a good or a service to a customer is a principal or an agent is based upon whether the entity controls the good or the service before it is transferred to the customer. ASU 2016-08 has the same effective date and transition requirements as ASU 2014-09, as amended by ASU 2015-14. See ASU 2014-09, below, for a discussion of the effect that adoption of ASU 2016-08 is expected to have on the Company’s financial statements.

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. ASU 2016-02 requires a lessee to recognize in its balance sheet a liability to make lease payments and a right-of-use asset representing its right to use the underlying asset for the lease term. The lease liability will be equal to the present value of lease payments and the right-of-use asset will be based on the lease liability, subject to adjustment such as for initial direct costs. For income statement purposes, the new standard retains a dual model similar to ASC 840, requiring leases to be classified as either operating leases or capital leases. For lessees, operating leases will result in straight-line expense (similar to current accounting by lessees for operating leases under ASC 840) while capital leases will result in a front-loaded expense pattern (similar to current accounting by lessees for capital leases under ASC 840). ASU 2016-02 requires a lessee to disclose qualitative and quantitative information about its leasing arrangements. 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 has approved an amendment to ASU 2016-02, which allows entities to elect to continue 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 option would record the cumulative effect of adoption on the effective date rather than at the beginning of the earliest comparative period presented. The Company is continuing to evaluate the impact that ASU 2016-02 will have on its financial statements.

In May 2014, the FASB issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), or ASU 2014-09, which changes the principle under which the Company will recognize revenue from contracts with customers from one which requires the Company to satisfy specific criteria before recognizing revenue to one which requires the Company to recognize 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. ASU 2014-09, as amended, defines a five-step process to achieve this core principle: (1) identify the contract with the customer, (2) identify the performance obligations in the contract, (3) determine the transaction price, (4) allocate the transaction price to the performance obligations in the contract, and (5) recognize revenue when (or as) the entity satisfies a performance obligation.

During the years ended December 31, 2017, 2016 and 2015, the Company recognized revenue only from a license agreement with Maruishi, or the Maruishi Agreement, and a license agreement with CKDP, or the CKDP Agreement. Under each of these agreements, the Company has recognized revenue from upfront and milestone payments and may earn additional future milestone payments upon the achievement of defined clinical development and regulatory events. The Company has also recognized revenue from a sub-license fee under the Maruishi Agreement. Since all defined milestones will not have been achieved and the related revenue will not have been recognized under ASC 605 as of the date of adoption of ASU 2014-09, as amended, those contracts will be included within the scope of ASU 2014-09, as amended.

The Company is currently accounting for the Maruishi Agreement and the CKDP Agreement under ASC 605-25, Multiple-Element Arrangements and ASC 605-28, Milestone Method. The Company has analyzed the terms and conditions of each of these contracts in light of the guidance under ASU 2014-09, including amendments under ASU 2016-08, 2016-10, 2016-12 and 2016-20, and has concluded that, due to the similarity of the application of the guidance under ASC 605-25 and ASC 605-28 and under ASU 2014-09, as amended, as it relates to revenue recognition for licenses of intellectual property, or IP, as applied to each of these contracts, the distinct performance obligations, transaction prices, amount of the transaction price allocated to the performance obligations and timing and amount of revenue recognition under ASU 2014-09, as amended, will be the same as under ASC 605-25 and ASC 605-28.

In particular, the following aspects of ASU 2014-09, as amended, are the same as those under ASC 605-25 and ASC 605-28 in respect of the Maruishi Agreement and the CKDP Agreement. The Maruishi Agreement has two distinct performance obligations, granting of the license and the R&D services and the CKDP Agreement has one distinct performance obligation, granting of the license. The methodology for determining the relative standalone selling price of the performance obligations and the allocation of the transaction price to the performance obligations is the same under both standards. The licenses granted to the counterparties under these two contracts are deemed to be functional IP for which revenue is recognized at a point in time, which has been determined to be inception of the respective license agreements, the same as under ASC 605. The R&D services under the Maruishi Agreement were performed from inception of the agreement in 2013 through the third quarter of 2015. Accordingly, under ASU 2014-09, as amended, revenue related to the R&D services under the Maruishi Agreement would be recognized proportionately as those services were performed, as it was under ASC 605-25.

Although the milestone method guidance under ASC 605-28 no longer applies under ASU 2014-09, as amended, the guidance under ASU 2014-09, as amended, for milestones and sales-based royalties related to licenses of IP is effectively the same as pertains to milestones achieved by the Company and those achieved by the counterparty to each license agreement. In addition, due to the probability, at inception of each of the two license agreements, that revenue recognized related to the achievement of milestones and sales-based royalty payments will be reversed in the future, the constraint on including those potential payments in the transaction price at that time applies under ASU 2014-09, as amended. Under ASU 2014-09, as amended, recognition of revenue for achievement of any milestone and sales-based royalty payment will occur at the time that it becomes probable that those events will be achieved. Application of the guidance under ASU 2014-09, as amended, to the milestones achieved under the Maruishi Agreement and the CKDP Agreement prior to adoption of that standard will not change the amount or timing of revenue recognized under ASC 605 for any reporting period presented at or after the date of adoption of ASU 2014-09, as amended. As a result of the foregoing considerations, the Company has concluded that upon adoption of ASU 2014-09, as amended, there will be no impact on its results of operations, financial position or cash flows for any period presented from the CKDP Agreement or the Maruishi Agreement.

ASU 2014-09, as amended by ASU 2015-14, is effective for annual reporting periods beginning after December 15, 2017, and interim periods within those annual reporting periods. ASU 2014-09, as amended, allows for two transition methods: (1) retrospectively to each prior reporting period presented, or (2) using a modified retrospective approach, with the cumulative effect of initially applying ASU 2014-09, as amended, recognized as an adjustment to the opening balance of retained earnings at the date of initial adoption. The Company adopted ASU 2014-09, as amended, using the full retrospective method on January 1, 2018.

Available-for-Sale Marketable Securities

12 Months Ended

Dec. 31, 2017

Investments Debt And Equity Securities [Abstract]

Available-for-Sale Marketable Securities

3. Available-for-Sale Marketable Securities

As of December 31, 2017, and 2016, the Company’s available-for-sale marketable securities consisted of money market funds and debt securities issued by U.S. government-sponsored entities and by investment grade institutions. As of December 31, 2016, the Company’s available-for-sale marketable securities also included debt securities issued by the U.S. Treasury.

The following tables summarize the Company's available-for-sale marketable securities by major type of security as of December 31, 2017, and 2016:

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

As of December 31, 2016

			Gross Unrealized					Estimated Fair
Type of Security	Amortized Cost		Gains		Losses			Value
Money market funds	$	8,268	$	8	$	—		$	8,276
U.S. Treasury securities		2,523		—		(1	)		2,522
U.S. government agency obligations		3,501		1		—			3,502
Corporate bonds		16,683		—		(6	)		16,677
Commercial paper		15,206		3		(2	)		15,207
Total available-for-sale marketable securities	$	46,181	$	12	$	(9	)	$	46,184

All available-for-sale marketable securities are classified in the Company’s Balance Sheets as Marketable securities.

The Company classifies its marketable debt securities based on their contractual maturity dates. As of December 31, 2017, the Company’s marketable debt securities mature at various dates through July 2018. 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				2016
Contractual maturity	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Less than one year	$	43,263	$	43,230	$	37,913	$	37,908

During the years ended December 31, 2017 and 2016, the Company sold shares of its investments in available-for-sale marketable securities with total fair values of $8,755 and $34,003, 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 gains, totaling $5 and $23, 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, 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	)

	Less than 12 Months					12 Months or Greater				Total
			Gross					Gross				Gross
			Unrealized					Unrealized				Unrealized
As of December 31, 2016	Fair Value		Losses			Fair Value		Losses		Fair Value		Losses
U.S. Treasury securities	$	2,522	$	(1	)	$	—	$	—	$	2,522	$	(1	)
Corporate bonds		9,919		(6	)		—		—		9,919		(6	)
Commercial paper		5,227		(2	)		—		—		5,227		(2	)
Total	$	17,668	$	(9	)	$	—	$	—	$	17,668	$	(9	)

As of December 31, 2017, and 2016, the Company held a total of 30 out of 31 positions and 18 out of 34 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, 2017, or 2016. 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, 2017

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, 2017, 2016 and 2015.

