Achillion Pharmaceuticals 10-K 25 Feb 16 2015 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2015	Feb. 19, 2016	Jun. 30, 2015
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	FY
Trading Symbol	ACHN
Entity Registrant Name	ACHILLION PHARMACEUTICALS INC
Entity Central Index Key	1,070,336
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	No
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		136,640,019
Entity Public Float			$ 1,014,028,559

Balance Sheets

Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Current assets:
Cash and cash equivalents	$ 81,725	$ 73,664
Marketable securities	377,616	79,215
Accounts and other receivables	506	95
Prepaid expenses and other current assets	2,741	1,901
Total current assets	462,588	154,875
Fixed assets, net	1,735	1,726
Other assets	50	54
Restricted cash	152	152
Total assets	464,525	156,807
Current liabilities:
Accounts payable	4,133	6,418
Accrued expenses	10,302	6,446
Current portion of long-term debt	223	195
Total current liabilities	14,658	13,059
Long-term debt	231	279
Total liabilities	$ 14,889	$ 13,338
Commitments (Notes 14 and 15)
Stockholders' Equity:
Common Stock, $.001 par value; 200,000 shares authorized at December 31, 2015 and 2014; 136,640 and 103,594 shares issued and outstanding at December 31, 2015 and 2014, respectively	$ 137	$ 104
Additional paid-in capital	905,256	599,796
Stock subscription receivable		(5,737)
Accumulated deficit	(455,712)	(450,682)
Accumulated other comprehensive loss	(45)	(12)
Total stockholders' equity	449,636	143,469
Total liabilities and stockholders' equity	$ 464,525	$ 156,807

Balance Sheets (Parenthetical)

Balance Sheets (Parenthetical) - $ / shares shares in Thousands	Dec. 31, 2015	Dec. 31, 2014
Common stock, par value	$ 0.001	$ 0.001
Common Stock, shares authorized	200,000	200,000
Common Stock, shares issued	136,640	103,594
Common Stock, shares outstanding	136,640	103,594

Statements of Comprehensive Los

Statements of Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue	$ 66,122
Operating expenses
Research and development	56,553	$ 53,515	$ 46,736
General and administrative	24,676	15,911	12,741
Total operating expenses	81,229	69,426	59,477
Loss from operations	(15,107)	(69,426)	(59,477)
Other income (expense)
Interest income	1,188	455	582
Interest expense	(55)	(37)	(52)
Other income	8,944
Net loss	(5,030)	(69,008)	(58,947)
Unrealized loss on marketable securities	(33)	(18)	(13)
Total other comprehensive loss	(33)	(18)	(13)
Total comprehensive loss	$ (5,063)	$ (69,026)	$ (58,960)
Basic and diluted net loss per share attributable to common stockholders (Note 4)	$ (0.04)	$ (0.70)	$ (0.63)
Weighted average shares used in computing basic and diluted net loss per share attributable to common stockholders	125,592	98,367	93,983

Statements of Stockholders' Equ

Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Public Offering	Stock Purchase Agreement	Common Stock	Common StockPublic Offering	Common StockStock Purchase Agreement	Additional Paid-in Capital	Additional Paid-in CapitalPublic Offering	Additional Paid-in CapitalStock Purchase Agreement	Accumulated Deficit	Stock Subscription Receivable	Stock Subscription ReceivablePublic Offering	Accumulated Other Comprehensive Income (Loss)
Beginning Balance (in shares) at Dec. 31, 2012				79,626
Beginning Balance at Dec. 31, 2012	$ 72,047			$ 80			$ 394,675			$ (322,727)			$ 19
Net loss	(58,947)									(58,947)
Other comprehensive loss	(13)												(13)
Stock compensation	5,920						5,920
Issuance of common stock upon exercise of warrants (in shares)				4
Issuance of common stock upon exercise of stock options (in shares)				224
Issuance of common stock upon exercise of stock options	555						555
Issuance of common stock under the Employee Stock Purchase Plan (in shares)				44
Issuance of common stock under the Employee Stock Purchase Plan	185						185
Issuance of common stock, net of issuance costs (in shares)					16,894
Issuance of common stock, net of issuance costs		$ 133,211			$ 17			$ 133,194
Ending Balance (in shares) at Dec. 31, 2013				96,792
Ending Balance at Dec. 31, 2013	152,958			$ 97			534,529			(381,674)			6
Net loss	(69,008)									(69,008)
Other comprehensive loss	(18)												(18)
Stock compensation	7,273						7,273
Issuance of common stock upon exercise of warrants (in shares)				2,099
Issuance of common stock upon exercise of warrants	5,251			$ 2			5,249
Issuance of common stock upon exercise of stock options (in shares)				1,364
Issuance of common stock upon exercise of stock options	4,067			$ 1			4,192				$ (126)
Issuance of common stock under the Employee Stock Purchase Plan (in shares)				102
Issuance of common stock under the Employee Stock Purchase Plan	241			$ 1			240
Issuance of common stock, net of issuance costs (in shares)					3,237
Issuance of common stock, net of issuance costs		42,705			$ 3			48,313				$ (5,611)
Ending Balance (in shares) at Dec. 31, 2014				103,594
Ending Balance at Dec. 31, 2014	143,469			$ 104			599,796			(450,682)	(5,737)		(12)
Net loss	(5,030)									(5,030)
Other comprehensive loss	(33)												(33)
Stock compensation	$ 10,072						10,072
Issuance of common stock upon exercise of warrants (in shares)				8
Issuance of common stock upon exercise of stock options (in shares)	835			835
Issuance of common stock upon exercise of stock options	$ 3,857			$ 1			3,730				$ 126
Issuance of common stock under the Employee Stock Purchase Plan (in shares)				36
Issuance of common stock under the Employee Stock Purchase Plan	290						290
Issuance of common stock, net of issuance costs (in shares)					13,800	18,367
Issuance of common stock, net of issuance costs		$ 138,134	$ 158,877		$ 14	$ 18		$ 132,509	$ 138,859			$ 5,611
Ending Balance (in shares) at Dec. 31, 2015				136,640
Ending Balance at Dec. 31, 2015	$ 449,636			$ 137			$ 905,256			$ (455,712)			$ (45)

Statements of Cash Flows

Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities
Net loss	$ (5,030)	$ (69,008)	$ (58,947)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	694	489	399
Noncash stock-based compensation	10,072	7,273	5,920
(Gain)/loss on disposal of equipment	2	(2)
Premium on purchases of marketable securities	(2,486)	(947)	(3,387)
Amortization of premium on marketable securities	2,002	1,959	2,360
Changes in operating assets and liabilities:
Accounts and other receivables	(411)	385	(203)
Prepaid expenses and other current assets	(837)	575	(104)
Accounts payable	(2,285)	1,827	315
Accrued expenses	3,856	1,925	11
Net cash provided by (used in) operating activities	5,577	(55,524)	(53,636)
Cash flows from investing activities
Purchase of fixed assets	(704)	(947)	(408)
Purchase of marketable securities	(692,525)	(79,338)	(168,117)
Maturities of marketable securities	394,575	123,625	103,491
Net cash provided by (used in) investing activities	(298,654)	43,340	(65,034)
Cash flows from financing activities
Proceeds from exercise of stock options	3,730	4,067	555
Proceeds from exercise of warrants		5,251
Proceeds from sale of stock under the Employee Stock Purchase Plan	290	241	185
Borrowings of debt	229	440
Repayments of debt	(249)	(313)	(350)
Net cash provided by financing activities	301,138	52,391	133,601
Net increase in cash and cash equivalents	8,061	40,207	14,931
Cash and cash equivalents, beginning of period	73,664	33,457	18,526
Cash and cash equivalents, end of period	81,725	73,664	33,457
Supplemental disclosure of cash flow information
Cash paid for interest	50	35	46
Supplemental disclosure of noncash financing activities
Cashless exercise of warrants	53	2,848	47
Public Offering
Cash flows from financing activities
Proceeds from issuance of common stock, net of issuance costs	138,260	$ 42,705	$ 133,211
Stock Purchase Agreement
Cash flows from financing activities
Proceeds from issuance of common stock, net of issuance costs	$ 158,878

