Chimerix (CMRX) 10-K 1 Mar 18 2017 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Feb. 22, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Entity Registrant Name	CHIMERIX INC
Entity Central Index Key	1,117,480
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 151,017,173
Entity Common Stock, Shares Outstanding		47,652,332

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 18,548	$ 51,463
Short-term investments, available-for-sale	132,972	180,558
Accounts receivable	1,682	1,599
Prepaid expenses and other current assets	3,331	2,845
Total current assets	156,533	236,465
Long-term investments	76,731	47,407
Property and equipment, net of accumulated depreciation	1,894	2,843
Other long-term assets	72	55
Total assets	235,230	286,770
Current liabilities:
Accounts payable	3,812	3,890
Accrued liabilities	9,384	6,215
Total current liabilities	13,196	10,105
Lease-related obligations	224	441
Total liabilities	13,420	10,546
Stockholders’ equity:
Preferred stock, $0.001 par value, 10,000,000 shares authorized at December 31, 2017 and 2016; no shares issued and outstanding as of December 31, 2017 and 2016	0	0
Common stock, $0.001 par value; 200,000,000 shares authorized at December 31, 2017 and 2016; 47,505,532 and 46,522,475 shares issued and outstanding at December 31, 2017 and 2016, respectively	47	46
Additional paid-in capital	709,514	692,422
Accumulated other comprehensive loss, net	(963)	(440)
Accumulated deficit	(486,788)	(415,804)
Total stockholders’ equity	221,810	276,224
Total liabilities and stockholders’ equity	$ 235,230	$ 286,770

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Preferred stock, par value (in USD per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in USD per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares, issued (in shares)	47,505,532	46,522,475
Common stock, shares, outstanding (in shares)	47,505,532	46,522,475

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Contract revenue	$ 4,494,000	$ 5,702,000	$ 9,214,000
Collaboration and licensing revenue	0	0	1,548,000
Total revenues	4,494,000	5,702,000	10,762,000
Operating expenses:
Research and development	49,448,000	58,647,000	97,717,000
General and administrative	27,148,000	25,007,000	31,296,000
Total operating expenses	76,596,000	83,654,000	129,013,000
Loss from operations	(72,102,000)	(77,952,000)	(118,251,000)
Other (expense) income:
Other-than-temporary impairment of investment	(1,160,000)	0	0
Interest income, net	2,278,000	1,562,000	879,000
Net loss	(70,984,000)	(76,390,000)	(117,372,000)
Other comprehensive loss:
Unrealized (loss) gain on investments, net	(523,000)	324,000	(799,000)
Comprehensive loss	$ (71,507,000)	$ (76,066,000)	$ (118,171,000)
Per share information:
Net loss, basic and diluted (in USD per share)	$ (1.51)	$ (1.65)	$ (2.67)
Weighted-average shares outstanding, basic and diluted (in shares)	46,963,430	46,267,064	43,878,326

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2014	$ 274,636	$ 41	$ 496,602	$ 35	$ (222,042)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	12,959		12,959
Exercise of stock options	2,107		2,107
Exercise of warrants	1,001	1	1,000
Employee stock purchase plan purchases	1,048		1,048
Issuance of common stock, value	161,879	4	161,875
Comprehensive loss:
Unrealized (loss) gain on investments, net	(799)			(799)
Net loss	(117,372)				(117,372)
Comprehensive loss	(118,171)
Ending Balance at Dec. 31, 2015	335,459	46	675,591	(764)	(339,414)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	16,223		16,223
Exercise of stock options	168		168
Employee stock purchase plan purchases	440		440
Comprehensive loss:
Unrealized (loss) gain on investments, net	324			324
Net loss	(76,390)				(76,390)
Comprehensive loss	(76,066)
Ending Balance at Dec. 31, 2016	276,224	46	692,422	(440)	(415,804)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	16,110	1	16,109
Exercise of stock options	121		121
Employee stock purchase plan purchases	712		712
Issuance of common stock, value	150		150
Comprehensive loss:
Unrealized (loss) gain on investments, net	(523)			(523)
Net loss	(70,984)				(70,984)
Comprehensive loss	(71,507)
Ending Balance at Dec. 31, 2017	$ 221,810	$ 47	$ 709,514	$ (963)	$ (486,788)