	Total Accumulated Other Comprehensive Income (Loss)
Balance, December 31, 2014	$	—
Other comprehensive loss before reclassifications		(35	)
Amount reclassified from accumulated other comprehensive income		—
Net current period other comprehensive loss		(35	)
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	)

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	2017		2016		2015		Comprehensive Loss
Unrealized gains on available-for- sale marketable securities
Realized gains on sale of securities	$	5	$	23	$	—	Other income
		—		—		—	Income tax benefit
	$	5	$	23	$	—

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

Prepaid Expenses	12 Months Ended
Prepaid Expenses	Dec. 31, 2017
Text Block [Abstract]
Prepaid Expenses	5. Prepaid Expenses 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. As of December 31, 2016, the amount of prepaid expenses was $1,530, consisting of $1,256 of prepaid R&D clinical costs, $112 of prepaid insurance, and $162 of other costs.

Property and Equipment, Net

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Property and Equipment, Net

6. Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2017		2016
Computer and office equipment	$	158	$	149
Laboratory equipment		628		795
Furniture and fixtures		27		173
Leasehold improvements		1,128		8,582
	$	1,941	$	9,699
Less accumulated depreciation and amortization		764		8,085
Property and equipment, net	$	1,177	$	1,614

Depreciation and amortization expense included in R&D expense and General and administrative expense was $495, $1,465 and $839 for the years ended December 31, 2017, 2016 and 2015, 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 16, 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
Restricted Cash	Dec. 31, 2017
Receivables [Abstract]
Restricted Cash	7. Restricted Cash The Company is required to maintain stand-by letters of credit as a security deposit under each of the Shelton Lease and the Stamford Lease (refer to Note 16, Commitments and Contingencies). The fair value of each letter of credit approximates its contract value. In each case, the Company’s bank requires the Company to maintain restricted cash balances to serve as collateral for the letter of credit issued to the respective landlords by the bank. During December 2017, the letter of credit for the Shelton Lease expired and the related restricted cash balance was reclassified to cash and cash equivalents on the Company’s Balance Sheet. As of December 31, 2017, 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 remains 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, 2016, the Company had $700 of restricted cash related to the Shelton lease in current assets. As of December 31, 2017 and 2016, the Company had $769 of restricted cash related to the Stamford lease in long-term assets.

Accounts Payable and Accrued Expenses

12 Months Ended

Dec. 31, 2017

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,
	2017		2016

Accounts payable	$	3,829	$	4,738
Accrued research projects		2,356		4,352
Accrued professional fees		384		163
Accrued compensation and benefits		1,864		1,514
Accrued other		73		766
	$	8,506	$	11,533

Stockholders' Equity	12 Months Ended
Stockholders' Equity	Dec. 31, 2017
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, 2017, there were 32,662,255 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 July 29, 2015, the Company entered into an underwriting agreement, or the Underwriting Agreement, with Stifel, Nicolaus & Company, Incorporated and Piper Jaffray & Co., as representatives of the several underwriters named therein, relating to the issuance and sale by the Company of 3,763,440 shares of its common stock, or the 2015 Offering. The 2015 Offering was made pursuant to the Company’s Registration Statement on Form S-3 (File No. 333-203072), filed with the SEC on March 27, 2015 and declared effective on May 13, 2015, and a related prospectus supplement dated July 29, 2015, which was filed with the SEC on July 30, 2015. As part of the 2015 Offering, the Company granted the underwriters an option to purchase 564,516 additional shares of common stock. On August 4, 2015, the Company closed the 2015 Offering, including the full exercise of the underwriters’ option to purchase 564,516 additional shares of common stock, at a public offering price of $18.60 per share. The Company received net proceeds of approximately $75,231, after deducting the underwriting discounts and commissions and offering expenses paid by the Company of $5,269. 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 approximately $86,224, after deducting the underwriting discounts and commissions and offering expenses paid by the Company of $5,891.

Fair Value Measurements

12 Months Ended

Dec. 31, 2017

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, 2017 and 2016 and by level within the fair value hierarchy:

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	$	—

Fair value measurement as of December 31, 2016:

			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	$	12,092	$	12,092	$	—	$	—
Available-for-sale marketable securities:
Money market fund		8,276		—		8,276		—
U.S. Treasury securities		2,522		—		2,522		—
U.S. government agency obligations		3,502		—		3,502		—
Corporate bonds		16,677		—		16,677		—
Commercial paper		15,207		—		15,207		—
Restricted cash:
Commercial money market account		1,469		1,469		—		—
Total financial assets	$	59,745	$	13,561	$	46,184	$	—

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, 2017, 2016 and 2015. There were no transfers of financial assets between Levels 1, 2, or 3 classifications during the years ended December 31, 2017, 2016 and 2015.