Nature of the Business

Nature of the Business	12 Months Ended
	Dec. 31, 2015
Nature of the Business	1. Nature of the Business Achillion Pharmaceuticals, Inc. (the “Company”) was incorporated on August 17, 1998 in Delaware. The Company is seeking to transform innovation into novel treatments that address the needs of patients by discovering and developing small molecule therapeutics for the treatment of infectious diseases and immune system disorders. The Company is devoting substantially all of its efforts towards product research and development. The Company incurred losses of $441,850 from inception through December 31, 2015 and had an accumulated deficit of $455,712 at December 31, 2015, which includes preferred stock dividends recognized until the Company’s initial public offering in 2006. The Company has funded its operations primarily through the sale of equity securities. Based on the Company’s current clinical plan, the Company believes that its existing cash, cash equivalents and marketable securities will be sufficient to meet its current projected operating requirements for at least the next 12 months. However, the Company’s future capital requirements may change and will depend upon numerous factors, including but not limited to: • the costs involved in the preclinical and clinical development of the Company’s complement inhibitor drug candidates; • the costs involved in obtaining regulatory approvals for the Company’s drug candidates; • the scope, prioritization and number of programs the Company pursues; • the costs involved in preparing, filing, prosecuting, maintaining, enforcing and defending patent and other intellectual property claims; • the timing and amount of proceeds received from milestones achieved and royalties earned, if any, by the Company under its exclusive collaboration and license agreement with Janssen Pharmaceuticals, Inc. (“Janssen”); • the Company’s ability to raise debt or equity capital, including any changes in the credit or equity markets that may impact its ability to obtain capital in the future; • the costs associated with, and the outcome of, lawsuits against the Company, if any; • the Company’s acquisition and development of new technologies and drug candidates; and • competing technological and market developments currently unknown to the Company.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2015
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States (“GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. Revenue Recognition The Company recognizes revenue from contract research and development and research progress payments in accordance with Accounting Standards Codification (“ASC”) 605, Revenue Recognition When the Company determines that an arrangement should be accounted for as a single unit of accounting, it must determine the period over which the performance obligations will be performed and revenue related to upfront license payments will be recognized. Revenue will be recognized using either a proportionate performance or straight-line method. The Company recognizes revenue using the proportionate performance method provided that it can reasonably estimate the level of effort required to complete its performance obligations under an arrangement and such performance obligations are provided on a best-efforts basis. Under the proportionate performance method, periodic revenue related to up-front license payments is recognized as the percentage of actual effort expended in that period to total effort expected for all of its performance obligations under the arrangement. Actual effort is generally determined based upon actual direct labor hours or full-time equivalents (“FTE”) incurred and include research and development activities performed by internal scientists. Total expected effort is generally based upon the total direct labor hours of FTEs incorporated into the detailed budget and project plan that is agreed to by both parties to the collaboration. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company expects to complete the related performance obligations. In the event that a change in estimate occurs, the change will be accounted for using the cumulative catch-up method which provides for an adjustment to revenue in the current period. Estimates of the Company’s level of effort may change in the future, resulting in a material change in the amount of revenue recognized in future periods, including negative revenue in some periods. Generally under collaboration arrangements, payments received during the period of performance may include up-front payments, time-or performance-based milestones and reimbursement of internal and external costs. The proportion of actual performance to total expected performance is applied to these payments in determining periodic revenue, but will be limited by the aggregate cash received or receivable to date. Substantive milestone payments are recognized upon achievement of the milestone. Determining whether a milestone is substantive requires judgment that should be made at the inception of the arrangement. To meet the definition of a substantive milestone, the consideration earned by achieving the milestone (1) would have to be commensurate with either the level of effort required to achieve the milestone or the enhancement in the value of the item delivered, (2) would have to relate solely to past performance, and (3) should be reasonable relative to all deliverables and payment terms in the arrangement. No bifurcation of an individual milestone is allowed and there can be more than one milestone in an arrangement. In May 2015, the Company entered into parallel transactions with Janssen and its affiliate, Johnson & Johnson Innovation-JJDC, Inc. (“JJDC”), consisting of (i) an exclusive collaboration and license agreement with Janssen (the “Janssen Agreement”) pursuant to which, upon the closing of the transactions contemplated by the Janssen Agreement on June 29, 2015, the Company granted Janssen exclusive worldwide rights to develop and commercialize products that contain one or more of the Company’s drug candidates for the treatment of chronic hepatitis C virus (“HCV”) infection, namely odalasvir, a second-generation NS5A inhibitor, ACH-3422, a NS5B HCV polymerase inhibitor, and sovaprevir, a NS3/4A HCV protease inhibitor, and (ii) a stock purchase agreement with JJDC (the “Stock Purchase Agreement”) pursuant to which, upon the closing of the transactions contemplated by the Stock Purchase Agreement, JJDC purchased 18,367 shares of the Company’s common stock (the “Shares”) at a price of $12.25 per share, for an aggregate purchase price of $225,000. The Janssen Agreement became effective on June 29, 2015 upon the early termination of applicable waiting periods under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (the “HSR Act”). The Stock Purchase Agreement became effective July 1, 2015. In connection with the closing of the transactions contemplated by the Stock Purchase Agreement, the Company and JJDC entered into an Investor Agreement (the “Investor Agreement”) on July 1, 2015. Pursuant to the terms of the Janssen Agreement, the Company was required to provide technology transfer services related to the chemistry, manufacturing and know-how to Janssen for up to 180 days after the effective date of the agreement. In accordance with ASC 605-25, which provides guidance on accounting for multiple-element arrangements, including the determination of the units of accounting and allocation of total arrangement consideration, the Company identified all of the obligations at the inception of the Janssen Agreement. The significant obligations were determined to be the license and the technology transfer services. The Company determined that license and technology transfer services represent a single unit of accounting because they were not viewed to have standalone value. The Janssen Agreement entered into by the Company and Janssen, the Stock Purchase Agreement, and the Investor Agreement were entered into by the Company and Janssen’s affiliate in contemplation of each other. The only upfront amount received by the Company in exchange for the license and technology transfer services and the issuance of the Shares was the $225,000. The Company determined that the amount received in excess of the fair value of the Shares upon issuance of $66,122 was attributed to the license and technology services. The Company also determined that there was no discernable pattern in which the technology services would be provided during the 180 day period after the effective date. In accordance with ASC 605-10, the Company determined that straight-line attribution of the license and technology services revenues would be used to recognize revenue. As such, revenue of $66,122 was recorded during the year ended December 31, 2015 associated with this transaction. The development, regulatory and sales milestones represent non-refundable amounts that would be paid by Janssen to the Company if certain milestones are achieved in the future. The Company has elected to apply the guidance in ASC 605-28 to the milestones. These milestones, if achieved, are substantive as they relate solely to past performance and are commensurate with estimated enhancement of value associated with the achievement of each milestone as a result of the Company’s performance; however, there can be no assurance that Janssen will achieve the milestones or that the Company will receive the related revenue. Stock-Based Compensation—Employee Stock-Based Awards The Company applies the provisions of ASC 718, Stock Compensation, The Company primarily grants qualified stock options for a fixed number of shares to employees with an exercise price equal to the market value of the shares at the date of grant. To the extent that the amount of the aggregate fair market value of qualified stock options that become exercisable for an individual exceeds $100 during any tax year, those stock options are treated as non-qualified stock options. Under the fair value recognition provisions, stock-based compensation cost is based on the fair value of the portion of stock-based awards that is ultimately expected to vest. The Company utilizes the Black-Scholes option pricing model for determining the estimated fair value for stock-based awards. The Black-Scholes model requires the use of assumptions which determine the fair value of the stock-based awards. Determining the fair value of stock-based awards at the grant date requires judgment, including estimating the expected term of stock options, the expected volatility of our stock and expected dividends. For the years ended December 31, 2015 and 2014, the Company based its estimate of the expected term on historical data for similar stock option grants. The Company utilized the simplified method in developing an estimate of the expected term of “plain vanilla” share options for the year ended December 31, 2013. This method was considered appropriate given the Company’s limited exercise history. For the years ended December 31, 2015, 2014 and 2013, the Company calculated volatility based on actual volatility for the expected term of the option. The Company estimates forfeitures at the grant date and recognizes compensation costs for only those awards that are expected to vest. Accrued Expenses As part of the process of preparing financial statements, the Company is required to estimate accrued expenses. This process involves identifying services which have been performed on its behalf and estimating the level of service performed and the associated cost incurred for such service as of each balance sheet date in its financial statements. In accruing service fees, the Company estimates the time period over which services will be provided and the level of effort in each period. If the actual timing of the provision of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. The majority of service providers invoice the Company monthly in arrears for services performed. Some service providers require upfront or milestone payments. If the estimate of services performed is less than the upfront or milestone payments, the difference is accounted for as a prepaid expense. In the event that the Company does not identify costs that have begun to be incurred or the Company underestimates or overestimates the level of services performed or the costs of such services, actual expenses could differ from such estimates. The date on which some services commence, the level of services performed on or before a given date and the cost of such services are often subjective determinations. The Company makes judgments based upon facts and circumstances known to it in accordance with GAAP. Cash, Cash Equivalents and Restricted Cash Cash and cash equivalents are stated at cost, which approximates fair value, and include short-term, highly-liquid investments with original maturities of less than three months. The Company also holds certificates of deposit, which collateralize the Company’s facility lease which are classified as restricted cash in the accompanying balance sheets. The restricted cash will be released from restriction in 2020. At December 31, 2015 and 2014, the Company had $81,725 and $73,664, respectively, of cash and cash equivalents. Marketable Securities and Equity Investments The Company applies the provisions of ASC 820, Fair Value Measurements and Disclosures, Level 1: Quoted prices in active markets for identical assets and liabilities that the reporting entity has the ability to access at the measurement date; Level 2: Inputs other than quoted prices in active markets, that are observable either directly or indirectly, such as quoted prices for similar assets or liabilities, quoted in markets that are not active, or other inputs that are observable; or Level 3: Unobservable inputs. The fair value of the Company’s marketable securities of $377,616 and $79,215 as of December 31, 2015 and 2014, respectively, was valued based on level 2 inputs. The Company’s investments consist mainly of corporate debt securities and government sponsored bond obligations. Fair value is determined by taking into consideration valuations obtained from third-party pricing services. The third-party pricing services utilize industry standard valuation models, 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; issuer credit spreads; benchmark securities; and other observable inputs. The Company has assessed these as level 2 within the fair value hierarchy of ASC 820. The Company classifies its entire investment portfolio as available for sale as defined in ASC 320, “ Debt and Equity Securities Fair Value of Financial Instruments The Company’s financial instruments, including cash, cash equivalents, accounts receivable, and accounts payable are carried at cost, which approximates their fair value because of the short-term maturity of these instruments. The Company believes that the carrying value of its debt balance outstanding approximates fair value. Fair value is determined using a discounted cash flow model based on current interest rates. Concentration of Risk Concentration of credit risk exists with respect to cash and cash equivalents and investments. The Company maintains its cash and cash equivalents and investments with high quality financial institutions. At times, amounts may exceed federally insured deposit limits. For the years ended December 31, 2015, 2014, and 2013, 100%, 0% and 0%, respectively, of the Company’s revenue was generated from the Janssen Agreement. At December 31, 2015, 79% of the Company’s accounts receivable was from one collaboration partner. Fixed Assets Property and equipment are recorded at cost and are depreciated and amortized over the shorter of their remaining lease term or their estimated useful lives on a straight-line basis as follows: Laboratory equipment 4-7 years Office equipment 3-5 years Leasehold improvements Lesser of life of Expenditures for maintenance and repairs, which do not improve or extend the useful lives of the respective assets, are expensed as incurred. When assets are sold or retired, the related cost and accumulated depreciation are removed from their respective accounts and any resulting gain or loss is included in income (loss) from operations. Long-lived Assets ASC 360, Property, Plant and Equipment, Research and Development Expenses All costs associated with internal research and development, research and development services for which the Company has externally contracted and licensed technology are expensed as incurred. Research and development expense includes direct and indirect costs for salaries, employee benefits, subcontractors, including clinical research organizations (“CROs”), operating supplies, facility-related expenses and depreciation. Patent Costs The Company expenses the costs of obtaining and maintaining patents. Income Taxes The Company uses an asset and liability approach for financial accounting and reporting of income taxes. Deferred tax assets and liabilities are determined based on temporary differences between financial reporting and tax basis of assets and liabilities and are measured by applying enacted rates and laws to taxable years in which differences are expected to be recovered or settled. Further, the effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that the rate change is enacted. A valuation allowance is required when it is “more likely than not” that all or a portion of deferred tax assets will not be realized. The Company applies the provisions of ASC 740, Income Taxe The Company did not have any unrecognized tax benefits as of December 31, 2015. The Company reviews all tax positions to ensure the tax treatment selected is sustainable based on its technical merits and that the position would be sustained if challenged. Segment Information The Company is engaged solely in the discovery and development of innovative small molecule drug therapies. Accordingly, the Company has determined that it operates in one operating segment. Accounting Standards Updates In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, “Revenue from Contracts with Customers (Topic 606),” which supersedes all existing revenue recognition requirements, including most industry-specific guidance. ASU No. 2014-09 requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. In August 2015, the FASB issued ASU No. 2015-14, “Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date,” which delays the effective date of ASU No. 2014-09 by one year. The new standard is effective for reporting periods beginning after December 15, 2017. The Company is currently evaluating the impact ASU No. 2014-09 will have on its financial position and results of operations. In August 2014, FASB issued ASU No. 2014-15, “Presentation of Financial Statements – Going Concern.” ASU No. 2014-15 provides guidance regarding management’s responsibility to evaluate whether there exists substantial doubt about an organization’s ability to continue as a going concern and to provide related footnote disclosures in certain circumstances. ASU No. 2014-15 is effective for annual reporting periods beginning after December 15, 2016, and interim periods thereafter. The Company does not believe ASU No. 2014-15 will have a material effect on its financial position and results of operations or disclosures. In April 2015, FASB issued ASU No. 2015-03, “Interest – Imputation of Interest.” ASU No. 2015-03 requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. ASU 2015-03 is effective for financial statements issued for fiscal years beginning after December 15, 2015 (and interim periods within those fiscal years) with early adoption permitted and retrospective application required. The Company does not believe ASU No. 2015-03 will have a material effect on its financial position and results of operations.

Financing Activities

Financing Activities	12 Months Ended
	Dec. 31, 2015
Financing Activities	3. Financing Activities Public Offerings In February 2015, the Company entered into an underwriting agreement (the “Underwriting Agreement”) with Leerink Partners LLC and Deutsche Bank Securities Inc., as representatives of the several underwriters named therein (collectively, the “Underwriters”), relating to a public offering of shares of the Company’s common stock, par value $0.001 per share, at a price of $10.25 per share less underwriting discounts and commissions (the “Offering”). The Company issued and sold to the Underwriters an aggregate of 13,800 shares of common stock in connection with the Offering. The Offering resulted in net proceeds to the Company of $132,558. Between December 22, 2014 and December 31, 2014, the Company sold 3,236 shares of its common stock pursuant to the Sales Agreement, dated November 8, 2012, between the Company and Cantor Fitzgerald & Co. (the “Cantor Sales Agreement”). In connection with these sales, the Company received $48,316 in net proceeds. As of December 31, 2014, the Company had $5, 611 in subscriptions receivable, as a portion of the sales under the Cantor Sales Agreement closed in January 2015. In January 2015, the Company received the remaining $5,611 in net proceeds from the portion of the sales that closed in January 2015. In February 2013, the Company entered into an underwriting agreement (the “Underwriting Agreement”) with Citigroup Global Markets, Inc. and Leerink Swann LLC as representatives of the several underwriters named therein (the “Underwriters”), related to a public offering of shares of the Company’s common stock, par value $0.001 per share, at a price of $8.40 per share less underwriting discounts and commissions. The Company issued and sold to the Underwriters an aggregate of 16,894 shares of common stock in connection with the Offering. The offering resulted in net proceeds to the Company of $133,211.

Earnings (Loss) Per Share

Earnings (Loss) Per Share	12 Months Ended
	Dec. 31, 2015
Earnings (Loss) Per Share	4. Earnings (Loss) Per Share Basic earnings (loss) per share (“EPS”) is calculated in accordance with Accounting Standards Codification (“ASC”) 260, Earnings Per Share Years Ended December 31, 2015 2014 2013 (in thousands) Net loss (numerator) $ (5,030 ) $ (69,008 ) $ (58,947 ) Weighted-average shares, in thousands (denominator) 125,592 98,367 93,983 Basic and diluted net loss per share $ (0.04 ) $ (0.70 ) $ (0.63 ) Potentially dilutive securities outstanding as of December 31, 2015, 2014 and 2013 are as follows: Years Ended December 31, 2015 2014 2013 (in thousands) Stock Options 8,501 9,493 9,083 Warrants 2,833 2,844 5,338