Consolidated Statements of Sto6

Consolidated Statements of Stockholders' Equity (Deficit) (Parenthetical) $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)$ / sharesshares
Stock issuance costs	$ 0
Common Stock Offering, June 16, 2015
New issues of stock during period (in shares) | shares	4,341,250
Offering price per share (in USD per share) | $ / shares	$ 39.75
Stock issuance costs	$ 10,685

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (70,984,000)	$ (76,390,000)	$ (117,372,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation of property and equipment	1,091,000	1,063,000	657,000
Amortization of debt costs	0	0	64,000
Amortization of premium/discount on investments	(5,000)	1,223,000	1,622,000
Share-based compensation	16,110,000	16,223,000	12,959,000
Other-than-temporary impairment of investment	1,160,000	0	0
Amortization of lease-related obligations	(319,000)	132,000	19,000
Changes in operating assets and liabilities:
Accounts receivable	(83,000)	861,000	(2,354,000)
Prepaid expenses and other assets	(168,000)	3,215,000	(3,028,000)
Accounts payable and accrued liabilities	3,073,000	(10,142,000)	7,725,000
Net cash used in operating activities	(50,125,000)	(63,815,000)	(99,708,000)
Cash flows from investing activities:
Purchases of property and equipment	(151,000)	(841,000)	(2,393,000)
Purchases of short-term investments	0	(23,992,000)	(60,297,000)
Purchases of long-term investments	(162,613,000)	(79,381,000)	(234,791,000)
Proceeds from sales of short-term investments	13,500,000	0	1,003,000
Proceeds from maturities of short-term investments	165,695,000	198,279,000	126,742,000
Proceeds from maturities of long-term investments	0	0	240,000
Net cash provided by (used in) investing activities	16,431,000	94,065,000	(169,496,000)
Cash flows from financing activities:
Proceeds from exercise of stock options	121,000	168,000	2,107,000
Proceeds from employee stock purchase plan	712,000	440,000	1,048,000
Proceeds from exercise of warrants	0	0	1,001,000
Proceeds from public offerings, net of offering costs	0	0	161,879,000
Payments of Stock Issuance Costs	(54,000)	0	0
Payments for deferred financing costs	0	0	(338,000)
Repayments of debt	0	0	(4,350,000)
Net cash provided by financing activities	779,000	608,000	161,347,000
Net (decrease) increase in cash and cash equivalents	(32,915,000)	30,858,000	(107,857,000)
Beginning of period	51,463,000	20,605,000	128,462,000
End of period	18,548,000	51,463,000	20,605,000
Supplemental disclosure of cash flow information
Cash paid for interest	0	0	158,000
Non-cash addition to deferred offering costs	$ 276,000	$ 0	$ 0