- 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

Collaborations	12 Months Ended
Collaborations	Dec. 31, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Collaborations	11. Collaborations Chong Kun Dang Pharmaceutical Corporation In April, 2012, the Company entered into a license agreement with CKDP, or the CKDP Agreement, that provides CKDP with the exclusive rights to develop, manufacture and commercialize products containing CR845/difelikefalin in South Korea. At inception of the CKDP Agreement, the Company received a non-refundable and non-creditable amount of $1,000 and is eligible to receive milestone payments totaling $3,750, relating to pre-defined clinical development ($2,250) and regulatory events ($1,500), as well as royalties on sales of any marketed products containing CR845/difelikefalin. The Company is accounting for the milestones under ASC 605-28 Revenue Recognition – Milestone Method. At the time of execution of the CKDP Agreement, there was significant uncertainty as to whether the stated milestones would be achieved. In conjunction with this uncertainty, the Company has determined that the milestones are substantive in nature as they are commensurate with the enhancement of value of the delivered license because they relate to clinical success and advancement within the FDA drug development platform. The milestones also relate solely to past performance and monetary investment of the Company to achieve the clinical advancement. During the year ended December 31, 2015, the Company met the milestone criteria, as set forth in the CKDP Agreement, for the completion of both a Phase 1b trial of Oral CR845/difelikefalin in the United States and a Phase 2 trial of I.V. CR845/difelikefalin in uremic pruritus patients in the United States. Both milestones were considered to be substantive and, therefore, the total amount of the milestone payments earned, $626 (net of South Korean withholding tax of $124), was recognized as milestone revenue upon achievement of the milestones. The next potential milestone that the Company could be entitled to receive under the CKDP Agreement will be a clinical development milestone for the completion by the Company in the United States of a Phase 3 trial of CR845/difelikefalin in uremic pruritus. If achieved, this milestone will result in a payment of $750, before South Korean withholding taxes, being due to the Company. 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 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 between 2013 and 2015. Under the terms of the agreement, the Company received an upfront non-refundable, non-creditable license fee of $15,000. As indicated in Note 2, Summary of Significant Accounting Policies – Revenue Recognition, the Company accounts for arrangements of this type under ASC 605-25, Revenue Recognition - Multiple Element Arrangements. The Company has identified two deliverables under this guidance: (1) the license; and (2) the R&D services specific to the uremic pruritus field of use. The Company has determined that the license has standalone value because Maruishi has the right to sublicense and manufacture CR845/difelikefalin in Japan. The second deliverable is the R&D services, which also have standalone value as similar services are sold separately by other vendors. Since both license and R&D services separability criteria have been met, they are being accounted for as separate units of accounting from the outset of the arrangement. As a result, the total value of the arrangement was allocated between the two units of accounting based upon their relative standalone selling prices. The Company used its best estimate of the selling price of these units of accounting, since, as described in Note 2, Summary of Significant Accounting Policies – Revenue Recognition, neither VSOE nor TPE was available. To determine these estimates, the Company used a discounted cash flow method that forecasted and analyzed CR845/difelikefalin in the Japanese 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. As a result, at inception of the Maruishi Agreement, the management of the Company determined that the license and the R&D services deliverables had estimated selling prices of $10,200 and $6,200, respectively. The resulting percentage allocations were applied to the total consideration. The amount assigned to the license deliverable was recognized immediately as license revenue. The amount assigned to the R&D services unit of accounting was initially recorded as deferred revenue and was recognized as collaborative revenue as the services were provided through July 2015. Under the terms of the Maruishi Agreement, the Company is also entitled to receive aggregate milestone payments of $8,000 for events performed by Maruishi in Japan and $2,500 for events performed by the Company in the United States. At the time of execution of the Maruishi Agreement, there was significant uncertainty as to whether the stated milestones would be achieved. In conjunction with this uncertainty, the Company has determined that the milestones achieved in the United States are substantive in nature as they are commensurate with the enhancement of value of the delivered license as they relate to clinical success and advancement within the FDA drug development platform. The Company accounts for those milestone payments under ASC 605-28 Revenue Recognition – Milestone Method. However, the milestones achieved by Maruishi in Japan are not substantive and are accounted for in accordance with the multiple-element arrangement guidance in ASC 605-25. During June 2014, Maruishi completed a Phase 1 clinical trial in Japan related to CR845/difelikefalin in acute post-operative pain, which constituted achievement of one of the milestones specified in the Maruishi Agreement and was considered not to be substantive. Accordingly, the Company allocated the non-refundable payment of $480, net of a contractual foreign currency exchange adjustment of $20, to the two deliverables in the same proportion as the initial upfront payment had been allocated. The portion of the payment allocated to the previously delivered license deliverable ($302) was recognized as license revenue entirely at the time of achievement of the milestone. A portion of the payment allocated to the R&D services deliverable ($88) was recognized as collaborative revenue at the time of achievement of the milestone to the extent of R&D services provided through that date and the remainder ($90) was deferred and was recognized as collaborative revenue through July 2015, which was the period during which the Company provided R&D services to Maruishi. In September 2015, Maruishi initiated a Phase 2 clinical trial of CR845/difelikefalin in Japan for uremic pruritus, which triggered a $1,725 milestone payment (net of contractual foreign currency exchange adjustments of $275) to the Company. At the time of achievement of the milestone, the Company had delivered all deliverables under the Maruishi Agreement. Since the milestone was achieved in Japan, it was deemed not to be substantive. Accordingly, the Company recognized $1,084 as license and milestone revenue and $641 as collaborative revenue in connection with achievement of this milestone. The next potential milestones that the Company could be entitled to receive under the Maruishi Agreement will be a clinical development milestone for the completion by the Company of the first Phase 3 trial of CR845/difelikefalin in acute pain in the United States and the initiation by Maruishi of a Phase 3 clinical trial of CR845/difelikefalin in Japan for uremic pruritus. If achieved, these milestones will result in payments of $1,000 and $2,000, respectively, before contractual foreign currency exchange adjustments, being due to the Company. The Maruishi Agreement includes a requirement for the Company to negotiate in good faith with the counterparty, after the execution of the Maruishi Agreement, a separate agreement to supply it with CR845/difelikefalin clinical compound or, at Maruishi’s election, CR845/difelikefalin drug substance. However, the Company has no obligation to execute a supply agreement. Furthermore, Maruishi may choose instead to manufacture its own requirements of CR845/difelikefalin drug product and/or drug substance by entering into agreements with contract manufacturing organizations that have the ability to manufacture CR845/difelikefalin. In fact, the parties have never negotiated a master supply agreement. The Company did not identify the potential supply agreement as a unit of accounting within the overall Maruishi Agreement since the significant terms of the supply agreement had not been negotiated between the parties and no executed agreement exists with designated responsibilities. The Company views this aspect of the Maruishi Agreement as a substantive option subject to negotiation that is separate and independent of the Maruishi Agreement and should not be considered a deliverable. In the event that a supply agreement is negotiated, the price to be paid is 110% of the Company’s fully-burdened cost of manufacturing CR845/difelikefalin. The Company concluded that this price does not represent a significant and incremental discount because it is consistent with the amount that other third parties would charge Maruishi. In the absence of a master supply agreement, the Company and Maruishi entered into separate supply agreements for the Company to supply Maruishi with CR845/difelikefalin. Thus, the only deliverable under these separate supply agreements was the delivery of CR845/difelikefalin clinical compound. The Company had previously entered into manufacturing and service agreements with third parties to manufacture CR845/difelikefalin. Payments made by the Company to third parties based on firm and fixed commitments by Maruishi to purchase CR845/difelikefalin from the Company are capitalized as prepaid expense. During the manufacturing process, title and risk of loss remains with the third party until the Company paid in full for the material. Once the Company has title to the CR845/difelikefalin and has delivered it to Maruishi, prepaid expense related to that CR845/difelikefalin is reduced with an offset to R&D expense. At that time, Maruishi reimburses the Company for its external and internal costs for purchasing CR845/difelikefalin and processing the sale to Maruishi and the Company recognizes clinical compound revenue for the reimbursement amount. Deposits received from Maruishi prior to delivery of CR845/difelikefalin are recorded as deferred revenue. During the years ended December 31, 2017, 2016 and 2015, the Company recognized clinical compound revenue of $68, $86 and $0, respectively. The Company is also eligible to receive tiered, low double-digit royalties with respect to any sales of the licensed product sold in Japan by Maruishi. Additionally, the Company can receive sub-license fees (subject to certain credits for milestone payments already made) if Maruishi enters into a sub-license agreement regarding the product candidates. In March 2017, Maruishi entered into a sub-license agreement with Kissei Pharmaceutical Co. Ltd. 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, during 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 deliverables 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. The Company incurred R&D expense related to the Maruishi Agreement of $61, $78 (both related to the cost of clinical compound sold to Maruishi) and $1,583 (consisting of clinical trial costs related to the R&D services deliverable) during years ended December 31, 2017, 2016 and 2015, respectively.

Stock-Based Compensation

12 Months Ended

Dec. 31, 2017

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

Stock-Based Compensation

12. 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. However, as of January 1, 2015 for officers and January 1, 2016 for employees and non-employee consultants, subsequent grants of Stock Awards vest monthly over a period of four years from the grant date. Stock options initially granted to members of the Company’s Board of Directors vest on the date of the Annual Meeting of Stockholders at which their initial term expires based on the class of Director. 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, 2018, the aggregate number of shares of common stock that may be issued pursuant to Stock Awards under the 2014 Plan automatically increased from 3,920,613 to 4,900,481. 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.

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, 2017 is as follows:

				Weighted-		Aggregate
	Number of			Average		Intrinsic
	Options			Exercise Price		Value
Outstanding at December 31, 2016		2,548,408		$	8.75
Granted		1,328,500			16.16
Exercised		(247,892	)		6.85
Forfeited		(136,875	)		7.55
Outstanding at December 31, 2017		3,492,141		$	11.75	$	7,739
Weighted average remaining contractual life as of December 31, 2017 (in years)		8.00
Options exercisable at December 31, 2017		1,556,102		$	10.25	$	4,451
Weighted average remaining contractual life as of December 31, 2017 (in years)		6.96
Options vested and expected to vest as of December 31, 2017		3,492,141		$	11.75	$	7,739
Weighted average remaining contractual life as of December 31, 2017 (in years)		8.00

The total fair value of options vested during the years ended December 31, 2017, 2016 and 2015 was $5,303, $3,589 and $2,489, respectively. The intrinsic value of options exercised during the years ended December 31, 2017, 2016 and 2015 was $2,285, $126 and $1,748, respectively.