Collaboration Arrangements

Collaboration Arrangements	12 Months Ended
	Dec. 31, 2015
Collaboration Arrangements	5. Collaboration Arrangements Janssen Pharmaceuticals, Inc. In May 2015, the Company entered into parallel transactions with Janssen and its affiliate, JJDC, Inc. , consisting of (i) an exclusive collaboration and license agreement with Janssen pursuant to which, upon the closing of the transactions contemplated by the Janssen Agreement on June 29, 2015, the Company granted Janssen exclusive worldwide rights to develop and commercialize products that contain one or more of the Company’s drug candidates for the treatment of HCV, namely odalasvir, a second-generation NS5A inhibitor, ACH-3422, a NS5B HCV polymerase inhibitor, and sovaprevir, a NS3/4A HCV protease inhibitor, and (ii) a stock purchase agreement with pursuant to which, upon the closing of the transactions contemplated by the Stock Purchase Agreement, JJDC purchased 18,367 shares of the Company’s common stock at a price of $12.25 per share, for an aggregate purchase price of $225,000. The Janssen Agreement became effective on June 29, 2015 upon the early termination of applicable waiting periods under the HSR Act. The Stock Purchase Agreement became effective July 1, 2015. In connection with the closing of the transactions contemplated by the Stock Purchase Agreement, the Company and JJDC entered into the Investor Agreement on July 1, 2015. Under the terms of the Janssen Agreement, the Company is eligible to receive (1) up to $115,000 of milestone payments based upon achievement of clinical enrollment and dosing in specified studies, substantially all of which is related to dosing in one study, (2) up to an additional $290,000 of milestone payments based upon regulatory approvals and first commercial sale in specified territories, the majority of which relates to regulatory approval and the first commercial sale in the U.S., and (3) up to an additional $500,000 of milestone payments based upon achieving worldwide sales targets. The Company is also eligible to receive royalties on worldwide annual net sales of licensed products, if any, at tiered royalty rate percentages beginning in the mid-teens and rising to the low-twenties, subject to customary reductions. The royalty term is determined on a licensed-product-by-licensed-product and country-by-country basis and begins on the first commercial sale of a licensed product in a country and ends on the expiration of the last to expire of specified patents or regulatory exclusivity covering such licensed product in such country or, with a customary royalty reduction, ten years after such first commercial sale if there is no such exclusivity. Janssen will bear the future costs of worldwide development and commercialization of licensed products, subject to specified exceptions relating to the Company’s ongoing studies and technology transfer. The term of the Janssen Agreement will continue, unless earlier terminated, until expiration of the royalty term for licensed products or all payment obligations under the Janssen Agreement. Janssen may terminate the Janssen Agreement upon 60 days’ written notice to the Company at any time prior to submission of the first application for marketing approval for a licensed product in any of the specified major market countries. Janssen may also terminate the Janssen Agreement under specified circumstances relating to the safety or regulatory approvability of a licensed product. Either the Company or Janssen may terminate the Janssen Agreement if the other party is in material breach of the agreement and fails to cure such breach within specified cure periods. Either the Company or Janssen may terminate the Janssen Agreement in the event of specified insolvency events involving the other party. Upon any early termination, rights to the Company’s licensed drug candidates will revert to the Company. Pursuant to the terms of the Janssen Agreement, the Company was required to provide technology transfer services related to the chemistry, manufacturing and know-how to Janssen for up to 180 days after the effective date. In accordance with ASC 605-25, which provides guidance on accounting for multiple-element arrangements, including the determination of the units of accounting and allocation of total arrangement consideration, the Company identified all of the obligations at the inception of the Janssen Agreement. The significant obligations were determined to be the license and the technology transfer services. The Company has determined that license and technology transfer services represent a single unit of accounting because they were not viewed to have standalone value. The Janssen Agreement entered into by the Company and Janssen, the Stock Purchase Agreement, and the Investor Agreement were entered into by the Company and Janssen’s affiliate in contemplation of each other. The only upfront amount received by the Company in exchange for the license and technology transfer services and the issuance of the Company’s common stock was the $225,000. The Company determined that the amount received in excess of the fair value of the Company’s common stock upon issuance of $66,122 was attributed to the license and technology services. The Company also determined that there was no discernable pattern in which the technology services would be provided during the 180 day period after the effective date. In accordance with ASC 605-10, the Company determined that straight-line attribution of the license and technology services revenues would be used to recognize revenue. As such, revenue of $66,122 was recorded during the year ended December 31, 2015 associated with this transaction. The development, regulatory and sales milestones represent non-refundable amounts that would be paid by Janssen to the Company if certain milestones are achieved in the future. The Company has elected to apply the guidance in ASC 605-28 to the milestones. These milestones, if achieved, are substantive as they relate solely to past performance and are commensurate with estimated enhancement of value associated with the achievement of each milestone as a result of the Company’s performance; however, there can be no assurance that Janssen will achieve the milestones or that the Company will receive the related revenue. Pursuant to the terms of the Investor Agreement, the Shares are subject to a lock-up restriction, such that JJDC agreed it will not, and will also cause its affiliates not to, without the prior approval of the Company, sell, transfer or otherwise dispose of the Shares until the earliest to occur of (i) a specified period after July 1, 2015, (ii) the expiration or earlier termination of the Janssen Agreement or (iii) other specified events. In addition, for the seven year period following the expiration of the lock-up period, subject to specified conditions, the Company has agreed to file a registration statement in order to register all or a portion of the Shares. The Company will not be required to effect more than two such demand registrations for JJDC in the aggregate and is not required to effect more than one such demand registration in any 12 month period. The Company has also agreed to provide JJDC with certain “piggyback” registration rights such that for the seven year period following the expiration of the lock-up period, subject to specified conditions, whenever the Company proposes to register shares of its common stock for its account, JJDC will have the right to include some or all of its Shares in such registration. The Investor Agreement also contains other customary terms and conditions of the parties with respect to the registration of the Shares. Pursuant to the terms of the Investor Agreement, the Shares are subject to a voting agreement, such that until the earliest to occur of (i) a specified period after July 1, 2015, (ii) the expiration or earlier termination of the Janssen Agreement or (iii) other specified events, and subject to specified conditions and excluding specified extraordinary matters, JJDC will, and will cause its permitted transferees to, vote in accordance with the recommendation of the Company’s Board of Directors, or in the case of a meetings of stockholders, if JJDC or a permitted transferee has delivered written notice to the Company at any time prior to the vote on any given matter (but in any event not less than five business days prior to such vote), setting forth its intent to vote on such matter, vote in the same proportion as the votes cast by all other holders of all classes of voting securities of the Company, and JJDC has granted the Company an irrevocable proxy with respect to the foregoing. In addition, pursuant to the terms of the Investor Agreement, the Shares are subject to a standstill agreement, such that until the earliest to occur of (i) a specified period after July 1, 2015 or (ii) other specified events, neither JJDC nor any of its affiliates, except as expressly approved in writing by the Company, will, subject to specified conditions, directly or indirectly, acquire shares of the Company’s outstanding common stock, seek to have called any meeting of the stockholders of the Company, solicit proxies or consents in opposition to the recommendation of a majority of the Company’s Board of Directors with respect to any matter or undertake other specified actions related to the potential acquisition of additional equity interests in the Company. GCA Therapeutics, Ltd. In February 2010, the Company entered into a license agreement (the “Agreement”) with GCA Therapeutics, Ltd. (“GCAT”) for elvucitabine, the Company’s nucleoside reverse transcriptase inhibitor for the treatment of both hepatitis B virus (“HBV”) infection and human immunodeficiency virus (“HIV”) infection. The Agreement was amended and restated in March 2010. The exclusive license grants GCAT the right, through a Chinese joint venture with Tianjing Institute of Pharmaceutical Research, to clinically develop and commercialize elvucitabine in mainland China, Hong Kong and Taiwan. Under the terms of the Agreement, GCAT, through a sublicense agreement with a Chinese joint venture, T&T Pharma Co., Ltd., will assume all development and regulatory responsibility and associated costs for elvucitabine. The Company did not receive any payment upon the signing of the agreement. Upon the first commercial sale of a licensed product GCAT is obligated to pay $100 to the Company. Further, the Company will be eligible to receive royalties up to 15% of net sales in those territories. The Company does not believe that the milestone specified under the Agreement is substantive as achievement of the milestone is based solely on the performance of GCAT and does not relate to any past or future performance by the Company. Because the Company has no performance obligations under the Agreement, it intends to recognize revenue related to the milestone payment upon achievement of the milestone by GCAT. However, there can be no assurance that GCAT will achieve the milestone or that the Company will receive the related revenue. This Agreement shall be effective, unless earlier terminated, until the expiration of the last to expire royalty term. Ora, Inc. In October 2012, the Company entered into a license and development agreement (the “Ora Agreement”) with Ora, Inc. (“Ora”) for the worldwide development and commercialization of ACH-702 delivered topically or locally. The Ora Agreement was amended in April 2013. Under the terms of the Ora Agreement, Ora has assumed development and regulatory responsibility and associated costs for ACH-702. Upon initiation of the agreement, the Company received a one-time license fee of $100, which was recognized as revenue upon the completion of the technology transfer by the Company. The Company is eligible to receive up to $4,000 in development milestones and up to $7,000 in commercialization milestones as well as royalties up to 3.5% of net sales. The Company has no further obligations under the Ora Agreement. The Ora Agreement includes the right to sublicense any or all of the licensed rights, subject to the Company’s approval. Ora has agreed to pay the Company 15% of all up-front licensing payments and any other payment allocated to or received by Ora pursuant to any sublicense agreement granted by Ora under the Ora Agreement; provided that such payment is not a royalty on net sales and not a development or commercial milestone already due to Achillion. In December 2012, Ora entered into a sublicense agreement with Taejoon Pharmaceutical Co. for the development of ACH-702. The Company does not believe that the milestones specified under the Ora Agreement are substantive as achievement of the milestones is based solely on the performance of Ora and its sublicensee(s) and does not relate to any past or future performance by the Company. Because the Company has no performance obligations under the Ora Agreement, it intends to recognize revenue related to any milestone payments upon achievement of the milestone by Ora or its sublicensee(s). The Ora Agreement shall be effective and, unless earlier terminated, will continue until the last sale of each and every licensed product to an unrelated third party by Ora, its affiliate or sublicensee.

Marketable Securities

Marketable Securities	12 Months Ended
	Dec. 31, 2015
Marketable Securities	6. Marketable Securities The fair value of the Company’s marketable securities of $377,616 and $79,215 as of December 31, 2015 and 2014, respectively, is valued based on level 2 inputs. The Company’s investments consist mainly of corporate debt securities and government sponsored bond obligations. Fair value is determined by taking into consideration valuations obtained from third-party pricing services. The third-party pricing services utilize industry standard valuation models, 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; issuer credit spreads; benchmark securities; and other observable inputs. There were no transfers between levels within the hierarchy during the years ended December 31, 2014 and 2015. The Company has assessed these as level 2 within the fair value hierarchy of ASC 820. The Company classifies its entire investment portfolio as available for sale as defined in ASC 320, Debt and Equity Securities The unrealized (loss) gain from marketable securities was $(45), $(12) and $6 at December 31, 2015, 2014 and 2013, respectively. As of December 31, 2015, none of the Company’s investments were determined to be other than temporarily impaired. The following table summarizes the Company’s investments: December 31, 2015 December 31, 2014 Amortized Unrealized Unrealized Estimated Amortized Unrealized Unrealized Estimated Commercial Paper $ 137,483 $ 158 (6 ) $ 137,635 $ 4,747 $ 3 — $ 4,750 Corporate Debt Securities 164,884 — (205 ) 164,679 58,452 1 (16 ) 58,437 Government and Agency Securities 75,294 16 (8 ) 75,302 16,028 1 (1 ) 16,028 Total $ 377,661 $ 174 (219 ) $ 377,616 $ 79,227 $ 5 (17 ) $ 79,215 The following additional table summarizes, by industry, the fair value of investments: As of December 31, 2015 2014 Government 75,301 16,028 Banking 93,040 18,925 Industrial 209,275 44,262 Total $ 377,616 $ 79,215

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Dec. 31, 2015
Prepaid Expenses and Other Current Assets	7. Prepaid Expenses and Other Current Assets A summary of prepaid expenses and other current assets is as follows: As of December 31, 2015 2014 Prepaid research and development costs $ 487 $ 221 Tax credit receivable 640 678 Maintenance agreements 269 387 Interest receivable 1,199 551 Other prepaid expenses 146 64 Total $ 2,741 $ 1,901

Fixed Assets, net

Fixed Assets, net	12 Months Ended
	Dec. 31, 2015
Fixed Assets, net	8. Fixed Assets, net A summary of property and equipment is as follows: As of December 31, 2015 2014 Laboratory equipment $ 3,642 $ 3,461 Office equipment 1,302 989 Leasehold improvements 3,083 3,017 Construction in process 6 — 8,033 7,467 Less—accumulated depreciation and amortization (6,298 ) (5,741 ) Total $ 1,735 $ 1,726 Depreciation expense was $693, $488 and $390 for the years ended December 31, 2015, 2014 and 2013, respectively.

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2015
Accrued Expenses	9. Accrued Expenses Accrued expenses consist of the following: As of December 31, 2015 2014 Accrued compensation $ 4,326 $ 846 Accrued research and development expenses 4,743 4,727 Accrued professional expenses 846 649 Other accrued expenses 387 224 Total $ 10,302 $ 6,446 Accrued research and development expenses are comprised of amounts owed to third-party contract research organizations, (“CROs”), clinical investigators, laboratories and data managers for research and development work performed on behalf of the Company.

Debt

Debt	12 Months Ended
	Dec. 31, 2015
Debt	10. Debt Debt consists of the following: As of December 31, 2015 2014 2011 Credit Facility, payable in equal monthly installments through March 2015, with fixed interest of 6.44% per annum $ — $ 56 2014 Credit Facility, payable in equal monthly installments through October 2017, with fixed interest of 6.30% per annum 279 418 2014 Credit Facility, payable in equal monthly installments through March 2018, with fixed interest of 6.20% per annum 175 — Total long-term debt 454 474 Less: current portion (223 ) (195 ) Total long-term debt, net of current portion $ 231 $ 279 In March 2011, the Company entered into a Master Security Agreement for a $2,000 Capital Expenditure Line of Credit, (the “2011 Credit Facility”) with Webster Bank (“Webster”). Under the 2011 Credit Facility, the Company could draw down equipment loan advances for the purchase of new laboratory equipment through March 2013. In connection with the Master Security Agreement, the Company granted Webster a security interest in equipment to be purchased under the Credit Facility In October 2014, the Company entered into a Master Security Agreement for a $1,000 Capital Expenditure Line of Credit (the “2014 Credit Facility”) with Webster. Under the 2014 Credit Facility, the Company can draw down equipment loan advances for the purchase of new laboratory equipment through October 2015. In connection with the Master Security Agreement, the Company granted Webster a security interest in equipment to be purchased under the Credit Facility. In October 2014 and March 2015, Webster advanced $440 and $229, respectively, to the Company under the 2014 Credit Facility. The fair value for this debt is classified as a level 2 measurement. Fair value is computed using a discounted cash flow model based on current interest rates. At this time, the carrying value approximates fair value.