The Business and Summary of Sig

The Business and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
The Business and Summary of Significant Accounting Policies	The Business and Summary of Significant Accounting Policies Description of Business Chimerix, Inc. (the Company) is a biopharmaceutical company committed to discovering, developing and commercializing medicines that improve outcomes for immunocompromised patients. The Company was founded in 2000 based on the promise of our proprietary lipid conjugate technology to unlock the potential of some of the most broad-spectrum antivirals by enhancing their antiviral activity and safety profiles in convenient dosing regimens. The Company's lead compound, brincidofovir, is in development as an oral and intravenous (IV) formulation for the prevention and treatment of DNA viruses, including smallpox, adenoviruses, and the human herpesviruses. The Company is also advancing the development of CMX521 for the treatment and prevention of norovirus. In addition, the Company has an active discovery program focusing on viral targets for which limited or no therapies are currently available. Basis of Presentation The consolidated financial statements include the accounts of the Company, and its wholly owned subsidiaries. The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (GAAP). The preparation of the Company’s consolidated financial statements requires estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and the disclosure of contingent assets and liabilities in the consolidated financial statements and accompanying notes. Although these estimates are based on knowledge of current events and actions the Company may undertake in the future, actual results may ultimately differ from these estimates and assumptions. Reclassifications Certain prior period amounts in the accompanying consolidated financial statements have been reclassified to conform to the current year presentation. These reclassifications had no effect on previously reported net income or stockholders' equity (deficit). Cash and Cash Equivalents The Company considers any highly liquid instrument with an original maturity of three months or less at acquisition to be a cash equivalent. Cash equivalents consist of money market funds and commercial paper. Investments Investments consist primarily of brokered certificates of deposit, U.S. Treasury securities and stock of a U.S. corporation. The Company invests in high-credit quality investments in accordance with its investment policy which minimizes the probability of loss. Available-for-sale securities are carried at fair value as determined by quoted market prices, with the unrealized gains and losses, net of tax, reported as a separate component of stockholders equity. Realized gains and losses are determined using the specific identification method and transactions are recorded on a settlement date basis in interest income or expense, net. Investments with original maturities beyond three months at the date of purchase and which mature on, or less than twelve months from, the balance sheet date are classified as short-term. Investments with a maturity beyond twelve months from the balance sheet date are classified as long-term. The Company periodically reviews available-for-sale securities for other-than-temporary declines in fair value below the cost basis and whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. The Company evaluates, among other things, the duration and extent to which the fair value of a security is less than its cost; the financial condition of the issuer and any changes thereto; and the Company’s intent to sell, or whether it will more likely than not be required to sell, the security before recovery of its amortized cost basis. The Company does not intend to sell, and is not likely to be required to sell, the available-for-sale securities in an unrealized loss position before recovery of the amortized cost bases of the securities, which may be maturity. Any such declines in value judged to be other-than-temporary on available-for-sale securities are reported in other-than-temporary impairment of investment. For the year ended December 31, 2017, the Company determined the decline in value for its investment in ContraVir Pharmaceuticals common stock to be other-than temporary. As such, during the fourth quarter of 2017, the Company reclassified a loss of $1.2 million from accumulated other comprehensive loss, net in the Consolidated Balance Sheets to other-than-temporary impairment of investment in the Consolidated Statements of Operations and Comprehensive Loss. There were no such declines in value for the years ended December 31, 2016 and 2015 . The Company recognizes interest income on an accrual basis in interest income, net in the Consolidated Statements of Operations and Comprehensive Loss. Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk consist of cash, cash equivalents, short-term investments, long-term investments and accounts receivable. The Company is exposed to credit risk, subject to federal deposit insurance, in the event of default by the financial institutions holding its cash and cash equivalents to the extent of amounts recorded on the balance sheets. Accounts receivable represent amounts due from an agency of the federal government. Accounts Receivable Accounts receivable at December 31, 2017 and December 31, 2016 consisted of amounts billed under the Company’s contract with the Biomedical Advanced Research and Development Authority (BARDA). Receivables under the BARDA contract are recorded as qualifying research activities are conducted and invoices from the Company’s vendors are received. The Company carries its accounts receivable at cost less an allowance for doubtful accounts. On a periodic basis, the Company evaluates its accounts receivable and establishes an allowance based on its history of collections and write-offs and the current status of all receivables. The Company does not accrue interest on trade receivables. If accounts become uncollectible, they will be written off through a charge to the allowance for doubtful accounts. The Company has not recorded a charge to allowance for doubtful accounts as management believes all receivables are fully collectible. Fair Value of Financial Instruments The carrying amounts of certain financial instruments, including accounts receivable, accounts payable and accrued expenses approximate their fair values due to the short-term nature of such instruments. For assets and liabilities recorded