During the years ended December 31, 2017, 2016 and 2015, the Company granted 1,328,500, 1,078,000 and 774,000 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,
	2017	2016	2015
Risk-free interest rate	1.85% -2.57%	1.19% - 1.93%	1.43% - 1.89%
Expected volatility	75.3% - 84.5%	67.8% - 77.8%	64.0% - 67.4%
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	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, 2017, 2016 and 2015 was $11.46, $4.28 and $7.17, respectively.

At the end of each fiscal quarter during the years ended December 31, 2017, 2016 and 2015, 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,
	2017	2016	2015
Risk-free interest rate	1.28% - 2.39%	1.35% - 2.38%	1.81% - 2.15%
Expected volatility	74.6% - 87.3%	70.8% - 75.5%	70.6% - 72.2%
Expected dividend yield	0%	0%	0%
Expected life of non-employee options (in years)	0.62 - 9.94	7.08 - 9.60	8.1 - 8.8

Under ASC 505-50, upon re-measurement of each award, income or expense is recognized during its vesting term.

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, 2017, 2016 and 2015 was $10.16, $4.81 and $10.05, 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, 2017, 2016 and 2015, the Company recognized compensation expense in the accompanying Statements of Comprehensive Loss relating to stock options, as follows:

	Year Ended December 31,
	2017		2016		2015
Research and development	$	2,433	$	1,301	$	1,073
General and administrative		3,897		1,499		1,441
Total stock option expense	$	6,330	$	2,800	$	2,514

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,
	2017		2016			2015
Research and development	$	170	$	(79	)	$	250
General and administrative		200		(20	)		100
Total stock option expense	$	370	$	(99	)	$	350

In connection with the retirement of the Company’s former Chief Financial Officer, effective August 15, 2017, or the Modification Date, the Company modified the terms of the former Chief Financial Officer’s outstanding Stock Awards to: (1) accelerate 50% of the 98,771 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 for an aggregate of 183,000 shares 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 that would have vested based on their original vesting terms through the term of the consulting services was a Type 1 modification 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 Financial Officer provides continuous service as a consultant.

During the year ended December 31, 2017, with respect to these modifications, the Company recognized $537 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 General and administrative expense in the table above.

As of December 31, 2017, 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 $15,913, which is expected to be realized over a weighted average period of 2.96 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, 2017, 2016 and 2015.

Income Taxes

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Income Taxes

13. Income Taxes

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

	December 31,
	2017			2016			2015
Current:
Federal	$	—		$	—		$	—
State		(204	)		(468	)		(397	)
		(204	)		(468	)		(397	)
Deferred:
Federal	—			—			—
State	—			—			—
	—			—			—
Benefit from income taxes	$	(204	)	$	(468	)	$	(397	)

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,
	2017			2016			2015
Income taxes using U.S. federal statutory rate		34.00	%		34.00	%		34.00	%
State income taxes, net of federal benefit		5.33	%		5.44	%		5.95	%
Tax Cuts and Jobs Act		-44.43	%		0.00	%		0.00	%
Impact of R&D tax credit on effective tax rate		3.25	%		3.24	%		3.14	%
Stock option shortfalls and cancellations		0.21	%		-0.07	%		-0.03	%
Permanent items and other		-0.56	%		-0.64	%		-0.41	%
Change in valuation allowance		2.55	%		-41.17	%		-41.07	%
		0.35	%		0.80	%		1.58	%

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

	December 31,
	2017			2016
Deferred tax assets:
Net operating loss carryforwards	$	54,831		$	57,887
Federal and state tax credits		8,401			6,221
Accelerated depreciation		-			259
Stock-based compensation expense		2,382			1,783
Other		582			641
		66,196			66,791
Deferred tax liabilities:
Accelerated depreciation		(23	)		-
Valuation allowance		(66,173	)		(66,791	)
Net deferred tax asset	$	—		$	—

A 100% valuation allowance has been recorded on the deferred tax asset as of December 31, 2017 and 2016 because management believes it is more likely than not that the asset will not be realized. The change in the valuation allowance during 2017 and 2016 was $618 and $23,758, respectively.

The Company has a 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. The financial statements reflect expected future tax consequences of such positions presuming the taxing authorities possess full knowledge of the position and all relevant facts. As of December 31, 2017 and 2016, 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 interest expense.

At December 31, 2017, the Company had federal and state net operating loss carryforwards of approximately $205,135 and $198,350, respectively. The federal and state tax loss carryforwards will begin to expire in 2026 and 2027, respectively, unless previously utilized. The 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 $7,378 and $1,033, 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 2017 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%. On December 31, 2017, the Company did not have any foreign subsidiaries and the international aspects of the Act are not applicable.

On December 22, 2017, Staff Accounting Bulletin 118, or SAB 118, was issued by the SEC due to the complexities involved in accounting for the Act. SAB 118 requires the Company to include in its financial statements a reasonable estimate of the impact of the Act on earnings to the extent such estimate has been determined. Accordingly, the U.S. provision for income tax for 2017 is based on the reasonable estimate guidance provided by SAB 118. The Company is continuing to assess the impact from the Act and will record adjustments in 2018, if necessary.

Net Loss Per Share

12 Months Ended

Dec. 31, 2017

Earnings Per Share [Abstract]

Net Loss Per Share

14. 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,
	2017		2016		2015
Basic:
Weighted average shares outstanding		31,202,842		27,279,008		24,620,372
Diluted:
Weighted average shares outstanding - Basic		31,202,842		27,279,008		24,620,372
Common stock options *		—		—		—
Denominator for diluted net loss per share		31,202,842		27,279,008		24,620,372

*	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,
	2017			2016			2015
Net loss	$	(58,125	)	$	(57,280	)	$	(24,690	)
Weighted-average common shares outstanding:
Basic and Diluted		31,202,842			27,279,008			24,620,372
Net loss per share:
Basic and Diluted	$	(1.86	)	$	(2.10	)	$	(1.00	)

As of December 31, 2017 and 2016, 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
Employee Benefit Plan	Dec. 31, 2017
Postemployment Benefits [Abstract]
Employee Benefit Plan	15. 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. Effective January 1, 2015, 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, 2017, 2016 and 2015, employer contributions to the plan were $174, $118 and $80, respectively.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Commitments And Contingencies Disclosure [Abstract]

Commitments and Contingencies

16. Commitments and Contingencies

Contractual obligations and commitments as of December 31, 2017, comprising future minimum lease payments under the Company’s Stamford lease, were as follows:

	Payment Due for the Year Ending December 31,
	2018		2019		2020		2021		2022		Thereafter		Total
Stamford operating lease	$	1,091	$	1,215	$	1,240	$	1,264	$	1,288	$	1,164	$	7,262

Stamford Operating Lease

In December 2015, the Company entered into a lease agreement, or the Stamford Lease, with Four Stamford Plaza Owner LLC, or the Landlord, 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, 2017 and 2016, the balance of deferred lease obligation, representing the difference between cash rent paid and straight-line rent expense, was $876 and $583, respectively. The Stamford Lease is renewable for one five-year term.

As of the Commencement Date, the Stamford landlord had made tenant improvements of approximately $1,094 to the leased premises. Such amount was included in Property and equipment, net and in Deferred lease obligation on the Company’s Balance Sheet on that date. 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, 2017 and 2016, the balance of Deferred lease obligation related to tenant improvements was $842 and $987, respectively.

Total rent expense under the Stamford Lease was $935 and $797 for the years ended December 31, 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 and $665 for the years ended December 31, 2016 and 2015, respectively.