Capital Structure

Capital Structure	12 Months Ended
	Dec. 31, 2015
Capital Structure	11. Capital Structure Preferred Stock At December 31, 2015, the Company had 5,000 authorized shares of undesignated preferred stock of which no shares were issued and outstanding. Common Stock At December 31, 2015, the Company had 200,000 authorized shares of $0.001 par value common stock of which 136,640 shares were issued and outstanding and 21,813 shares were reserved for future issuance. Warrants At December 31, 2015, there were 2,833 warrants outstanding with a weighted average exercise price of $3.12 and a weighted average remaining contractual life of 1.64 years. Stock Subscription Receivable In December 2014, the Company issued 3,236 shares of common stock under a sales agreement with Cantor Fitzgerald. Sales of the Company’s common stock under the agreement with Cantor Fitzgerald were deemed to be “at-the-market” equity offerings as defined in Rule 415 under the Securities Act of 1933, as amended, or the Securities Act. The offering resulted in net proceeds to the Company of $48,316. Of the total proceeds, $5,611 was not received until January 2015 and was recorded as a stock subscription receivable as of December 31, 2014. In December 2014, the Company issued 19 shares of common stock upon the exercise of stock options. The proceeds of $126 were not received until January 2015 and were recorded as a stock subscription receivable as of December 31, 2014.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2015
Stock-Based Compensation	12. Stock-Based Compensation 2006 Stock Incentive Plan The Company’s 2006 Stock Incentive Plan (“the 2006 Plan”), was adopted by the Company’s board of directors in May 2006, amended by its board of directors in September 2006, approved by its stockholders in September 2006 and became effective in October 2006, upon the closing of the Company’s initial public offering. The Company originally reserved for issuance 750 shares of common stock under the 2006 Plan. In addition, the Plan contained an “evergreen” provision, which allowed for an annual increase in the number of shares available for issuance under the Plan on the first day of each fiscal year during the period beginning on the first day of fiscal year 2007 and ending on the second day of fiscal year 2010. Under the evergreen provision, the Company registered an additional 2,673 shares of common stock to be issued under the 2006 Plan. On June 10, 2010, stockholders of the Company approved an amendment to the 2006 Plan to increase by 3,000 shares the number of shares of common stock reserved for issuance under the 2006 Plan from 3,423 shares to 6,423 shares. On June 5, 2012, stockholders of the Company approved an amendment to the 2006 Plan to increase by 7,000 shares the number of shares of common stock reserved for issuance under the 2006 Plan from 6,423 shares to 13,423 shares. The 2006 Plan provided for the grant of incentive stock options, nonstatutory stock options, restricted stock, restricted stock units, stock appreciation rights and other stock-based awards. The Company’s officers, employees, consultants, advisors and directors, and those of any subsidiaries, were eligible to receive awards under the 2006 Plan. The Company’s board of directors administered the 2006 Plan. Following the adoption of the 2015 Stock Incentive Plan, the Company no longer grants stock options or other awards under the 2006 Plan. 2015 Stock Incentive Plan The Company’s 2015 Stock Incentive Plan (“the 2015 Plan”), was approved by the Company’s stockholders in June 2015. The 2015 Plan replaced the Company’s 2006 Stock Incentive Plan, as amended, (the “2006 Plan”). Upon the approval of the 2015 Plan by stockholders, the 2006 Plan terminated, and all then outstanding awards under the 2006 Plan remained in effect, but no additional awards will be made under the 2006 Plan. However, the terms of the 2006 Plan will continue to apply to awards previously granted under the 2006 Plan. The 2015 Plan allows for the issuance of 6,900 new shares of common stock plus up to 1,894 shares of common stock that remained available for issuance under the previously approved 2006 Plan immediately prior to the effectiveness of the 2015 Plan, all of which shares rolled over and became available for issuance under the 2015 Plan upon its effectiveness. Solely to the extent that any of the 8,727 shares of common stock subject to awards that were issued and outstanding under the 2006 Plan immediately prior to the effectiveness of the 2015 Plan expire, terminate, are surrendered, cancelled or forfeited, such shares also will become available for the future grant of awards under the 2015 Plan. All of the foregoing share numbers are subject, in the case of incentive stock options, to any limitations under the Internal Revenue Code of 1986, as amended (the “Code”), and are also subject to adjustment upon stock splits, stock dividends, and other specified events. Certain sub-limitations apply to the shares available for issuance under the 2015 Plan. The 2015 Plan allows for the issuance of incentive stock options intended to qualify under Section 422 of the Code, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The maximum number of shares with respect to which awards may be granted to any participant under the 2015 Plan may not exceed 1,500 shares per fiscal year (subject to adjustment upon stock splits, stock dividends, and other specified events). The maximum aggregate number of shares with respect to which awards may be granted to directors who are not employees of the Company at the time of grant will be 10% of the maximum number of shares authorized for issuance under the 2015 Plan. The 2015 Plan is administered by the Company’s Board of Directors and allows for the issuance of incentive stock options intended to qualify under Section 422 of the Code, nonstatutory stock options, stock appreciation rights, restricted stock awards, and other stock-based awards. The Company’s officers, employees, consultants, advisors and directors are eligible to receive awards under the 2015 Plan; however, incentive stock options may only be granted to employees. Options granted under the Company’s 1998 Stock Option Plan, 2006 Stock Incentive Plan and the 2015 Stock Incentive Plan (the “Plans”), are exercisable for a period determined by the Company, but in no event longer than ten years from the date of the grant. Options generally vest ratably over four years. As of December 31, 2015, there were 8,773 shares available to be granted under the 2015 Plan. A summary of the status of the Company’s stock option activity for the year ended December 31, 2015 is presented in the table and narrative below: 2015 Options Weighted Outstanding at January 1, 2015 9,493 $ 6.88 Granted 213 9.14 Exercised (835 ) 4.47 Forfeited (297 ) 7.93 Cancelled (73 ) 12.60 Outstanding at December 31, 2015 8,501 $ 7.08 Options exercisable at December 31, 2015 5,642 $ 6.39 Options vested and expected to vest at December 31, 2015 8,083 $ 7.05 The following table summarizes information about stock options outstanding at December 31, 2015: Options Outstanding Options Vested Range of Exercise Prices Number Weighted Average Weighted Number Weighted $0.00 – $2.00 137 3.0 $ 1.05 137 $ 1.05 $2.01 – $4.00 3,279 6.6 3.07 2,288 3.08 $4.01 – $6.00 367 2.2 5.16 364 5.15 $6.01 – $8.00 1,754 6.8 7.45 1,347 7.42 $8.01 – $10.00 1,033 7.3 8.73 742 8.68 $10.01 – $12.00 125 8.6 10.51 41 10.70 $12.01 – $14.00 1,502 8.9 13.44 419 13.43 $14.01 – $16.00 301 1.0 14.75 301 14.75 $16.01 – $20.00 3 1.1 19.00 3 19.00 8,501 6.7 $ 7.08 5,642 $ 6.39 As of December 31, 2015, the intrinsic value of the options outstanding and options vested was $36,733 and $27,139, respectively. The intrinsic value for stock options is calculated based on the difference between the exercise prices of the underlying awards and the quoted stock price of the Company’s common stock as of the reporting date. The total intrinsic value of stock options exercised for the years ended December 31, 2015, 2014 and 2013 was $5,302, $7,855 and $790, respectively. The weighted-average, grant-date fair value of options granted during the years ended December 31, 2015, 2014 and 2013 was $6.66, $8.50 and $3.29, respectively. The weighted-average, grant-date fair value of options vested at December 31, 2015, 2014 and 2013 was $4.64, $4.13 and $3.52, respectively. The weighted average remaining contractual life is 5.9 years for options exercisable and 6.7 years for options vested and expected to vest. Stock -Based Compensation Under the provisions of ASC 718, stock-based compensation cost is based on the fair value of the portion of stock-based awards that is ultimately expected to vest during the period. The Company utilizes the Black-Scholes option pricing model for determining the estimated fair value for stock-based awards. The Black-Scholes model requires the use of assumptions which determine the fair value of the stock-based awards. Determining the fair value of stock-based awards at the grant date requires judgment, including estimating the expected term of stock options, the expected volatility of our stock and expected dividends. The Company is also required to estimate forfeitures at the grant date and recognize compensation costs for only those awards that are expected to vest. Judgment is required in estimating the amount of stock-based awards that are expected to be forfeited. For the years ended December 31, 2015 and 2014, the Company based its estimate of the expected term on historical data for similar stock option grants. The Company utilized the simplified method in developing an estimate of the expected term of “plain vanilla” share options for the year ended December 31, 2013. This method was considered appropriate given the Company’s limited exercise history. For the years ended December 31, 2015, 2014 and 2013, the Company calculated volatility based on actual volatility for the expected term of the option. The assumptions used to value options granted are as follows: For the Years Ended December 31, 2015 2014 2013 Expected term of option 6.25 years 5.0 – 6.25 years 5.0 – 6.1 years Expected volatility 84% – 91% 92% – 96% 87% – 94% Risk free interest rate 1.54 – 1.93% 1.81 – 2.02% 1.01 – 2.10% Expected dividend yield 0% 0% 0% Total compensation expense recorded in the accompanying statements of comprehensive loss associated with option grants made to employees for the years ended December 31, 2015, 2014 and 2013 was $9,887, $7,177 and $5,760, respectively. Total compensation expense recorded in the accompanying statements of comprehensive loss associated with option grants made to consultants for the years ended December 31, 2015, 2014 and 2013 was $25, $0 and $69, respectively. The Company recorded no tax benefit related to these options as the Company is currently in a net operating loss position and maintains a full valuation allowance. As of December 31, 2015, the total compensation cost related to options not yet recognized in the financial statements is approximately $15,333, net of estimated forfeitures, and the weighted average period over which it is expected to be recognized is 1.4 years. Compensation expense related to option grants made to employees and consultants is included in research and development and general and administrative expense as follows: For the Years Ended December 31, 2015 2014 2013 Research and development $ 4,346 $ 2,713 $ 2,146 General and administrative 5,567 4,464 3,682 Total $ 9,913 $ 7,177 $ 5,828 2006 Employee Stock Purchase Plan The Company established an Employee Stock Purchase Plan effective December 1, 2006 (the “2006 ESPP Plan”). Eligible employees can purchase common stock pursuant to payroll deductions at a price equal to 85% of the lower of the fair market value of the common stock at the beginning or end of each six-month offering period. The Company originally reserved for issuance 250 shares of common stock under the 2006 ESPP Plan. On June 10, 2010, stockholders of the Company approved an amendment to the 2006 ESPP Plan to increase by 250 shares the number of shares of common stock reserved for issuance under the 2006 ESPP Plan from 250 shares to 500 shares. On June 2, 2015, stockholders of the Company approved an amendment to the 2006 ESPP Plan to increase by 1,700 shares the number of shares of common stock reserved for issuance under the 2006 ESPP Plan from 500 to 2,200 shares. The Company measures the fair value of issuances under the 2006 ESPP Plan using the Black-Scholes option pricing model at the end of each reporting period. The compensation cost for the Plan consists of the 15% of the grant date stock price discount and the fair value of the option features. The Company recorded compensation cost related to 2006 ESPP Plan of $160, $96 and $92 for the years ended December 31, 2015, 2014 and 2013, respectively. As of December 31, 2015, there were 1,706 shares available for future issuance under the 2006 ESPP Plan. The assumptions used to value options granted under the 2006 ESPP Plan are as follows: For the Years Ended December 31, 2015 2014 2013 Expected term of option 6 months 6 months 6 months Expected volatility 57% – 88% 67% – 130% 47% – 141% Risk free interest rate 0.07 – 0.08% 0.06 – 0.08% 0.11 – 0.14% Expected dividend yield 0% 0% 0%

Other Income

Other Income	12 Months Ended
	Dec. 31, 2015
Other Income	13. Other Income In August 2015, a stockholder of the Company paid $8,944 to the Company relating to the disgorgement of short swing profits under Section 16(b) of the Securities Exchange Act.

Other License and Research and

Other License and Research and Development Agreements	12 Months Ended
	Dec. 31, 2015
Other License and Research and Development Agreements	14. Other License and Research and Development Agreements The Company has entered into certain non-exclusive HCV license and collaborative research agreements with third parties relating to the Company’s drug discovery and development initiatives. Under these agreements, the Company has been granted certain worldwide non-exclusive licenses to use the licensed compounds or technologies. Included in the accompanying 2015, 2014 and 2013 statements of comprehensive loss is $115, $140 and $140, respectively, of research and development expense resulting from these arrangements. In order to maintain its rights under these agreements, provided that the Company does not terminate such agreements, the Company will also be required to pay an additional $100 of aggregate minimum payments over the next five years. In February 2000, the Company entered into a license agreement with Vion Pharmaceuticals, (“Vion”), pursuant to which it obtained a worldwide exclusive sublicense from Vion on the composition of matter and use of elvucitabine. Vion’s license rights were granted to it by Yale University, (“Yale”). Upon the dissolution of Vion in a 2011 bankruptcy, the Company became a direct licensee of Yale. This license covers the use of elvucitabine alone, as a pharmaceutical composition containing elvucitabine alone, or its use as monotherapy to treat HIV. Yale has retained rights to utilize the intellectual property licensed by this agreement for its own noncommercial purposes. Through December 31, 2015, the Company has made aggregate payments of $35 to Yale under this agreement, including a $10 initial license fee and a $25 development milestone payment. Under the terms of the agreement, the Company may be required to make additional milestone payments to Yale of up to an aggregate of $850 for each licensed product based on the achievement of specified development and regulatory approval milestones. The Company is also required to pay Yale specified royalties on net product sales and a specified share of sublicensing fees that it receives under any sublicenses that it grants. No other payments are included in the Company’s financial statements as these payments are contingent on the achievement of certain milestones that have not yet been reached. In July 2002, the Company entered into a license agreement with Emory University (“Emory”), pursuant to which it obtained a worldwide exclusive license under specified licensed patents to use elvucitabine in combination with other antivirals. Under the license, Emory retains a right to use the intellectual property for educational and research purposes only and also retains the right to approve sublicenses under specified circumstances. Through December 31, 2015, the Company has made aggregate payments of $150 to Emory under this agreement, including an initial license fee of $100 and a development milestone payment of $50. The Company may also be required to make additional payments of up to an aggregate of $400 based on the achievement of specified development and regulatory approval milestones. Under this agreement, the Company is also required to pay Emory specified royalties on net product sales and a specified share of sublicensing fees that it receives under any sublicenses that it grants. As these payments are contingent on the achievement of certain milestones that have not yet been reached, the related amounts are not recognized as expense in the accompanying financial statements.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2015
Commitments and Contingencies	15. Commitments and Contingencies 401(k) Retirement Plan The Company has a 401(k) defined contribution retirement plan covering substantially all full-time employees. The Company currently matches employee contributions at a rate of $0.50 cents for each dollar contribution, up to 6% of salary deferrals. However, the decision to match any employee contributions is at the sole discretion of the Company. The Company made matching contributions of $296, $262 and $238 for the years ended December 31, 2015, 2014 and 2013, respectively. Operating Leases The Company leases its operating facility located in New Haven, Connecticut. The lease agreement requires monthly lease payments through March 2020. The Company is recording the expense associated with the lease on a straight-line basis over the expected term of the lease and, as a result, has accrued $72 and $83 at December 31, 2015 and 2014, respectively. The future minimum annual lease payments under this operating lease at December 31, 2015 are as follows: Year Ended December 31, 2016 $ 770 2017 $ 814 2018 $ 814 2019 $ 814 2020 $ 203 Total $ 3,415 Rent expense under operating leases was $629, $617, and $617, for the years ended December 31, 2015, 2014 and 2013, respectively. From time to time, in the ordinary course of business, the Company is subject to litigation and regulatory examinations as well as information gathering requests, inquiries and investigations. As of December 31, 2015, there are no active matters. On May 5, 2014, without any settlement payment by the Company, any individual defendant or any third party on their behalf, the lead plaintiffs in the previously disclosed consolidated class action lawsuit originally filed in October 2013 against the Company and certain of its current and former officers in the United States District Court for the District of Connecticut voluntarily dismissed all of their claims without prejudice. The Court approved the voluntary dismissal and closed the case on May 6, 2014. A dismissal without prejudice does not prevent the litigation of the same claims in a subsequent action.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2015
Income Taxes	16. Income Taxes The Company uses an asset and liability approach for financial accounting and reporting of income taxes. Deferred tax assets and liabilities are determined based on temporary differences between financial reporting and tax basis of assets and liabilities and are measured by applying enacted rates and laws to taxable years in which differences are expected to be recovered or settled. Further, the effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that the rate changes. The Company applies the provisions of ASC 740, Income Taxes, The Company does not have any interest or penalties accrued related to uncertain tax positions as it does not have any unrecognized tax benefits. In the event the Company determines that accrual of interest or penalties is necessary in the future, the amount will be presented as a component of income taxes. The income tax provision (benefit) consists of the following: As of December 31, 2015 2014 2013 Deferred: Federal and state $ 699 $ 29,287 $ 28,554 Valuation allowance (699 ) (29,287 ) (28,554 ) Total deferred $ — $ — $ — A reconciliation of the statutory tax rates to the effective tax rates is as follows: Years Ended December 31, 2015 2014 2013 Federal statutory rate (34.0 )% (34.0 )% (34.0 )% State tax, net of federal benefit (5.0 ) (5.0 ) (5.0 ) Other 0.52 0.07 0.07 Share-based compensation 2.84 (0.56 ) 2.90 Valuation allowance 35.64 39.49 36.03 0 % 0 % 0 % Future tax benefits (deferred tax assets) related to temporary differences are as follows: As of December 31, 2015 2014 Gross deferred tax assets: Net operating losses $ 151,076 $ 154,019 Tax credits (federal and state) 12,977 11,419 Share-based compensation 7,643 5,378 Other 419 601 $ 172,115 $ 171,417 Less—valuation allowance (172,115 ) (171,417 ) Net deferred tax asset $ — $ — At December 31, 2015 and 2014, the Company had gross deferred income tax assets of approximately $172,115 and $171,417, respectively, which result primarily from net operating loss and tax credit carryforwards. ASC 740 requires that a valuation allowance be established when it is “more likely than not” that all or a portion of deferred tax assets will not be realized. A review of all positive and negative evidence is required when measuring the need for a valuation allowance. The Company’s cumulative loss from inception represents sufficient negative evidence to require a valuation allowance. The Company concluded that it is appropriate to maintain a full valuation allowance for its net deferred tax assets. Additionally, the Company intends to maintain a valuation allowance until sufficient positive evidence exists to support its reversal. At December 31, 2015 and 2014, the Company had available the following net operating loss and credit carryforwards: As of December 31, 2015 2014 Federal net operating loss carryforwards $ 351,977 $ 360,598 State net operating loss carryforwards 418,720 418,879 Federal research and development credit carryforwards 8,337 7,007 State research and development credit carryforwards 4,640 4,412 The Company’s federal net operating loss carryforwards expire commencing in 2018 through 2035 and state net operating loss carryforwards which expire commencing in 2020 through 2035. The Company’s federal research and development credit carryforwards expire commencing in 2028 through 2034. The Connecticut research and development carryforwards have no expiration period. Deferred tax assets relating to tax benefits of employee stock options have been reduced to reflect exercises. Some exercises resulted in tax deductions in excess of previously recorded benefits based on the option value at the time of grant (“windfalls”). Although these windfalls are reflected in net operating loss carryforwards, the additional tax benefit associated with the windfall is not recognized until the deduction reduces taxes payable. Accordingly, approximately $14,345 of the net operating loss carryforwards available, if realized, would be credited to additional paid-in capital. Utilization of the net operating losses and research and development credit carryforwards may be subject to a substantial annual limitation under Section 382 of the Internal Revenue Code of 1986, or Section 382, due to changes in ownership of the Company that have occurred previously or that could occur in the future. These ownership changes may limit the amount of net operating losses and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax. In general, an ownership change, as defined by Section 382, results from transactions increasing the ownership of certain shareholders or public groups in the stock of a corporation by more than 50 percentage points over a three-year period. The Company completed a review of its changes in ownership through December 31, 2011, and determined that it had three ownership changes since inception. The changes of ownership will result in approximately $55,429 of net operating loss carryforwards that the Company expects to expire unutilized and approximately $4,066 of research and development credit carryforwards that the Company expects to expire unutilized. The Company had historically recorded a valuation allowance against the net operating losses and research and development carryforwards. This resulted in no change to the income statement, with a change to footnote disclosure only. The Company will continue to update its analysis of ownership changes and the potential limitations on its deferred tax assets. The federal and state tax authorities could challenge tax positions taken by the Company for the periods for which there are open tax years. Years subject to audit are years in which unused net operating losses were generated that remain open by the statute of limitations. The Company is open to challenge for the periods of 2004 through 2015 in federal and the State of Connecticut jurisdictions. The Company did not have any unrecognized tax benefits as of December 31, 2014 and 2015. The State of Connecticut provides companies with the opportunity to exchange certain research and development credit carryforwards for cash in exchange for foregoing the carryforward of the research and development credit. The program provides for such exchange of the research and development credits at a rate of 65% of the annual research and development credit, as defined. During the years ended December 31, 2015, 2014 and 2013, the Company recorded a benefit of approximately $640, $678 and $183, respectively, for the estimated proceeds from this exchange. This benefit is recorded as a reduction of research and development expenditures.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2015
Related Party Transactions	17. Related Party Transactions Nicole Vitullo In connection with Domain Associates, LLC’s (“Domain”) agreement to invest in the Company, the board of directors of the Company elected Nicole Vitullo of Domain as a Class II member of the board of directors on September 30, 2010 to serve until her successor is duly elected and qualified. Ms. Vitullo is a partner at Domain. In August 2010, Domain purchased 8,032 shares of common stock and warrants to purchase 2,811 shares of common stock for an aggregate purchase price of $20.4 million. As of December 31, 2015, Domain was the beneficial owner of approximately 4% of the Company’s total issued and outstanding shares of common stock.