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. As of December 31, 2016, the balance of the letter of credit was $700, which was secured with restricted cash (refer to Note 7, 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 and $67 for the year ended December 31, 2015.

Legal Matters	12 Months Ended
Legal Matters	Dec. 31, 2017
Commitments And Contingencies Disclosure [Abstract]
Legal Matters	17. Legal Matters From time to time, the Company may become involved in arbitrations or legal proceedings that arise in the ordinary course of its business. The Company cannot predict the timing or outcome of these claims and proceedings. Currently, the Company is not involved in any such arbitration and/or legal proceeding that it expects to have a material effect on its financial condition, results of operations or business.

Quarterly Results of Operations (Unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Quarterly Results of Operations (Unaudited)

18. Quarterly Results of Operations (Unaudited)

The following tables contain selected financial data for each quarter of the years ended December 31, 2017 and 2016. 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, 2017 and 2016. The operating results for any period are not necessarily indicative of results for any future periods.

	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).

	Year Ended December 31, 2016
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
Revenues	$	7		$	79		$	—		$	—
Net loss - Basic and Diluted		(10,692	)		(13,075	)		(11,542	)		(21,971	)
Loss per share - Basic and Diluted	$	(0.39	)	$	(0.48	)	$	(0.42	)	$	(0.81	)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

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
Laboratory equipment	8 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)

Long-Lived Assets

Revenue Recognition

Royalty revenue is recognized when earned. To date, no royalties have been earned or were otherwise due to the Company.

Research and Development Expenses

Income Taxes

Stock-Based Compensation

Income (Loss) Per Share

Segment Reporting

Leases

Litigation Reserves

Recent Accounting Pronouncements

Accounting Pronouncements Recently Adopted

Recent Accounting Pronouncements Not Yet Adopted

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Summary of Useful Lives of Property and Equipment

Asset Category	Useful Lives
Computer and office equipment	5 years
Laboratory equipment	8 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, 2017

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, 2017, and 2016:

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

As of December 31, 2016

			Gross Unrealized					Estimated Fair
Type of Security	Amortized Cost		Gains		Losses			Value
Money market funds	$	8,268	$	8	$	—		$	8,276
U.S. Treasury securities		2,523		—		(1	)		2,522
U.S. government agency obligations		3,501		1		—			3,502
Corporate bonds		16,683		—		(6	)		16,677
Commercial paper		15,206		3		(2	)		15,207
Total available-for-sale marketable securities	$	46,181	$	12	$	(9	)	$	46,184

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

	As of December 31,
	2017				2016
Contractual maturity	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Less than one year	$	43,263	$	43,230	$	37,913	$	37,908

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, 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	)

	Less than 12 Months					12 Months or Greater				Total
			Gross					Gross				Gross
			Unrealized					Unrealized				Unrealized
As of December 31, 2016	Fair Value		Losses			Fair Value		Losses		Fair Value		Losses
U.S. Treasury securities	$	2,522	$	(1	)	$	—	$	—	$	2,522	$	(1	)
Corporate bonds		9,919		(6	)		—		—		9,919		(6	)
Commercial paper		5,227		(2	)		—		—		5,227		(2	)
Total	$	17,668	$	(9	)	$	—	$	—	$	17,668	$	(9	)

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Accumulated Other Comprehensive Income (Loss) (Tables)

12 Months Ended

Dec. 31, 2017

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, 2014	$	—
Other comprehensive loss before reclassifications		(35	)
Amount reclassified from accumulated other comprehensive income		—
Net current period other comprehensive loss		(35	)
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	)

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	2017		2016		2015		Comprehensive Loss
Unrealized gains on available-for- sale marketable securities
Realized gains on sale of securities	$	5	$	23	$	—	Other income
		—		—		—	Income tax benefit
	$	5	$	23	$	—

Property and Equipment, Net (Tables)

12 Months Ended

Dec. 31, 2017

Property Plant And Equipment [Abstract]

Property and Equipment, Net

Property and equipment, net consists of the following:

	December 31,
	2017		2016
Computer and office equipment	$	158	$	149
Laboratory equipment		628		795
Furniture and fixtures		27		173
Leasehold improvements		1,128		8,582
	$	1,941	$	9,699
Less accumulated depreciation and amortization		764		8,085
Property and equipment, net	$	1,177	$	1,614

Accounts Payable and Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2017

Payables And Accruals [Abstract]

Schedule of Accounts Payable and Accrued Expenses

Accounts payable and accrued expenses consist of the following:

	December 31,
	2017		2016

Accounts payable	$	3,829	$	4,738
Accrued research projects		2,356		4,352
Accrued professional fees		384		163
Accrued compensation and benefits		1,864		1,514
Accrued other		73		766
	$	8,506	$	11,533

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

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, 2017 and 2016 and by level within the fair value hierarchy:

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	$	—

Fair value measurement as of December 31, 2016:

			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	$	12,092	$	12,092	$	—	$	—
Available-for-sale marketable securities:
Money market fund		8,276		—		8,276		—
U.S. Treasury securities		2,522		—		2,522		—
U.S. government agency obligations		3,502		—		3,502		—
Corporate bonds		16,677		—		16,677		—
Commercial paper		15,207		—		15,207		—
Restricted cash:
Commercial money market account		1,469		1,469		—		—
Total financial assets	$	59,745	$	13,561	$	46,184	$	—

Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Summary of Stock Option Activity

				Weighted-		Aggregate
	Number of			Average		Intrinsic
	Options			Exercise Price		Value
Outstanding at December 31, 2016		2,548,408		$	8.75
Granted		1,328,500			16.16
Exercised		(247,892	)		6.85
Forfeited		(136,875	)		7.55
Outstanding at December 31, 2017		3,492,141		$	11.75	$	7,739
Weighted average remaining contractual life as of December 31, 2017 (in years)		8.00
Options exercisable at December 31, 2017		1,556,102		$	10.25	$	4,451
Weighted average remaining contractual life as of December 31, 2017 (in years)		6.96
Options vested and expected to vest as of December 31, 2017		3,492,141		$	11.75	$	7,739
Weighted average remaining contractual life as of December 31, 2017 (in years)		8.00

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,
	2017	2016	2015
Risk-free interest rate	1.85% -2.57%	1.19% - 1.93%	1.43% - 1.89%
Expected volatility	75.3% - 84.5%	67.8% - 77.8%	64.0% - 67.4%
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	10

Summary of Compensation Expense Relating to Stock Options

During the years ended December 31, 2017, 2016 and 2015, the Company recognized compensation expense in the accompanying Statements of Comprehensive Loss relating to stock options, as follows:

	Year Ended December 31,
	2017		2016		2015
Research and development	$	2,433	$	1,301	$	1,073
General and administrative		3,897		1,499		1,441
Total stock option expense	$	6,330	$	2,800	$	2,514

Non-employee Consultants [Member]

Summary of Compensation Expense Relating to Stock Options

	Year Ended December 31,
	2017		2016			2015
Research and development	$	170	$	(79	)	$	250
General and administrative		200		(20	)		100
Total stock option expense	$	370	$	(99	)	$	350

Re-Measurement [Member]

Summary of Assumptions Used in Black-Scholes Option Pricing Model

	Year Ended December 31,
	2017	2016	2015
Risk-free interest rate	1.28% - 2.39%	1.35% - 2.38%	1.81% - 2.15%
Expected volatility	74.6% - 87.3%	70.8% - 75.5%	70.6% - 72.2%
Expected dividend yield	0%	0%	0%
Expected life of non-employee options (in years)	0.62 - 9.94	7.08 - 9.60	8.1 - 8.8

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Components of Income Taxes (Benefit)

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

	December 31,
	2017			2016			2015
Current:
Federal	$	—		$	—		$	—
State		(204	)		(468	)		(397	)
		(204	)		(468	)		(397	)
Deferred:
Federal	—			—			—
State	—			—			—
	—			—			—
Benefit from income taxes	$	(204	)	$	(468	)	$	(397	)

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,
	2017			2016			2015
Income taxes using U.S. federal statutory rate		34.00	%		34.00	%		34.00	%
State income taxes, net of federal benefit		5.33	%		5.44	%		5.95	%
Tax Cuts and Jobs Act		-44.43	%		0.00	%		0.00	%
Impact of R&D tax credit on effective tax rate		3.25	%		3.24	%		3.14	%
Stock option shortfalls and cancellations		0.21	%		-0.07	%		-0.03	%
Permanent items and other		-0.56	%		-0.64	%		-0.41	%
Change in valuation allowance		2.55	%		-41.17	%		-41.07	%
		0.35	%		0.80	%		1.58	%

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,
	2017			2016
Deferred tax assets:
Net operating loss carryforwards	$	54,831		$	57,887
Federal and state tax credits		8,401			6,221
Accelerated depreciation		-			259
Stock-based compensation expense		2,382			1,783
Other		582			641
		66,196			66,791
Deferred tax liabilities:
Accelerated depreciation		(23	)		-
Valuation allowance		(66,173	)		(66,791	)
Net deferred tax asset	$	—		$	—

Net Loss Per Share (Tables)

12 Months Ended

Dec. 31, 2017

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,
	2017		2016		2015
Basic:
Weighted average shares outstanding		31,202,842		27,279,008		24,620,372
Diluted:
Weighted average shares outstanding - Basic		31,202,842		27,279,008		24,620,372
Common stock options *		—		—		—
Denominator for diluted net loss per share		31,202,842		27,279,008		24,620,372

*	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,
	2017			2016			2015
Net loss	$	(58,125	)	$	(57,280	)	$	(24,690	)
Weighted-average common shares outstanding:
Basic and Diluted		31,202,842			27,279,008			24,620,372
Net loss per share:
Basic and Diluted	$	(1.86	)	$	(2.10	)	$	(1.00	)

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

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, 2017, comprising future minimum lease payments under the Company’s Stamford lease, were as follows:

	Payment Due for the Year Ending December 31,
	2018		2019		2020		2021		2022		Thereafter		Total
Stamford operating lease	$	1,091	$	1,215	$	1,240	$	1,264	$	1,288	$	1,164	$	7,262

Quarterly Results of Operations (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Information Disclosure [Abstract]

Schedule of Selected Financial Data for Each Quarter

	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)

	Year Ended December 31, 2016
	First Quarter			Second Quarter			Third Quarter			Fourth Quarter
Revenues	$	7		$	79		$	—		$	—
Net loss - Basic and Diluted		(10,692	)		(13,075	)		(11,542	)		(21,971	)
Loss per share - Basic and Diluted	$	(0.39	)	$	(0.48	)	$	(0.42	)	$	(0.81	)

Business - Additional Information (Detail) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Apr. 05, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Nature Of Business [Line Items]
Date of formation										Jul. 02, 2004
Proceeds from equity and debt financing		$ 291,100								$ 291,100
Upfront and milestone payments received in connection with license agreements		33,500								33,500
Unrestricted cash and cash equivalents and marketable securities		92,569								92,569
Accumulated deficit		220,341				$ 162,171				220,341	$ 162,171
Net loss		$ 14,177	$ 12,444	$ 9,300	$ 22,204	$ 21,971	$ 11,542	$ 13,075	$ 10,692	58,125	57,280	$ 24,690
Net cash used in operating activities										(54,827)	$ (47,381)	(21,478)
Follow-on Public Offering [Member]
Nature Of Business [Line Items]
Proceeds from offering of common stock	$ 86,224									$ 86,224		$ 75,231

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Financial_Institution Segment	Dec. 31, 2016 USD ($)	Dec. 31, 2015	Jan. 01, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Royalty revenue	$ 0
Prepaid R&D expense	$ 1,287,000	$ 1,256,000
Expected dividend yield	0.00%	0.00%	0.00%
Number of operating segments \| Segment	1
ASU 2016-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative-effect adjustment to stockholders' equity				$ 45,000
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	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	2

Summary of Significant Accounting Policies - Summary of Useful Lives of Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2017
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	5 years
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated useful lives, years	8 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, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 83,251	$ 46,181
Gross Unrealized Gains		12
Gross Unrealized Losses	(70)	(9)
Estimated Fair Value	83,181	46,184
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,988	8,268
Gross Unrealized Gains		8
Gross Unrealized Losses	(37)
Estimated Fair Value	39,951	8,276
US Government Corporations and Agencies Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,799	3,501
Gross Unrealized Gains		1
Gross Unrealized Losses	(5)
Estimated Fair Value	7,794	3,502
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,919	16,683
Gross Unrealized Losses	(12)	(6)
Estimated Fair Value	15,907	16,677
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,545	15,206
Gross Unrealized Gains		3
Gross Unrealized Losses	(16)	(2)
Estimated Fair Value	$ 19,529	15,207
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		2,523
Gross Unrealized Losses		(1)
Estimated Fair Value		$ 2,522

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, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Amortized Cost of marketable debt securities, contractual maturity, Less than one year	$ 43,263	$ 37,913
Fair value of marketable debt securities, contractual maturity, Less than one year	$ 43,230	$ 37,908

Available-for-Sale Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Position	Dec. 31, 2016 USD ($) Position
Schedule of Available-for-sale Securities [Line Items]
Proceeds from sale of available-for-sale marketable securities	$ 8,755,000	$ 34,003,000
Realized gains	$ 5,000	$ 23,000
Number of available-for-sale marketable securities in unrealized loss positions \| Position	30	18
Total number of positions \| Position	31	34
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	8,755,000	34,003,000
Realized gains	$ 5,000	$ 23,000

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, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Fair Value	$ 82,683	$ 17,668
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(70)	(9)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Fair Value		0
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses		0
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	82,683	17,668
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(70)	(9)
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)
US Government Corporations and Agencies 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	7,794
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(5)
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	7,794
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(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	15,907	9,919
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(12)	(6)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Fair Value		0
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses		0
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	15,907	9,919
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	(12)	(6)
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	19,031	5,227
Available-for-sale marketable securities, continuous unrealized loss position, Less than 12 Months, Gross Unrealized Losses	(16)	(2)
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Fair Value		0
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses		0
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value	19,031	5,227
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses	$ (16)	(2)
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		2,522
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, 12 Months or Greater, Fair Value		0
Available-for-sale marketable securities, continuous unrealized loss position, 12 Months or Greater, Gross Unrealized Losses		0
Available-for-sale marketable securities, continuous unrealized loss position, Fair Value		2,522
Available-for-sale marketable securities, continuous unrealized loss position, Gross Unrealized Losses		$ (1)

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	$ 50,725	$ 105,044	$ 51,662
Other comprehensive income (loss) before reclassifications	(68)	61	(35)
Amount reclassified from accumulated other comprehensive income	(5)	(23)
Net current period other comprehensive income (loss)	(73)	38	(35)
Balance, Value	86,780	50,725	105,044
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Balance, Value	3	(35)
Net current period other comprehensive income (loss)	(73)	38	(35)
Balance, Value	$ (70)	$ 3	$ (35)

Accumulated Other Comprehensive Income (Loss) - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Other income	$ 1,156	$ 652	$ 101
Income tax benefit	(204)	(468)	(397)
Net of tax	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	23
Income tax benefit	$ 0	$ 0	$ 0

Prepaid Expenses - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred Costs Capitalized Prepaid And Other Assets Disclosure [Abstract]
Prepaid expenses	$ 1,635	$ 1,530
Prepaid R&D clinical costs	1,287	1,256
Prepaid insurance	124	112
Other prepaid costs	$ 224	$ 162