Unaudited Quarterly Results

Unaudited Quarterly Results	12 Months Ended
	Dec. 31, 2015
Unaudited Quarterly Results	18. Unaudited Quarterly Results The following tables summarize unaudited quarterly financial data for the years ended December 31, 2015 and 2014. This data has been derived from unaudited financial statements that, in the Company’s opinion, include all adjustments necessary for a fair statement of such information. The operating results for any quarter are not necessarily indicative of results for any future period. 2015 Quarters First Second Third (1) Fourth (1) Total operating revenue $ — $ 711 $ 33,820 $ 31,591 Total operating expenses 19,399 29,899 16,839 15,092 Net income (loss) (19,263 ) (28,978 ) 26,259 16,952 Net loss per share—basic and diluted $ (0.17 ) $ (0.25 ) $ 0.19 $ 0.12 Weighted average number of shares outstanding—basic 111,202 117,770 136,438 136,558 Weighted average number of shares outstanding—diluted 111,202 117,770 140,024 140,475 2014 Quarters First Second Third Fourth Total operating revenue $ — $ — $ — $ — Total operating expenses 16,235 15,766 15,764 21,661 Net loss (16,088 ) (15,657 ) (15,667 ) (21,596 ) Net loss per share—basic and diluted $ (0.17 ) $ (0.16 ) $ (0.16 ) $ (0.21 ) Weighted average number of shares outstanding—basic 96,792 97,017 99,031 100,579 Weighted average number of shares outstanding—diluted 96,792 97,017 99,031 100,579 (1) In May 2015, the Company entered into an exclusive collaboration and license agreement with Janssen, and its affiliate, Johnson & Johnson Innovation-JJDC, Inc., or JJDC, which we refer to as the Janssen Agreement for the further clinical advancement of a portfolio of antivirals the Company discovered and developed for the treatment of HCV. In addition, upon the closing of the transactions contemplated by the Janssen Agreement, the Company entered into a stock purchase agreement with JJDC. Pursuant to the JJDC stock purchase agreement, on July 1, 2015, the Company issued 18,367 shares of common stock to JJDC at a price of $12.25 per share for an aggregate purchase price of $225,000. The Company recorded revenue of $66,122 during the year ended December 31, 2015 associated with this transaction. Also refer to footnote 5.

Summary of Significant Accoun25

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2015
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the United States (“GAAP”) requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Revenue Recognition	Revenue Recognition The Company recognizes revenue from contract research and development and research progress payments in accordance with Accounting Standards Codification (“ASC”) 605, Revenue Recognition When the Company determines that an arrangement should be accounted for as a single unit of accounting, it must determine the period over which the performance obligations will be performed and revenue related to upfront license payments will be recognized. Revenue will be recognized using either a proportionate performance or straight-line method. The Company recognizes revenue using the proportionate performance method provided that it can reasonably estimate the level of effort required to complete its performance obligations under an arrangement and such performance obligations are provided on a best-efforts basis. Under the proportionate performance method, periodic revenue related to up-front license payments is recognized as the percentage of actual effort expended in that period to total effort expected for all of its performance obligations under the arrangement. Actual effort is generally determined based upon actual direct labor hours or full-time equivalents (“FTE”) incurred and include research and development activities performed by internal scientists. Total expected effort is generally based upon the total direct labor hours of FTEs incorporated into the detailed budget and project plan that is agreed to by both parties to the collaboration. Significant management judgment is required in determining the level of effort required under an arrangement and the period over which the Company expects to complete the related performance obligations. In the event that a change in estimate occurs, the change will be accounted for using the cumulative catch-up method which provides for an adjustment to revenue in the current period. Estimates of the Company’s level of effort may change in the future, resulting in a material change in the amount of revenue recognized in future periods, including negative revenue in some periods. Generally under collaboration arrangements, payments received during the period of performance may include up-front payments, time-or performance-based milestones and reimbursement of internal and external costs. The proportion of actual performance to total expected performance is applied to these payments in determining periodic revenue, but will be limited by the aggregate cash received or receivable to date. Substantive milestone payments are recognized upon achievement of the milestone. Determining whether a milestone is substantive requires judgment that should be made at the inception of the arrangement. To meet the definition of a substantive milestone, the consideration earned by achieving the milestone (1) would have to be commensurate with either the level of effort required to achieve the milestone or the enhancement in the value of the item delivered, (2) would have to relate solely to past performance, and (3) should be reasonable relative to all deliverables and payment terms in the arrangement. No bifurcation of an individual milestone is allowed and there can be more than one milestone in an arrangement. In May 2015, the Company entered into parallel transactions with Janssen and its affiliate, Johnson & Johnson Innovation-JJDC, Inc. (“JJDC”), consisting of (i) an exclusive collaboration and license agreement with Janssen (the “Janssen Agreement”) pursuant to which, upon the closing of the transactions contemplated by the Janssen Agreement on June 29, 2015, the Company granted Janssen exclusive worldwide rights to develop and commercialize products that contain one or more of the Company’s drug candidates for the treatment of chronic hepatitis C virus (“HCV”) infection, namely odalasvir, a second-generation NS5A inhibitor, ACH-3422, a NS5B HCV polymerase inhibitor, and sovaprevir, a NS3/4A HCV protease inhibitor, and (ii) a stock purchase agreement with JJDC (the “Stock Purchase Agreement”) pursuant to which, upon the closing of the transactions contemplated by the Stock Purchase Agreement, JJDC purchased 18,367 shares of the Company’s common stock (the “Shares”) at a price of $12.25 per share, for an aggregate purchase price of $225,000. The Janssen Agreement became effective on June 29, 2015 upon the early termination of applicable waiting periods under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (the “HSR Act”). The Stock Purchase Agreement became effective July 1, 2015. In connection with the closing of the transactions contemplated by the Stock Purchase Agreement, the Company and JJDC entered into an Investor Agreement (the “Investor Agreement”) on July 1, 2015. Pursuant to the terms of the Janssen Agreement, the Company was required to provide technology transfer services related to the chemistry, manufacturing and know-how to Janssen for up to 180 days after the effective date of the agreement. In accordance with ASC 605-25, which provides guidance on accounting for multiple-element arrangements, including the determination of the units of accounting and allocation of total arrangement consideration, the Company identified all of the obligations at the inception of the Janssen Agreement. The significant obligations were determined to be the license and the technology transfer services. The Company determined that license and technology transfer services represent a single unit of accounting because they were not viewed to have standalone value. The Janssen Agreement entered into by the Company and Janssen, the Stock Purchase Agreement, and the Investor Agreement were entered into by the Company and Janssen’s affiliate in contemplation of each other. The only upfront amount received by the Company in exchange for the license and technology transfer services and the issuance of the Shares was the $225,000. The Company determined that the amount received in excess of the fair value of the Shares upon issuance of $66,122 was attributed to the license and technology services. The Company also determined that there was no discernable pattern in which the technology services would be provided during the 180 day period after the effective date. In accordance with ASC 605-10, the Company determined that straight-line attribution of the license and technology services revenues would be used to recognize revenue. As such, revenue of $66,122 was recorded during the year ended December 31, 2015 associated with this transaction. The development, regulatory and sales milestones represent non-refundable amounts that would be paid by Janssen to the Company if certain milestones are achieved in the future. The Company has elected to apply the guidance in ASC 605-28 to the milestones. These milestones, if achieved, are substantive as they relate solely to past performance and are commensurate with estimated enhancement of value associated with the achievement of each milestone as a result of the Company’s performance; however, there can be no assurance that Janssen will achieve the milestones or that the Company will receive the related revenue.
Stock-Based Compensation-Employee Stock-Based Awards	Stock-Based Compensation—Employee Stock-Based Awards The Company applies the provisions of ASC 718, Stock Compensation, The Company primarily grants qualified stock options for a fixed number of shares to employees with an exercise price equal to the market value of the shares at the date of grant. To the extent that the amount of the aggregate fair market value of qualified stock options that become exercisable for an individual exceeds $100 during any tax year, those stock options are treated as non-qualified stock options. Under the fair value recognition provisions, stock-based compensation cost is based on the fair value of the portion of stock-based awards that is ultimately expected to vest. The Company utilizes the Black-Scholes option pricing model for determining the estimated fair value for stock-based awards. The Black-Scholes model requires the use of assumptions which determine the fair value of the stock-based awards. Determining the fair value of stock-based awards at the grant date requires judgment, including estimating the expected term of stock options, the expected volatility of our stock and expected dividends. For the years ended December 31, 2015 and 2014, the Company based its estimate of the expected term on historical data for similar stock option grants. The Company utilized the simplified method in developing an estimate of the expected term of “plain vanilla” share options for the year ended December 31, 2013. This method was considered appropriate given the Company’s limited exercise history. For the years ended December 31, 2015, 2014 and 2013, the Company calculated volatility based on actual volatility for the expected term of the option. The Company estimates forfeitures at the grant date and recognizes compensation costs for only those awards that are expected to vest.
Accrued Expenses	Accrued Expenses As part of the process of preparing financial statements, the Company is required to estimate accrued expenses. This process involves identifying services which have been performed on its behalf and estimating the level of service performed and the associated cost incurred for such service as of each balance sheet date in its financial statements. In accruing service fees, the Company estimates the time period over which services will be provided and the level of effort in each period. If the actual timing of the provision of services or the level of effort varies from the estimate, the Company will adjust the accrual accordingly. The majority of service providers invoice the Company monthly in arrears for services performed. Some service providers require upfront or milestone payments. If the estimate of services performed is less than the upfront or milestone payments, the difference is accounted for as a prepaid expense. In the event that the Company does not identify costs that have begun to be incurred or the Company underestimates or overestimates the level of services performed or the costs of such services, actual expenses could differ from such estimates. The date on which some services commence, the level of services performed on or before a given date and the cost of such services are often subjective determinations. The Company makes judgments based upon facts and circumstances known to it in accordance with GAAP.
Cash, Cash Equivalents and Restricted Cash	Cash, Cash Equivalents and Restricted Cash Cash and cash equivalents are stated at cost, which approximates fair value, and include short-term, highly-liquid investments with original maturities of less than three months. The Company also holds certificates of deposit, which collateralize the Company’s facility lease which are classified as restricted cash in the accompanying balance sheets. The restricted cash will be released from restriction in 2020. At December 31, 2015 and 2014, the Company had $81,725 and $73,664, respectively, of cash and cash equivalents.
Marketable Securities and Equity Investments	Marketable Securities and Equity Investments The Company applies the provisions of ASC 820, Fair Value Measurements and Disclosures, Level 1: Quoted prices in active markets for identical assets and liabilities that the reporting entity has the ability to access at the measurement date; Level 2: Inputs other than quoted prices in active markets, that are observable either directly or indirectly, such as quoted prices for similar assets or liabilities, quoted in markets that are not active, or other inputs that are observable; or Level 3: Unobservable inputs. The fair value of the Company’s marketable securities of $377,616 and $79,215 as of December 31, 2015 and 2014, respectively, was valued based on level 2 inputs. The Company’s investments consist mainly of corporate debt securities and government sponsored bond obligations. Fair value is determined by taking into consideration valuations obtained from third-party pricing services. The third-party pricing services utilize industry standard valuation models, 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; issuer credit spreads; benchmark securities; and other observable inputs. The Company has assessed these as level 2 within the fair value hierarchy of ASC 820. The Company classifies its entire investment portfolio as available for sale as defined in ASC 320, “ Debt and Equity Securities
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company’s financial instruments, including cash, cash equivalents, accounts receivable, and accounts payable are carried at cost, which approximates their fair value because of the short-term maturity of these instruments. The Company believes that the carrying value of its debt balance outstanding approximates fair value. Fair value is determined using a discounted cash flow model based on current interest rates.
Concentration of Risk	Concentration of Risk Concentration of credit risk exists with respect to cash and cash equivalents and investments. The Company maintains its cash and cash equivalents and investments with high quality financial institutions. At times, amounts may exceed federally insured deposit limits. For the years ended December 31, 2015, 2014, and 2013, 100%, 0% and 0%, respectively, of the Company’s revenue was generated from the Janssen Agreement. At December 31, 2015, 79% of the Company’s accounts receivable was from one collaboration partner.
Fixed Assets	Fixed Assets Property and equipment are recorded at cost and are depreciated and amortized over the shorter of their remaining lease term or their estimated useful lives on a straight-line basis as follows: Laboratory equipment 4-7 years Office equipment 3-5 years Leasehold improvements Lesser of life of Expenditures for maintenance and repairs, which do not improve or extend the useful lives of the respective assets, are expensed as incurred. When assets are sold or retired, the related cost and accumulated depreciation are removed from their respective accounts and any resulting gain or loss is included in income (loss) from operations.
Long-lived Asset	Long-lived Assets ASC 360, Property, Plant and Equipment,
Research and Development Expenses	Research and Development Expenses All costs associated with internal research and development, research and development services for which the Company has externally contracted and licensed technology are expensed as incurred. Research and development expense includes direct and indirect costs for salaries, employee benefits, subcontractors, including clinical research organizations (“CROs”), operating supplies, facility-related expenses and depreciation.
Patent Costs	Patent Costs The Company expenses the costs of obtaining and maintaining patents.
Income Taxes	Income Taxes The Company uses an asset and liability approach for financial accounting and reporting of income taxes. Deferred tax assets and liabilities are determined based on temporary differences between financial reporting and tax basis of assets and liabilities and are measured by applying enacted rates and laws to taxable years in which differences are expected to be recovered or settled. Further, the effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that the rate change is enacted. A valuation allowance is required when it is “more likely than not” that all or a portion of deferred tax assets will not be realized. The Company applies the provisions of ASC 740, Income Taxe The Company did not have any unrecognized tax benefits as of December 31, 2015. The Company reviews all tax positions to ensure the tax treatment selected is sustainable based on its technical merits and that the position would be sustained if challenged.
Segment Information	Segment Information The Company is engaged solely in the discovery and development of innovative small molecule drug therapies. Accordingly, the Company has determined that it operates in one operating segment.
Accounting Standards Updates	Accounting Standards Updates In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) No. 2014-09, “Revenue from Contracts with Customers (Topic 606),” which supersedes all existing revenue recognition requirements, including most industry-specific guidance. ASU No. 2014-09 requires a company to recognize revenue when it transfers goods or services to customers in an amount that reflects the consideration that the company expects to receive for those goods or services. In August 2015, the FASB issued ASU No. 2015-14, “Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date,” which delays the effective date of ASU No. 2014-09 by one year. The new standard is effective for reporting periods beginning after December 15, 2017. The Company is currently evaluating the impact ASU No. 2014-09 will have on its financial position and results of operations. In August 2014, FASB issued ASU No. 2014-15, “Presentation of Financial Statements – Going Concern.” ASU No. 2014-15 provides guidance regarding management’s responsibility to evaluate whether there exists substantial doubt about an organization’s ability to continue as a going concern and to provide related footnote disclosures in certain circumstances. ASU No. 2014-15 is effective for annual reporting periods beginning after December 15, 2016, and interim periods thereafter. The Company does not believe ASU No. 2014-15 will have a material effect on its financial position and results of operations or disclosures. In April 2015, FASB issued ASU No. 2015-03, “Interest – Imputation of Interest.” ASU No. 2015-03 requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. ASU 2015-03 is effective for financial statements issued for fiscal years beginning after December 15, 2015 (and interim periods within those fiscal years) with early adoption permitted and retrospective application required. The Company does not believe ASU No. 2015-03 will have a material effect on its financial position and results of operations.

Summary of Significant Accoun26

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2015
Property and Equipment, Net Estimated Useful Lives	Property and equipment are recorded at cost and are depreciated and amortized over the shorter of their remaining lease term or their estimated useful lives on a straight-line basis as follows: Laboratory equipment 4-7 years Office equipment 3-5 years Leasehold improvements Lesser of life of

Earnings (Loss) Per Share (Tabl

Earnings (Loss) Per Share (Tables)	12 Months Ended
	Dec. 31, 2015
Calculations of Basic and Diluted Net Loss per Share	The calculations of basic and diluted net loss per share are as follows: Years Ended December 31, 2015 2014 2013 (in thousands) Net loss (numerator) $ (5,030 ) $ (69,008 ) $ (58,947 ) Weighted-average shares, in thousands (denominator) 125,592 98,367 93,983 Basic and diluted net loss per share $ (0.04 ) $ (0.70 ) $ (0.63 )
Potentially Dilutive Securities Outstanding	Potentially dilutive securities outstanding as of December 31, 2015, 2014 and 2013 are as follows: Years Ended December 31, 2015 2014 2013 (in thousands) Stock Options 8,501 9,493 9,083 Warrants 2,833 2,844 5,338

Marketable Securities (Tables)

Marketable Securities (Tables)	12 Months Ended
	Dec. 31, 2015
Summary of the Investments	The following table summarizes the Company’s investments: December 31, 2015 December 31, 2014 Amortized Unrealized Unrealized Estimated Amortized Unrealized Unrealized Estimated Commercial Paper $ 137,483 $ 158 (6 ) $ 137,635 $ 4,747 $ 3 — $ 4,750 Corporate Debt Securities 164,884 — (205 ) 164,679 58,452 1 (16 ) 58,437 Government and Agency Securities 75,294 16 (8 ) 75,302 16,028 1 (1 ) 16,028 Total $ 377,661 $ 174 (219 ) $ 377,616 $ 79,227 $ 5 (17 ) $ 79,215
Fair Value of Investments	The following additional table summarizes, by industry, the fair value of investments: As of December 31, 2015 2014 Government 75,301 16,028 Banking 93,040 18,925 Industrial 209,275 44,262 Total $ 377,616 $ 79,215

Prepaid Expenses and Other Cu29

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Dec. 31, 2015
Summary of Prepaid Expenses and Other Current Assets	A summary of prepaid expenses and other current assets is as follows: As of December 31, 2015 2014 Prepaid research and development costs $ 487 $ 221 Tax credit receivable 640 678 Maintenance agreements 269 387 Interest receivable 1,199 551 Other prepaid expenses 146 64 Total $ 2,741 $ 1,901

Fixed Assets, net (Tables)

Fixed Assets, net (Tables)	12 Months Ended
	Dec. 31, 2015
Summary of Property and Equipment	A summary of property and equipment is as follows: As of December 31, 2015 2014 Laboratory equipment $ 3,642 $ 3,461 Office equipment 1,302 989 Leasehold improvements 3,083 3,017 Construction in process 6 — 8,033 7,467 Less—accumulated depreciation and amortization (6,298 ) (5,741 ) Total $ 1,735 $ 1,726

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2015
Summary of Accrued Expenses	Accrued expenses consist of the following: As of December 31, 2015 2014 Accrued compensation $ 4,326 $ 846 Accrued research and development expenses 4,743 4,727 Accrued professional expenses 846 649 Other accrued expenses 387 224 Total $ 10,302 $ 6,446

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2015
Schedule of Debt	Debt consists of the following: As of December 31, 2015 2014 2011 Credit Facility, payable in equal monthly installments through March 2015, with fixed interest of 6.44% per annum $ — $ 56 2014 Credit Facility, payable in equal monthly installments through October 2017, with fixed interest of 6.30% per annum 279 418 2014 Credit Facility, payable in equal monthly installments through March 2018, with fixed interest of 6.20% per annum 175 — Total long-term debt 454 474 Less: current portion (223 ) (195 ) Total long-term debt, net of current portion $ 231 $ 279

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2015
Summary of Stock Option Activity	A summary of the status of the Company’s stock option activity for the year ended December 31, 2015 is presented in the table and narrative below: 2015 Options Weighted Outstanding at January 1, 2015 9,493 $ 6.88 Granted 213 9.14 Exercised (835 ) 4.47 Forfeited (297 ) 7.93 Cancelled (73 ) 12.60 Outstanding at December 31, 2015 8,501 $ 7.08 Options exercisable at December 31, 2015 5,642 $ 6.39 Options vested and expected to vest at December 31, 2015 8,083 $ 7.05
Summary of Stock Options Outstanding	The following table summarizes information about stock options outstanding at December 31, 2015: Options Outstanding Options Vested Range of Exercise Prices Number Weighted Average Weighted Number Weighted $0.00 – $2.00 137 3.0 $ 1.05 137 $ 1.05 $2.01 – $4.00 3,279 6.6 3.07 2,288 3.08 $4.01 – $6.00 367 2.2 5.16 364 5.15 $6.01 – $8.00 1,754 6.8 7.45 1,347 7.42 $8.01 – $10.00 1,033 7.3 8.73 742 8.68 $10.01 – $12.00 125 8.6 10.51 41 10.70 $12.01 – $14.00 1,502 8.9 13.44 419 13.43 $14.01 – $16.00 301 1.0 14.75 301 14.75 $16.01 – $20.00 3 1.1 19.00 3 19.00 8,501 6.7 $ 7.08 5,642 $ 6.39
Assumptions Used to Value Options Granted	The assumptions used to value options granted are as follows: For the Years Ended December 31, 2015 2014 2013 Expected term of option 6.25 years 5.0 – 6.25 years 5.0 – 6.1 years Expected volatility 84% – 91% 92% – 96% 87% – 94% Risk free interest rate 1.54 – 1.93% 1.81 – 2.02% 1.01 – 2.10% Expected dividend yield 0% 0% 0%
Compensation Expense Related to Option Grants Made to Employees and Consultants	Compensation expense related to option grants made to employees and consultants is included in research and development and general and administrative expense as follows: For the Years Ended December 31, 2015 2014 2013 Research and development $ 4,346 $ 2,713 $ 2,146 General and administrative 5,567 4,464 3,682 Total $ 9,913 $ 7,177 $ 5,828
Assumptions Used to Value Options Granted Under 2006 ESPP Plan	The assumptions used to value options granted under the 2006 ESPP Plan are as follows: For the Years Ended December 31, 2015 2014 2013 Expected term of option 6 months 6 months 6 months Expected volatility 57% – 88% 67% – 130% 47% – 141% Risk free interest rate 0.07 – 0.08% 0.06 – 0.08% 0.11 – 0.14% Expected dividend yield 0% 0% 0%

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2015
Future Minimum Annual Lease Payments	The future minimum annual lease payments under this operating lease at December 31, 2015 are as follows: Year Ended December 31, 2016 $ 770 2017 $ 814 2018 $ 814 2019 $ 814 2020 $ 203 Total $ 3,415

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2015
Income Tax Provision (Benefit)	The income tax provision (benefit) consists of the following: As of December 31, 2015 2014 2013 Deferred: Federal and state $ 699 $ 29,287 $ 28,554 Valuation allowance (699 ) (29,287 ) (28,554 ) Total deferred $ — $ — $ —
Reconciliation of Provision for Income Taxes at Statutory Rates	A reconciliation of the statutory tax rates to the effective tax rates is as follows: Years Ended December 31, 2015 2014 2013 Federal statutory rate (34.0 )% (34.0 )% (34.0 )% State tax, net of federal benefit (5.0 ) (5.0 ) (5.0 ) Other 0.52 0.07 0.07 Share-based compensation 2.84 (0.56 ) 2.90 Valuation allowance 35.64 39.49 36.03 0 % 0 % 0 %
Future Tax Benefits (Deferred Tax Assets)	Future tax benefits (deferred tax assets) related to temporary differences are as follows: As of December 31, 2015 2014 Gross deferred tax assets: Net operating losses $ 151,076 $ 154,019 Tax credits (federal and state) 12,977 11,419 Share-based compensation 7,643 5,378 Other 419 601 $ 172,115 $ 171,417 Less—valuation allowance (172,115 ) (171,417 ) Net deferred tax asset $ — $ —
Net Operating Loss and Credit Carryforwards	At December 31, 2015 and 2014, the Company had available the following net operating loss and credit carryforwards: As of December 31, 2015 2014 Federal net operating loss carryforwards $ 351,977 $ 360,598 State net operating loss carryforwards 418,720 418,879 Federal research and development credit carryforwards 8,337 7,007 State research and development credit carryforwards 4,640 4,412

Unaudited Quarterly Results (Ta

Unaudited Quarterly Results (Tables)	12 Months Ended
	Dec. 31, 2015
Quarterly Financial Data	The following tables summarize unaudited quarterly financial data for the years ended December 31, 2015 and 2014. This data has been derived from unaudited financial statements that, in the Company’s opinion, include all adjustments necessary for a fair statement of such information. The operating results for any quarter are not necessarily indicative of results for any future period. 2015 Quarters First Second Third (1) Fourth (1) Total operating revenue $ — $ 711 $ 33,820 $ 31,591 Total operating expenses 19,399 29,899 16,839 15,092 Net income (loss) (19,263 ) (28,978 ) 26,259 16,952 Net loss per share—basic and diluted $ (0.17 ) $ (0.25 ) $ 0.19 $ 0.12 Weighted average number of shares outstanding—basic 111,202 117,770 136,438 136,558 Weighted average number of shares outstanding—diluted 111,202 117,770 140,024 140,475 2014 Quarters First Second Third Fourth Total operating revenue $ — $ — $ — $ — Total operating expenses 16,235 15,766 15,764 21,661 Net loss (16,088 ) (15,657 ) (15,667 ) (21,596 ) Net loss per share—basic and diluted $ (0.17 ) $ (0.16 ) $ (0.16 ) $ (0.21 ) Weighted average number of shares outstanding—basic 96,792 97,017 99,031 100,579 Weighted average number of shares outstanding—diluted 96,792 97,017 99,031 100,579 (1) In May 2015, the Company entered into an exclusive collaboration and license agreement with Janssen, and its affiliate, Johnson & Johnson Innovation-JJDC, Inc., or JJDC, which we refer to as the Janssen Agreement for the further clinical advancement of a portfolio of antivirals the Company discovered and developed for the treatment of HCV. In addition, upon the closing of the transactions contemplated by the Janssen Agreement, the Company entered into a stock purchase agreement with JJDC. Pursuant to the JJDC stock purchase agreement, on July 1, 2015, the Company issued 18,367 shares of common stock to JJDC at a price of $12.25 per share for an aggregate purchase price of $225,000. The Company recorded revenue of $66,122 during the year ended December 31, 2015 associated with this transaction. Also refer to footnote 5.