Property and Equipment, Net - Property and Equipment, Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,941	$ 9,699
Less accumulated depreciation and amortization	764	8,085
Property and equipment, net	1,177	1,614
Computer and Office Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	158	149
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	628	795
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	27	173
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 1,128	$ 8,582

Property and Equipment, Net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation and amortization expense	$ 495	$ 1,465	$ 839
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, 2017	Dec. 31, 2016
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash in current assets		$ 700
Restricted cash balance	$ 769	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	769
Shelton Lease [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash in current assets		$ 700

Accounts Payable and Accrued Expenses - Schedule of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accounts payable	$ 3,829	$ 4,738
Accrued research projects	2,356	4,352
Accrued professional fees	384	163
Accrued compensation and benefits	1,864	1,514
Accrued other	73	766
Total	$ 8,506	$ 11,533

Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Apr. 05, 2017	Aug. 04, 2015	Jul. 29, 2015	Dec. 31, 2017	Dec. 31, 2015	Mar. 30, 2017	Dec. 31, 2016
Subsidiary, Sale of Stock [Line Items]
Common stock, shares authorized				100,000,000			100,000,000
Common stock, par value				$ 0.001			$ 0.001
Preferred stock, shares authorized				5,000,000			5,000,000
Preferred stock, par value				$ 0.001			$ 0.001
Common stock, shares outstanding				32,662,255			27,296,863
Preferred stock, shares issued				0			0
Preferred stock, shares outstanding				0			0
Follow-on Public Offering [Member]
Subsidiary, Sale of Stock [Line Items]
Public offering price per share				$ 18.00	$ 18.60
Common Stock [Member] \| Maximum [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock, shares subscribed						5,117,500
Common Stock [Member] \| Follow-on Public Offering [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued			3,763,440	5,117,500	4,327,956
Public offering price per share		$ 18.60				$ 18.00
Proceeds from issuance of common stock net of underwriting discounts and offering costs	$ 86,224	$ 75,231
Offering expenses paid	$ 5,891	$ 5,269
Common Stock [Member] \| Underwriter's Over-Allotment [Member] \| Maximum [Member]
Subsidiary, Sale of Stock [Line Items]
Common stock, shares issued	667,500	564,516
Common stock, shares subscribed						667,500

Fair Value Measurements - Summary of Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial assets
Restricted cash	$ 769	$ 769
Recurring [Member]
Financial assets
Total financial assets	93,338	59,745
US Government Corporations and Agencies Securities [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	7,794	3,502
Corporate Bonds [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	15,907	16,677
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Recurring [Member]
Financial assets
Total financial assets	10,157	13,561
Significant Other Observable Inputs (Level 2) [Member] \| Recurring [Member]
Financial assets
Total financial assets	83,181	46,184
Significant Other Observable Inputs (Level 2) [Member] \| US Government Corporations and Agencies Securities [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	7,794	3,502
Significant Other Observable Inputs (Level 2) [Member] \| Corporate Bonds [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	15,907	16,677
Money Market Funds [Member] \| Recurring [Member]
Financial assets
Cash and cash equivalents	9,388	12,092
Available-for-sale marketable securities	39,951	8,276
Restricted cash	769	1,469
Money Market Funds [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Recurring [Member]
Financial assets
Cash and cash equivalents	9,388	12,092
Restricted cash	769	1,469
Money Market Funds [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	39,951	8,276
Commercial Paper [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	19,529	15,207
Commercial Paper [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities	$ 19,529	15,207
U.S. Treasury Securities [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities		2,522
U.S. Treasury Securities [Member] \| Significant Other Observable Inputs (Level 2) [Member] \| Recurring [Member]
Financial assets
Available-for-sale marketable securities		$ 2,522

Fair Value Measurements - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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

Collaborations - Additional Information (Detail) $ in Thousands	1 Months Ended				12 Months Ended
	Sep. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Apr. 30, 2013 USD ($)	Apr. 30, 2012 USD ($)	Dec. 31, 2017 USD ($) Deliverable	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License and milestone fee revenue					$ 530		$ 1,710
Withholding tax					(204)	$ (468)	(397)
Collaborative revenue					313		2,093
Clinical compound revenue					68	86
Maruishi Pharmaceutical Co., Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License and milestone fee revenue		$ 480			530		1,084
Upfront non-refundable, non-creditable license fee			$ 15,000		$ 843
Number of deliverables for revenue recognized \| Deliverable					2
Revenue recognized from non-substantive milestones					$ 8,000
Revenue recognized from substantive milestones					2,500
Contractual foreign currency exchange adjustments	$ 275	20
Collaborative revenue					313		641
Amount due from milestone payments net of contractual foreign currency exchange adjustments	$ 1,725
Clinical compound revenue					68	86	0
Cost of clinical compound related to R&D expense					61	$ 78
Clinical trial costs related to the R&D services deliverable							1,583
Chong Kun Dang Pharmaceutical Corporation [Member] \| South Korea
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Withholding tax							124
Clinical Development [Member] \| United States [Member] \| Maruishi Pharmaceutical Co., Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Next potential milestone that could be received					1,000
Clinical Development [Member] \| Japan [Member] \| Maruishi Pharmaceutical Co., Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Next potential milestone that could be received					$ 2,000
License [Member] \| Maruishi Pharmaceutical Co., Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License and milestone fee revenue		302
Estimated selling price			10,200
R & D Services [Member] \| Maruishi Pharmaceutical Co., Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Estimated selling price			$ 6,200
Collaborative revenue		88
Deferred R&D service revenue		$ 90
Supply Agreement [Member] \| Maruishi Pharmaceutical Co., Ltd. [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Price to be paid as percentage of fully burdened manufacturing cost					110.00%
Chong Kun Dang Pharmaceutical Agreement [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront non-refundable, non-creditable license fee				$ 1,000
Milestone payments receivable				3,750
License and milestone fee revenue							626
Next potential milestone that could be received							$ 750
Chong Kun Dang Pharmaceutical Agreement [Member] \| Clinical Development [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable				2,250
Chong Kun Dang Pharmaceutical Agreement [Member] \| Regulatory Events [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Milestone payments receivable				$ 1,500

Stock-Based Compensation - Additional Information (Detail) - USD ($)			9 Months Ended	12 Months Ended
	Aug. 15, 2017	Jan. 01, 2016	Sep. 30, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2018	Jan. 01, 2017	Jan. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of options vested				$ 5,303,000	$ 3,589,000	$ 2,489,000
Intrinsic value of options exercised				$ 2,285,000	$ 126,000	$ 1,748,000
Options granted				1,328,500	1,078,000	774,000
Fair value of options granted				$ 11.46	$ 4.28	$ 7.17
Compensation expense not yet recognized				$ 15,913,000
Weighted average period of compensation expense not yet recognized				2 years 11 months 15 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
Nonemployee Options [Member] \| Modified Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested shares outstanding as of modification date	98,771
Accelerate vesting percentage	50.00%
Number of stock options exercise period extended	183,000
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				$ 537,000
Nonemployee Consultants [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average fair values of outstanding options				$ 10.16	$ 4.81	$ 10.05
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								3,920,613
Options granted				1,328,500
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							4,900,481
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] \| Incentive Stock Options [Member] \| Maximum [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
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Options, Granted	1,328,500	1,078,000	774,000
2014 Equity Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Options, Outstanding at December 31, 2016	2,548,408
Number of Options, Granted	1,328,500
Number of Options, Exercised	(247,892)
Number of Options, Forfeited	(136,875)
Number of Options, Outstanding at December 31, 2017	3,492,141	2,548,408
Number of Options, Outstanding, Weighted average remaining contractual life as of December 31, 2017	8 years
Number of Options, Options exercisable at December 31, 2017	1,556,102
Number of Options, Exercisable, Weighted average remaining contractual life as of December 31, 2017	6 years 11 months 15 days
Number of Options, Options vested and expected to vest at December 31, 2017	3,492,141
Number of Options, Vested and expected to vest, Weighted average remaining contractual life as of December 31, 2017	8 years
Weighted-Average Exercise Price, Outstanding at December 31, 2016	$ 8.75
Weighted-Average Exercise Price, Granted	16.16
Weighted-Average Exercise Price, Exercised	6.85
Weighted-Average Exercise Price, Forfeited	7.55
Weighted-Average Exercise Price, Outstanding at December 31, 2017	11.75	$ 8.75
Weighted-Average Exercise Price, Options exercisable at December 31, 2017	10.25
Weighted-Average Exercise Price, Options vested and expected to vest at December 31, 2017	$ 11.75
Aggregate Intrinsic Value, Outstanding at December 31, 2017	$ 7,739
Aggregate Intrinsic Value, Options exercisable at December 31, 2017	4,451
Aggregate Intrinsic Value, Options vested and expected to vest at December 31, 2017	$ 7,739