Nature of Business - Additional

Nature of Business - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended										12 Months Ended			208 Months Ended
	Dec. 31, 2015		Sep. 30, 2015	[1]	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015
Nature Of Business [Line Items]
Losses incurred from inception	$ 16,952	[1]	$ 26,259		$ (28,978)	$ (19,263)	$ (21,596)	$ (15,667)	$ (15,657)	$ (16,088)	$ (5,030)	$ (69,008)	$ (58,947)	$ (441,850)
Accumulated deficit	$ (455,712)						$ (450,682)				$ (455,712)	$ (450,682)		$ (455,712)
[1]	In May 2015, the Company entered into an exclusive collaboration and license agreement with Janssen, and its affiliate, Johnson & Johnson Innovation-JJDC, Inc., or JJDC, which we refer to as the Janssen Agreement for the further clinical advancement of a portfolio of antivirals the Company discovered and developed for the treatment of HCV. In addition, upon the closing of the transactions contemplated by the Janssen Agreement, the Company entered into a stock purchase agreement with JJDC. Pursuant to the JJDC stock purchase agreement, on July 1, 2015, the Company issued 18,367 shares of common stock to JJDC at a price of $12.25 per share for an aggregate purchase price of $225,000. The Company recorded revenue of $66,122 during the year ended December 31, 2015 associated with this transaction. Also refer to footnote 5.

Summary of Significant Accoun38

Summary of Significant Accounting Policies - Additional Information (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	Jul. 01, 2015USD ($)$ / sharesshares	May. 31, 2015	Dec. 31, 2015USD ($)Segment	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2012USD ($)
Summary Of Significant Accounting Policies [Line Items]
Cash and cash equivalents			$ 81,725	$ 73,664	$ 33,457	$ 18,526
Fair value of marketable securities			$ 377,616	$ 79,215
Number of operating segment | Segment			1
Janssen Agreement | License and technology transfer services
Summary Of Significant Accounting Policies [Line Items]
Period of time pursuant to a collaboration agreement technology transfer services are required to be performed		180 days
The amount of cash received in excess of the fair value of the stock price for services to be performed and recognized as revenue in future periods	$ 66,122
Revenue recognized during the period			$ 66,122
Janssen Agreement | Stock Purchase Agreement
Summary Of Significant Accounting Policies [Line Items]
Issuance of common stock | shares	18,367
Number of stock purchased, price per share | $ / shares	$ 12.25
Issuance of common stock	$ 225,000
Credit Concentration Risk | License Revenue
Summary Of Significant Accounting Policies [Line Items]
Percentage concentration risk			100.00%	0.00%	0.00%
Credit Concentration Risk | Accounts Receivable
Summary Of Significant Accounting Policies [Line Items]
Percentage concentration risk			79.00%
Fair Value, Inputs, Level 2
Summary Of Significant Accounting Policies [Line Items]
Fair value of marketable securities			$ 377,616	$ 79,215
Minimum
Summary Of Significant Accounting Policies [Line Items]
Aggregate fair value of options exercisable			$ 100

Property and Equipment, Net Est

Property and Equipment, Net Estimated Useful Lives (Detail)	12 Months Ended
	Dec. 31, 2015
Laboratory Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property and Equipment useful lives	4 years
Laboratory Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property and Equipment useful lives	7 years
Office Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property and Equipment useful lives	3 years
Office Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property and Equipment useful lives	5 years
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property plant and equipment leasehold improvements useful life	Lesser of life of improvement or lease term

Financing Activities - Addition

Financing Activities - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Dec. 31, 2014	Feb. 28, 2015	Jan. 31, 2015	Feb. 28, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financing Activities [Line Items]
Common stock, par value	$ 0.001				$ 0.001	$ 0.001
Issuance of common stock, subscription receivable	$ 5,737					$ 5,737
Public Offering
Financing Activities [Line Items]
Common stock, par value		$ 0.001		$ 0.001
Share price		$ 10.25		$ 8.40
Number of common shares issued in public or private offering	3,236	13,800		16,894
Proceeds from issuance of common stock		$ 132,558	$ 5,611	$ 133,211	$ 138,260	42,705	$ 133,211
Proceeds from additional capital contribution to entity including amount of subscription receivable from investors	$ 48,316
Issuance of common stock, subscription receivable	$ 5,611					$ 5,611

Calculations of Basic and Dilut

Calculations of Basic and Diluted Net Loss per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended										12 Months Ended			208 Months Ended
	Dec. 31, 2015	[1]	Sep. 30, 2015	[1]	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015
Schedule Of Calculation Of Numerator And Denominator In Earnings Per Share [Line Items]
Net loss (numerator)	$ 16,952		$ 26,259		$ (28,978)	$ (19,263)	$ (21,596)	$ (15,667)	$ (15,657)	$ (16,088)	$ (5,030)	$ (69,008)	$ (58,947)	$ (441,850)
Weighted-average shares, in thousands (denominator)											125,592	98,367	93,983
Basic and diluted net loss per share	$ 0.12		$ 0.19		$ (0.25)	$ (0.17)	$ (0.21)	$ (0.16)	$ (0.16)	$ (0.17)	$ (0.04)	$ (0.70)	$ (0.63)
[1]	In May 2015, the Company entered into an exclusive collaboration and license agreement with Janssen, and its affiliate, Johnson & Johnson Innovation-JJDC, Inc., or JJDC, which we refer to as the Janssen Agreement for the further clinical advancement of a portfolio of antivirals the Company discovered and developed for the treatment of HCV. In addition, upon the closing of the transactions contemplated by the Janssen Agreement, the Company entered into a stock purchase agreement with JJDC. Pursuant to the JJDC stock purchase agreement, on July 1, 2015, the Company issued 18,367 shares of common stock to JJDC at a price of $12.25 per share for an aggregate purchase price of $225,000. The Company recorded revenue of $66,122 during the year ended December 31, 2015 associated with this transaction. Also refer to footnote 5.

Potentially Dilutive Securities

Potentially Dilutive Securities Outstanding (Detail) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding	8,501	9,493	9,083
Warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potentially dilutive securities outstanding	2,833	2,844	5,338

Collaboration Arrangements - Ad

Collaboration Arrangements - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Jul. 01, 2015	May. 31, 2015	Oct. 31, 2012	Feb. 28, 2010	Dec. 31, 2015	Dec. 31, 2012
Ora Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
License revenue						$ 100
Percentage of up front sub license fees			15.00%
Ora Agreement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of royalty receivable on net sales			3.50%
Janssen Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Period of time after the first commercial sale before a customary royalty reduction if there is no exclusivity in place		10 years
Period of time written notice is required to terminate the agreement		60 days
Period of time the company at the request of the purchaser is required to file a registration statement to register all or a portion of the shares issued under a stock purchase agreement		7 years
Janssen Agreement | License and technology transfer services
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Period of time pursuant to a collaboration agreement technology transfer services are required to be performed		180 days
The amount of cash received in excess of the fair value of the stock price for services to be performed and recognized as revenue in future periods	$ 66,122
Revenue recognized during the period					$ 66,122
Janssen Agreement | Clinical Milestone Events | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future milestone revenue		$ 115,000
Janssen Agreement | Regulatory Milestone Events | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future milestone revenue		290,000
Janssen Agreement | Sales Events Milestone | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future milestone revenue		$ 500,000
Janssen Agreement | Stock Purchase Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Issuance of common stock	18,367
Number of stock purchased, price per share	$ 12.25
Issuance of common stock	$ 225,000
GCAT Agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future milestone revenue				$ 100
GCAT Agreement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Percentage of royalty receivable on net sales				15.00%
Development Milestones | Ora Agreement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future milestone revenue			$ 4,000
Commercialization Milestones | Ora Agreement | Maximum
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Future milestone revenue			$ 7,000

Marketable Securities - Additio

Marketable Securities - Additional Information (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Fair value of marketable securities	$ 377,616	$ 79,215
Unrealized (loss) gain on marketable securities	(45)	(12)	$ 6
Fair Value, Inputs, Level 2
Schedule of Available-for-sale Securities [Line Items]
Fair value of marketable securities	$ 377,616	$ 79,215

Summary of Investments (Detail)

Summary of Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 377,661	$ 79,227
Unrealized Gain	174	5
Unrealized (Loss)	(219)	(17)
Estimated Fair Value	377,616	79,215
Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	164,884	58,452
Unrealized Gain		1
Unrealized (Loss)	(205)	(16)
Estimated Fair Value	164,679	58,437
Government and Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	75,294	16,028
Unrealized Gain	16	1
Unrealized (Loss)	(8)	(1)
Estimated Fair Value	75,302	16,028
Commercial Paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	137,483	4,747
Unrealized Gain	158	3
Unrealized (Loss)	(6)
Estimated Fair Value	$ 137,635	$ 4,750

Fair Value of Investments (Deta

Fair Value of Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fair value of investments	$ 377,616	$ 79,215
Government
Schedule of Available-for-sale Securities [Line Items]
Fair value of investments	75,301	16,028
Banking
Schedule of Available-for-sale Securities [Line Items]
Fair value of investments	93,040	18,925
Industrial
Schedule of Available-for-sale Securities [Line Items]
Fair value of investments	$ 209,275	$ 44,262

Summary of Prepaid Expenses and

Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Prepaid Expenses And Other Current Assets [Line Items]
Prepaid research and development costs	$ 487	$ 221
Tax credit receivable	640	678
Maintenance agreements	269	387
Interest receivable	1,199	551
Other prepaid expenses	146	64
Prepaid expenses and other current assets	$ 2,741	$ 1,901

Summary of Property and Equipme

Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 8,033	$ 7,467
Less-accumulated depreciation and amortization	(6,298)	(5,741)
Total	1,735	1,726
Laboratory Equipment
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	3,642	3,461
Office Equipment
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	1,302	989
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	3,083	$ 3,017
Construction in Progress
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 6

Fixed Assets, net - Additional

Fixed Assets, net - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Depreciation expense	$ 693	$ 488	$ 390

Accrued Expenses (Detail)

Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Accrued Liabilities [Line Items]
Accrued compensation	$ 4,326	$ 846
Accrued research and development expenses	4,743	4,727
Accrued professional expenses	846	649
Other accrued expenses	387	224
Total	$ 10,302	$ 6,446

Debt (Detail)

Debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Total long-term debt	$ 454	$ 474
Less: current portion	(223)	(195)
Total long-term debt, net of current portion	231	279
2011 Credit Facility
Line of Credit Facility [Line Items]
Total long-term debt		56
2014 Credit Facility | Credit Facility One
Line of Credit Facility [Line Items]
Total long-term debt	279	$ 418
2014 Credit Facility | Credit Facility Two
Line of Credit Facility [Line Items]
Total long-term debt	$ 175

Debt (Parenthetical) (Detail)

Debt (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
2011 Credit Facility
Line of Credit Facility [Line Items]
Credit Facility, interest rate		6.44%
Debt instrument, maturity year		Mar. 1, 2015
Credit Facility One | 2014 Credit Facility
Line of Credit Facility [Line Items]
Credit Facility, interest rate	6.30%	6.30%
Debt instrument, maturity year	Oct. 1, 2017	Oct. 1, 2017
Credit Facility Two | 2014 Credit Facility
Line of Credit Facility [Line Items]
Credit Facility, interest rate	6.20%
Debt instrument, maturity year	Mar. 1, 2018

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		12 Months Ended
	Mar. 31, 2015	Oct. 31, 2014	Dec. 31, 2015	Mar. 31, 2011
2011 Credit Facility
Line of Credit Facility [Line Items]
Line of Credit Facility, maximum borrowing capacity				$ 2,000
Line of Credit Facility, expiration date			Mar. 31, 2013
2014 Credit Facility
Line of Credit Facility [Line Items]
Line of Credit Facility, maximum borrowing capacity		$ 1,000
Line of Credit Facility, expiration date			Oct. 31, 2015
Line of Credit Facility advance drawn	$ 229	$ 440

Capital Structure - Additional

Capital Structure - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Dec. 31, 2014	Feb. 28, 2015	Dec. 31, 2014	Feb. 28, 2013	Dec. 31, 2015
Capital Structure [Line Items]
Preferred Stock, shares authorized					5,000
Preferred Stock, shares issued					0
Preferred Stock, shares outstanding					0
Common Stock, shares authorized	200,000		200,000		200,000
Common stock, par value	$ 0.001		$ 0.001		$ 0.001
Common Stock, shares issued	103,594		103,594		136,640
Common Stock, shares outstanding	103,594		103,594		136,640
Common stock, shares reserved					21,813
Warrants outstanding					2,833
Weighted average exercise price of warrants					$ 3.12
Weighted Average Remaining Contractual Life					1 year 7 months 21 days
Issuance of common stock, subscription receivable	$ 5,737		$ 5,737
Issuance of common stock upon exercise of stock options, shares					835
Stock Options
Capital Structure [Line Items]
Issuance of common stock, subscription receivable	$ 126		$ 126
Issuance of common stock upon exercise of stock options, shares			19
Public Offering
Capital Structure [Line Items]
Common stock, par value		$ 0.001		$ 0.001
Number of common shares issued in public or private offering	3,236	13,800		16,894
Proceeds from additional capital contribution to entity including amount of subscription receivable from investors	$ 48,316
Issuance of common stock, subscription receivable	$ 5,611		$ 5,611

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Jun. 02, 2015	Jun. 05, 2012	Jun. 10, 2010	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Jan. 02, 2010	Jun. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common Stock, shares issued and outstanding				8,501	9,493
Intrinsic value of options outstanding				$ 36,733
Intrinsic value of options vested				27,139
Intrinsic value of options exercised				$ 5,302	$ 7,855	$ 790
Weighted average grant date fair value of options granted				$ 6.66	$ 8.50	$ 3.29
Weighted average grant date fair value of options vested				$ 4.64	$ 4.13	$ 3.52
Weighted average remaining contractual life for option exercisable				5 years 10 months 24 days
Weighted average remaining contractual life for option vested				6 years 8 months 12 days
Total compensation cost related to stock options not yet recognized				$ 15,333
Total compensation cost related to stock options not yet recognized, Weighted average period for recognition				1 year 4 months 24 days
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense associated				$ 9,887	$ 7,177	$ 5,760
Consultants
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total compensation expense associated				$ 25	0	69
2006 Stock Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized for issuance				750
Additional shares of common stock authorized		7,000	3,000				2,673
Shares of common stock available for grant								1,894
Common Stock, shares issued and outstanding								8,727
2006 Stock Incentive Plan | Before Amendment
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized for issuance		6,423	3,423
2006 Stock Incentive Plan | After Amendment
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized for issuance		13,423	6,423
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized for issuance				6,900
Shares of common stock available for grant				8,773
Maximum number of Shares of common stock available for grant to any plan participant per fiscal year				1,500
Percentage on maximum number of authorized shares available for grant				10.00%
Two Thousand And Six Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional shares of common stock authorized	1,700		250
Shares of common stock available for grant				1,706
Total compensation expense associated				$ 160	$ 96	$ 92
Common stock lower price percentage				85.00%
Percentage of compensation cost grant date stock price discount				15.00%
Two Thousand And Six Employee Stock Purchase Plan | Before Amendment
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized for issuance	500		250
Two Thousand And Six Employee Stock Purchase Plan | After Amendment
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock authorized for issuance	2,200		500