Stock-Based Compensation - Summary of Assumptions Used in Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	1.85%	1.19%	1.43%
Risk-free interest rate, maximum	2.57%	1.93%	1.89%
Expected volatility, minimum	75.30%	67.80%	64.00%
Expected volatility, maximum	84.50%	77.80%	67.40%
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	10 years

Stock-Based Compensation - Summary of Re-measurement of Assumptions Used in Black-Scholes Option Pricing Model (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
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	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]
Expected dividend yield	0.00%	0.00%	0.00%
Re-Measurement [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	1.28%	1.35%	1.81%
Expected volatility	74.60%	70.80%	70.60%
Re-Measurement [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate	2.39%	2.38%	2.15%
Expected volatility	87.30%	75.50%	72.20%
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)	7 months 13 days	7 years 28 days	8 years 1 month 5 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)	9 years 11 months 8 days	9 years 7 months 5 days	8 years 9 months 18 days

Stock-Based Compensation - Summary of Compensation Expense Relating to Stock Options (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense	$ 6,330	$ 2,800	$ 2,514
Research and Development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense	2,433	1,301	1,073
General and Administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock option expense	$ 3,897	$ 1,499	$ 1,441

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, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 370	$ (99)	$ 350
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	170	(79)	250
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	$ 200	$ (20)	$ 100

Income Taxes - Components of Income Taxes (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ 0	$ 0	$ 0
State	(204)	(468)	(397)
Total Current	(204)	(468)	(397)
Deferred:
Federal	0	0	0
State	0	0	0
Total Deferred	0	0	0
Benefit from income taxes	$ (204)	$ (468)	$ (397)

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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		$ 618,000	$ 23,758,000
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.
Income tax rate		34.00%	34.00%	34.00%
Reduction in deferred tax assets		$ 25,913,000
Maximum [Member]
Income Taxes [Line Items]
Income tax rate		35.00%
Scenario Plan [Member]
Income Taxes [Line Items]
Income tax rate	21.00%
Earliest Tax Year [Member]
Income Taxes [Line Items]
Open tax years		2006
Latest Tax Year [Member]
Income Taxes [Line Items]
Open tax years		2017
Federal [Member]
Income Taxes [Line Items]
Net operating loss carryforwards		$ 205,135,000
Federal [Member] \| R&D Tax Credit Carryforwards [Member]
Income Taxes [Line Items]
Tax credit carryforwards		$ 7,378,000
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		$ 198,350,000
State and Local [Member] \| R&D Tax Credit Carryforwards [Member]
Income Taxes [Line Items]
Tax credit carryforwards		$ 1,033,000
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income taxes using U.S. federal statutory rate	34.00%	34.00%	34.00%
State income taxes, net of federal benefit	5.33%	5.44%	5.95%
Tax Cuts and Jobs Act	(44.43%)	0.00%	0.00%
Impact of R&D tax credit on effective tax rate	3.25%	3.24%	3.14%
Stock option shortfalls and cancellations	0.21%	(0.07%)	(0.03%)
Permanent items and other	(0.56%)	(0.64%)	(0.41%)
Change in valuation allowance	2.55%	(41.17%)	(41.07%)
Total	0.35%	0.80%	1.58%

Income Taxes - Schedule of Significant Components of Company's Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 54,831	$ 57,887
Federal and state tax credits	8,401	6,221
Accelerated depreciation		259
Stock-based compensation expense	2,382	1,783
Other	582	641
Gross deferred tax asset	66,196	66,791
Deferred tax liabilities:
Accelerated depreciation	(23)
Valuation allowance	(66,173)	(66,791)
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic:
Weighted average shares outstanding - Basic	31,202,842	27,279,008	24,620,372
Diluted:
Weighted average shares outstanding - Basic	31,202,842	27,279,008	24,620,372
Common stock options	0	0	0
Denominator for diluted net loss per share	31,202,842	27,279,008	24,620,372

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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net loss	$ (14,177)	$ (12,444)	$ (9,300)	$ (22,204)	$ (21,971)	$ (11,542)	$ (13,075)	$ (10,692)	$ (58,125)	$ (57,280)	$ (24,690)
Weighted-average common shares outstanding:
Basic and Diluted									31,202,842	27,279,008	24,620,372
Net loss per share:
Basic and Diluted	$ (0.43)	$ (0.38)	$ (0.29)	$ (0.81)	$ (0.81)	$ (0.42)	$ (0.48)	$ (0.39)	$ (1.86)	$ (2.10)	$ (1.00)

Net Loss per Share - Additional Information (Detail) - shares	12 Months Ended
	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	3,492,141	2,548,408

Employee Benefit Plan - Additional Information (Detail) - USD ($) $ in Thousands		12 Months Ended
	Jan. 01, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
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		$ 174	$ 118	$ 80
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, 2017 USD ($)
Operating Leased Assets [Line Items]
2018	$ 1,091
2019	1,215
2020	1,240
2021	1,264
2022	1,288
Thereafter	1,164
Total	$ 7,262

Commitments and Contingencies - Additional Information (Detail) - USD ($)		12 Months Ended
	May 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Commitments [Line Items]
Deferred lease obligation		$ 1,718,000	$ 1,570,000
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, 2017 and 2016, the balance of deferred lease obligation, representing the difference between cash rent paid and straight-line rent expense, was $876 and $583, 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		$ 876,000	583,000
Tenant improvement expenses	$ 1,094,000
Lease, rent expense		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		$ 842,000	987,000
Shelton Operating Lease [Member]
Other Commitments [Line Items]
Operating lease, expiration date		Nov. 30, 2017
Lease, rent expense			1,127,000	$ 665,000
Accrued cease use date liability	$ 1,312,000
Additional amortization expense			$ 899,000	$ 67,000
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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Revenues				$ 911			$ 79	$ 7	$ 911	$ 86	$ 3,803
Net loss - Basic and Diluted	$ (14,177)	$ (12,444)	$ (9,300)	$ (22,204)	$ (21,971)	$ (11,542)	$ (13,075)	$ (10,692)	$ (58,125)	$ (57,280)	$ (24,690)
Loss per share - Basic and Diluted	$ (0.43)	$ (0.38)	$ (0.29)	$ (0.81)	$ (0.81)	$ (0.42)	$ (0.48)	$ (0.39)	$ (1.86)	$ (2.10)	$ (1.00)

Quarterly Results of Operations (Unaudited) - Schedule of Selected Financial Data for Each Quarter (Parenthetical) (Detail) - $ / shares		3 Months Ended								12 Months Ended
	Jul. 29, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information [Line Items]
Sum of net loss per share calculated on quarterly basis										$ (1.91)
Loss per share - Basic and Diluted		$ (0.43)	$ (0.38)	$ (0.29)	$ (0.81)	$ (0.81)	$ (0.42)	$ (0.48)	$ (0.39)	$ (1.86)	$ (2.10)	$ (1.00)
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	3,763,440									5,117,500		4,327,956