Company's Stock Option Activity

Company's Stock Option Activity (Detail) shares in Thousands	12 Months Ended
	Dec. 31, 2015$ / sharesshares
Options
Outstanding at January 1, 2015 | shares	9,493
Stock Options, Granted | shares	213
Stock Options, Exercised | shares	(835)
Stock Options, Forfeited | shares	(297)
Stock Options, Cancelled | shares	(73)
Outstanding at December 31, 2015 | shares	8,501
Options exercisable at December 31, 2015 | shares	5,642
Options vested and expected to vest at December 31, 2015 | shares	8,083
Weighted Average Exercise Price
Weighted Average Exercise Price, Outstanding at January 1, 2015 | $ / shares	$ 6.88
Weighted Average Exercise Price, Granted | $ / shares	9.14
Weighted Average Exercise Price, Exercised | $ / shares	4.47
Weighted Average Exercise Price, Forfeited | $ / shares	7.93
Weighted Average Exercise Price, Cancelled | $ / shares	12.60
Weighted Average Exercise Price, Outstanding at December 31, 2015 | $ / shares	7.08
Weighted Average Exercise Price, Options exercisable at December 31, 2015 | $ / shares	6.39
Weighted Average Exercise Price, Options vested and expected to vest at December 31, 2015 | $ / shares	$ 7.05

Summary of Stock Options Outsta

Summary of Stock Options Outstanding (Detail) shares in Thousands	12 Months Ended
	Dec. 31, 2015$ / sharesshares
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options, Number Outstanding | shares	8,501
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	6 years 8 months 12 days
Options Outstanding, Weighted Average Exercise Price	$ 7.08
Options, Number Vested | shares	5,642
Options Vested, Weighted Average Exercise Price	$ 6.39
Range One
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options, Range of Exercise Prices, lower limit	0
Options, Range of Exercise Prices, upper limit	$ 2
Options, Number Outstanding | shares	137
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	3 years
Options Outstanding, Weighted Average Exercise Price	$ 1.05
Options, Number Vested | shares	137
Options Vested, Weighted Average Exercise Price	$ 1.05
Range Two
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options, Range of Exercise Prices, lower limit	2.01
Options, Range of Exercise Prices, upper limit	$ 4
Options, Number Outstanding | shares	3,279
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	6 years 7 months 6 days
Options Outstanding, Weighted Average Exercise Price	$ 3.07
Options, Number Vested | shares	2,288
Options Vested, Weighted Average Exercise Price	$ 3.08
Range Three
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options, Range of Exercise Prices, lower limit	4.01
Options, Range of Exercise Prices, upper limit	$ 6
Options, Number Outstanding | shares	367
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	2 years 2 months 12 days
Options Outstanding, Weighted Average Exercise Price	$ 5.16
Options, Number Vested | shares	364
Options Vested, Weighted Average Exercise Price	$ 5.15
Range Four
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options, Range of Exercise Prices, lower limit	6.01
Options, Range of Exercise Prices, upper limit	$ 8
Options, Number Outstanding | shares	1,754
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	6 years 9 months 18 days
Options Outstanding, Weighted Average Exercise Price	$ 7.45
Options, Number Vested | shares	1,347
Options Vested, Weighted Average Exercise Price	$ 7.42
Range Five
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options, Range of Exercise Prices, lower limit	8.01
Options, Range of Exercise Prices, upper limit	$ 10
Options, Number Outstanding | shares	1,033
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	7 years 3 months 18 days
Options Outstanding, Weighted Average Exercise Price	$ 8.73
Options, Number Vested | shares	742
Options Vested, Weighted Average Exercise Price	$ 8.68
Range Six
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options, Range of Exercise Prices, lower limit	10.01
Options, Range of Exercise Prices, upper limit	$ 12
Options, Number Outstanding | shares	125
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	8 years 7 months 6 days
Options Outstanding, Weighted Average Exercise Price	$ 10.51
Options, Number Vested | shares	41
Options Vested, Weighted Average Exercise Price	$ 10.70
Range Seven
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options, Range of Exercise Prices, lower limit	12.01
Options, Range of Exercise Prices, upper limit	$ 14
Options, Number Outstanding | shares	1,502
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	8 years 10 months 24 days
Options Outstanding, Weighted Average Exercise Price	$ 13.44
Options, Number Vested | shares	419
Options Vested, Weighted Average Exercise Price	$ 13.43
Range Eight
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options, Range of Exercise Prices, lower limit	14.01
Options, Range of Exercise Prices, upper limit	$ 16
Options, Number Outstanding | shares	301
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	1 year
Options Outstanding, Weighted Average Exercise Price	$ 14.75
Options, Number Vested | shares	301
Options Vested, Weighted Average Exercise Price	$ 14.75
Range Nine
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options, Range of Exercise Prices, lower limit	16.01
Options, Range of Exercise Prices, upper limit	$ 20
Options, Number Outstanding | shares	3
Options Outstanding, Weighted Average Remaining Contractual Life (Years)	1 year 1 month 6 days
Options Outstanding, Weighted Average Exercise Price	$ 19
Options, Number Vested | shares	3
Options Vested, Weighted Average Exercise Price	$ 19

Assumptions Used to Value Optio

Assumptions Used to Value Options Granted (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of option	6 years 3 months
Expected dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of option		5 years	5 years
Expected volatility	84.00%	92.00%	87.00%
Risk free interest rate	1.54%	1.81%	1.01%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of option		6 years 3 months	6 years 1 month 6 days
Expected volatility	91.00%	96.00%	94.00%
Risk free interest rate	1.93%	2.02%	2.10%

Compensation Expense Related to

Compensation Expense Related to Option Grants Made to Employees and Consultants (Detail) - Employees and Consultants - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	$ 9,913	$ 7,177	$ 5,828
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	4,346	2,713	2,146
General and Administrative Expense
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation	$ 5,567	$ 4,464	$ 3,682

Assumptions Used to Value Opt60

Assumptions Used to Value Options Granted under Two Thousand and Six Employee Stock Purchase Plan (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of option	6 years 3 months
Expected dividend yield	0.00%	0.00%	0.00%
Two Thousand And Six Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of option	6 months	6 months	6 months
Expected dividend yield	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of option		5 years	5 years
Expected volatility	84.00%	92.00%	87.00%
Risk free interest rate	1.54%	1.81%	1.01%
Minimum | Two Thousand And Six Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	57.00%	67.00%	47.00%
Risk free interest rate	0.07%	0.06%	0.11%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of option		6 years 3 months	6 years 1 month 6 days
Expected volatility	91.00%	96.00%	94.00%
Risk free interest rate	1.93%	2.02%	2.10%
Maximum | Two Thousand And Six Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	88.00%	130.00%	141.00%
Risk free interest rate	0.08%	0.08%	0.14%

Other Income - Additional Infor

Other Income - Additional Information (Detail) $ in Thousands	1 Months Ended
	Aug. 31, 2015USD ($)
Other Income
Other Income [Line Items]
Disgorgement of short swing profits under Section 16(b) of the Securities Exchange Act	$ 8,944

Other License and Research an62

Other License and Research and Development Agreements - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
HCV License Agreement
License Collaboration And Grant Agreements [Line Items]
License agreement payment	$ 115	$ 140	$ 140
Additional aggregate minimum payment under agreement	$ 100
Additional aggregate minimum payment, term	5 years
Yale University
License Collaboration And Grant Agreements [Line Items]
License agreement payment	$ 35
Initial license fee	10
Development milestone payment	25
Maximum additional milestone payment	850
Emory University
License Collaboration And Grant Agreements [Line Items]
License agreement payment	150
Initial license fee	100
Development milestone payment	50
Maximum additional milestone payment	$ 400

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Commitments and Contingencies Disclosure [Line Items]
Employer contributions for each dollar contribution	50.00%
Maximum employee contribution as a percentage of salary deferrals	6.00%
Employer matching contribution	$ 296	$ 262	$ 238
Period of monthly lease payment	Mar. 31, 2020
Accrued rent expense	$ 72	83
Rent expense under operating lease	$ 629	$ 617	$ 617

Future Minimum Annual Lease Pay

Future Minimum Annual Lease Payments (Detail) $ in Thousands	Dec. 31, 2015USD ($)
Operating Leased Assets [Line Items]
2,016	$ 770
2,017	814
2,018	814
2,019	814
2,020	203
Total	$ 3,415

Income Tax Provision (Benefit)

Income Tax Provision (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred Income Taxes And Tax Credits [Line Items]
Federal and state	$ 699	$ 29,287	$ 28,554
Valuation allowance	(699)	(29,287)	(28,554)
Total deferred	$ 0	$ 0	$ 0

Reconciliation of Provision for

Reconciliation of Provision for Income Taxes at Statutory Rates (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory rate	(34.00%)	(34.00%)	(34.00%)
State tax, net of federal benefit	(5.00%)	(5.00%)	(5.00%)
Other	0.52%	0.07%	0.07%
Share-based compensation	2.84%	(0.56%)	2.90%
Valuation allowance	35.64%	39.49%	36.03%
Effective Income Tax Rate Reconciliation, Percent, Total	0.00%	0.00%	0.00%

Future Tax Benefits (Deferred T

Future Tax Benefits (Deferred Tax Assets) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Gross deferred tax assets:
Net operating losses	$ 151,076	$ 154,019
Tax credits (federal and state)	12,977	11,419
Share-based compensation	7,643	5,378
Other	419	601
Gross deferred income tax asset	172,115	171,417
Less-valuation allowance	(172,115)	(171,417)
Net deferred tax asset	$ 0	$ 0

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Line Items]
Gross deferred income tax asset	$ 172,115	$ 171,417
Unrecognized tax benefit results in net operating loss carryforwards	14,345
Operating loss carryforwards, limitations on use	55,429
Income tax benefit for estimated proceeds	$ 640	$ 678	$ 183
State and Local Jurisdiction | Minimum
Income Tax Disclosure [Line Items]
Operating loss carryforward expiration year commencing	2,020
State and Local Jurisdiction | Maximum
Income Tax Disclosure [Line Items]
Operating loss carryforward expiration year commencing	2,035
Connecticut State
Income Tax Disclosure [Line Items]
Exchange percentage of research and development tax credit	65.00%
Research Tax Credit Carryforward
Income Tax Disclosure [Line Items]
Tax credit carryforwards, limitations on use	$ 4,066
Internal Revenue Service (IRS) | Minimum
Income Tax Disclosure [Line Items]
Operating loss carryforward expiration year commencing	2,018
Research and development credit carryforwards expiration	2,028
Internal Revenue Service (IRS) | Maximum
Income Tax Disclosure [Line Items]
Operating loss carryforward expiration year commencing	2,035
Research and development credit carryforwards expiration	2,034

Net Operating Loss and Credit C

Net Operating Loss and Credit Carryforwards (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 418,720	$ 418,879
State and Local Jurisdiction | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Research and development credit carryforwards	4,640	4,412
Internal Revenue Service (IRS)
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	351,977	360,598
Internal Revenue Service (IRS) | Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Research and development credit carryforwards	$ 8,337	$ 7,007

Related Party Transactions - Ad

Related Party Transactions - Additional Information (Detail) - Domain Associates LLC - USD ($) shares in Thousands, $ in Millions	1 Months Ended
	Aug. 31, 2010	Dec. 31, 2015
Related Party Transaction [Line Items]
Common stock shares purchased	8,032
Common stock warrants purchased	2,811
Common stock aggregate purchase price	$ 20.4
Ownership percentage of shares outstanding		4.00%

Quarterly Financial Data (Detai

Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended										12 Months Ended			208 Months Ended
	Dec. 31, 2015	[1]	Sep. 30, 2015	[1]	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015
Quarterly Financial Information [Line Items]
Total operating revenue	$ 31,591		$ 33,820		$ 711						$ 66,122
Total operating expenses	15,092		16,839		29,899	$ 19,399	$ 21,661	$ 15,764	$ 15,766	$ 16,235	81,229	$ 69,426	$ 59,477
Net income (loss)	$ 16,952		$ 26,259		$ (28,978)	$ (19,263)	$ (21,596)	$ (15,667)	$ (15,657)	$ (16,088)	$ (5,030)	$ (69,008)	$ (58,947)	$ (441,850)
Net income (loss) per share-basic and diluted	$ 0.12		$ 0.19		$ (0.25)	$ (0.17)	$ (0.21)	$ (0.16)	$ (0.16)	$ (0.17)	$ (0.04)	$ (0.70)	$ (0.63)
Weighted average number of shares outstanding-basic	136,558		136,438		117,770	111,202	100,579	99,031	97,017	96,792
Weighted average number of shares outstanding-diluted	140,475		140,024		117,770	111,202	100,579	99,031	97,017	96,792
[1]	In May 2015, the Company entered into an exclusive collaboration and license agreement with Janssen, and its affiliate, Johnson & Johnson Innovation-JJDC, Inc., or JJDC, which we refer to as the Janssen Agreement for the further clinical advancement of a portfolio of antivirals the Company discovered and developed for the treatment of HCV. In addition, upon the closing of the transactions contemplated by the Janssen Agreement, the Company entered into a stock purchase agreement with JJDC. Pursuant to the JJDC stock purchase agreement, on July 1, 2015, the Company issued 18,367 shares of common stock to JJDC at a price of $12.25 per share for an aggregate purchase price of $225,000. The Company recorded revenue of $66,122 during the year ended December 31, 2015 associated with this transaction. Also refer to footnote 5.

Quarterly Financial Data (Paren

Quarterly Financial Data (Parenthetical) (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	Jul. 01, 2015	Dec. 31, 2015	[1]	Sep. 30, 2015	[1]	Jun. 30, 2015	Dec. 31, 2015
Quarterly Financial Information [Line Items]
Revenue		$ 31,591		$ 33,820		$ 711	$ 66,122
Janssen Agreement
Quarterly Financial Information [Line Items]
Revenue							$ 66,122
Janssen Agreement | Stock Purchase Agreement
Quarterly Financial Information [Line Items]
Issuance of common stock	18,367
Number of stock purchased, price per share	$ 12.25
Number of stock purchased, aggregate purchase price	$ 225,000
[1]	In May 2015, the Company entered into an exclusive collaboration and license agreement with Janssen, and its affiliate, Johnson & Johnson Innovation-JJDC, Inc., or JJDC, which we refer to as the Janssen Agreement for the further clinical advancement of a portfolio of antivirals the Company discovered and developed for the treatment of HCV. In addition, upon the closing of the transactions contemplated by the Janssen Agreement, the Company entered into a stock purchase agreement with JJDC. Pursuant to the JJDC stock purchase agreement, on July 1, 2015, the Company issued 18,367 shares of common stock to JJDC at a price of $12.25 per share for an aggregate purchase price of $225,000. The Company recorded revenue of $66,122 during the year ended December 31, 2015 associated with this transaction. Also refer to footnote 5.