at fair value, it is the Company’s policy to maximize the use of observable inputs and minimize the use of unobservable inputs when developing fair value measurements, in accordance with the fair value hierarchy. Fair value measurements for assets and liabilities where there exists limited or no observable market data are based primarily upon estimates and are often calculated based on the economic and competitive environment, the characteristics of the asset or liability and other factors. Therefore, fair value measurements cannot be determined with precision and may not be realized in an actual sale or immediate settlement of the asset or liability. Additionally, there may be inherent weaknesses in any calculation technique and changes in the underlying assumptions used, including discount rates and estimates of future cash flows, could significantly affect the calculated current or future fair values. The Company utilizes fair value measurements to record fair value adjustments to certain assets and liabilities and to determine fair value disclosures. The Company groups assets and liabilities at fair value in three levels, based on the markets in which the assets and liabilities are traded and the reliability of the assumptions used to determine fair value. An adjustment to the pricing method used within either Level 1 or Level 2 inputs could generate a fair value measurement that effectively falls in a lower level in the hierarchy. These levels are: • Level 1 — Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access. • Level 2 — Valuations based on quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and models for which all significant inputs are observable, either directly or indirectly. • Level 3 — Valuations based on inputs that are unobservable and significant to the overall fair value measurement. The determination of where an asset or liability falls in the hierarchy requires significant judgment. The Company evaluates hierarchy disclosures and, based on various factors, it is possible that an asset or liability may be classified differently from period to period. However, the Company expects that changes in classification between levels will be rare. There were no transfers between Level 1 and Level 2 or transfers to or from Level 3 during the year ended December 31, 2017 . When the 18 month restriction on selling the Company's investment in ContraVir Pharmaceuticals ended on June 17, 2016, the investment was transferred from Level 3 to Level 2 as the fair value was based on quoted prices for similar assets and there were no significant unobservable inputs. The Company's investment in ContraVir Pharmaceuticals transferred from Level 2 to Level 1 when the Company converted its investment in Series B Preferred shares into common stock in September 2016. At December 31, 2017 and 2016 , the Company had cash equivalents, consisting of money market funds, and short-term and long-term investments consisting of U.S. Treasury securities, whose value is based on using quoted market prices. Accordingly, these securities are classified as Level 1. At December 31, 2017 , the Company had cash equivalents, consisting of commercial paper, and at December 31, 2016 , the Company had cash equivalents consisting of commercial paper, and short-term investments comprised of brokered certificates of deposits, for which quoted prices are not available that are valued using independent pricing models or other model-based valuation techniques such as the present value of future cash flows, adjusted for the security’s credit rating, prepayment assumptions and other factors such as credit loss assumptions. Accordingly, these securities are classified as Level 2. At December 31, 2015, the Company's preferred stock investment in ContraVir Pharmaceuticals was categorized as Level 3 as there were significant unobservable inputs. The valuation of the investment at December 31, 2015 was calculated on an as if converted to common share basis with a discount for lack of marketability applied due to the 18 month restriction from the date of the investment on selling the converted common shares, which ended on June 17, 2016. An option pricing model was used to determine the discount for lack of marketability of 10% at December 31, 2015. The key unobservable inputs used in the option pricing model at December 31, 2015 were (i) exercise price - $1.54 , (ii) dividend yield - 0% , (iii) expected holding period - 0.46 years , (iv) risk-free rate - 0.44% , and (v) volatility - 75% . On September 30, 2016, the Company converted its preferred stock investment in ContraVir into 1,071,429 shares of ContraVir common stock, which was categorized as a Level 1 asset and valued based on ContraVir's common stock value. The Company evaluates, among other things, the duration and extent to which the fair value of a security is less than its cost; the financial condition of the issuer and any changes thereto; and the Company’s intent to sell, or whether it will more likely than not be required to sell, the security before recovery of its market value. For the years ended December 31, 2016 and 2015 , the Company did not intend to sell, and was not more likely than not to be required to sell, its investment in ContraVir before recovery of its market value, therefore the changes in the fair value of ContraVir common shares was recorded to unrealized (loss) gain on investments, net in the Consolidated Statements of Operations and Comprehensive Loss. There was no material re-measurement to fair value of financial assets and liabilities that are not measured at fair value on a recurring basis. For additional information regarding the Company's investments, please refer to Note 2, "Investments." Below is a table that presents information about certain assets measured at fair value on a recurring basis (in thousands): Fair Value Measurements December 31, 2017 Total Quoted Prices in Significant Other Significant Cash equivalents Money market funds $ 10,816 $ 10,816 $ — $ — Commercial paper 3,995 — 3,995 — Total cash equivalents 14,811 10,816 3,995 — Short-term investments U.S. Treasury securities 132,586 132,586 — — Common stock of U.S. corporation 386 386 — — Total short-term investments 132,972 132,972 — — Long-term investments U.S. Treasury securities 76,731 76,731 — — Total long-term investments 76,731 76,731 — — Total assets $ 224,514 $ 220,519 $ 3,995 $ — Fair Value Measurements December 31, 2016 Total Quoted Prices in Significant Other Significant Cash equivalents Money market funds $ 15,733 $ 15,733 $ — $ — Commercial paper 35,097 — 35,097 — Total cash equivalents 50,830 15,733 35,097 — Short-term investments Certificates of deposit 7,450 — 7,450 — U.S. Treasury securities 171,822 171,822 — — Common stock of U.S. corporation 1,286 1,286 — — Total short-term investments 180,558 173,108 7,450 — Long-term investments U.S. Treasury securities 47,407 47,407 — — Total long-term investments 47,407 47,407 — — Total assets $ 278,795 $ 236,248 $ 42,547 $ — Below is a table that presents a reconciliation of the beginning and ending balances of assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) (in thousands): Fair Value Measurements (Level 3) Preferred stock of U.S. corporation: Fair value at December 31, 2015 1,485 Fair value decrease recorded in other comprehensive loss (371 ) Fair value transferred to Level 2 (1,114 ) Fair value at December 31, 2016 $ — Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2017 2016 Prepaid research and development expenses $ 1,138 $ 843 Interest receivable 601 772 Prepaid insurance 481 389 Other prepaid expenses and current assets 1,111 841 Total prepaid expenses and other current assets $ 3,331 $ 2,845 Property and Equipment Property and equipment are stated at cost, less accumulated depreciation. Depreciation is determined on a straight-line basis over the estimated useful lives of the assets, which generally range from three to five years . Leasehold improvements are amortized over the shorter of the useful life of the asset or the term of the related lease . Maintenance and repairs are charged against expense as incurred. Impairment of Long-Lived Assets The Company evaluates long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. If the estimated future cash flows (undiscounted and without interest charges) from the use of an asset are less than the carrying value, a write-down would be recorded to reduce the related asset to its estimated fair value. To date, no such write-downs have occurred. Deferred Lease Obligations The Company recognizes rent expense on a straight-line basis over the non-cancelable term of its operating lease and records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. The Company also records landlord-funded lease incentives, such as reimbursable leasehold improvements, as a deferred rent liability, which is amortized as a reduction of rent expense over the non-cancelable term of its operating lease. Accrued Liabilities Accrued liabilities consisted of the following (in thousands): December 31, 2017 2016 Accrued compensation $ 3,678 $ 2,906 Accrued research and development expenses 3,384 2,257 Accrued indemnification claim 1,000 — Other accrued liabilities 1,322 1,052 Total accrued liabilities $ 9,384 $ 6,215 Revenue Recognition The Company’s revenues generally consist of (i) contract and grant revenue – revenue generated under federal contracts and other awarded grants, and (ii) collaboration and licensing revenue – revenue related to non-refundable upfront fees, royalties and milestone payments earned under license agreements. Revenue is recognized in accordance with the criteria outlined in the Securities and Exchange Commission (SEC)’s Topic 13 and Accounting Standards Codification (ASC) 605-25 and by the Financial Accounting Standards Board. Following these accounting pronouncements, revenue is recognized when all four of the following criteria are met: (i) persuasive evidence of an arrangement exists; (ii) delivery of the products and/or services has occurred and risk of loss has passed; (iii) the selling price is fixed or determinable; and (iv) collectability is reasonably assured. For arrangements that involve the delivery of more than one element, each product, service and/or right to use assets is evaluated to determine whether it qualifies as a separate unit of accounting. This determination is based on whether the deliverable has “stand-alone value” to the customer. The consideration that is fixed or determinable is then allocated to each separate unit of accounting based on the relative selling prices of each deliverable. The consideration allocated to each unit of accounting is recognized as the related goods and services are delivered, limited to the consideration that is not contingent upon future deliverables. If the arrangement constitutes a single unit of accounting, the revenue recognition policy must be determined for the entire arrangement and the consideration received is recognized over the period of inception through the date the last deliverable within the single unit of accounting is expected to be delivered. Revisions to the estimated period of recognition are reflected in revenue prospectively. Non-refundable upfront fees are recorded as deferred revenue and recognized into revenue as license fees from collaborations on a straight-line basis over the estimated period of the Company’s substantive performance obligations. If the Company does not have substantive performance obligations, the Company recognizes non-refundable upfront fees into revenue through the date the deliverable is satisfied. Analyzing the arrangement to identify deliverables requires the use of judgment and each deliverable may be an obligation to deliver services, a right or license to use an asset, or another performance obligation. Milestone payments are recognized when earned, provided that (i) the milestone event is substantive; (ii) there is no ongoing performance obligation related to the achievement of the milestone earned; and (iii) it would result in additional payments. Milestone payments are considered substantive if all of the following conditions are met: the milestone payment is non-refundable; achievement of the milestone was not reasonably assured at the inception of the arrangement; substantive effort is involved to achieve the milestone; and the amount of the milestone appears reasonable in relation to the effort expended, the other milestones in the arrangement; and the related risk associated with the achievement of the milestone. Contingent based event payments the Company may receive under a license or collaboration agreement will be recognized when received. For the years ended December 31, 2017 , 2016 and 2015 , contract and grant revenue consisted only of revenue from the BARDA contract as there was no grant revenue. The Company recognizes contract and grant revenue as qualifying research activities are conducted based on invoices received from the Company’s vendors. Changes in fringe and indirect rates are recognized as a change in estimate in the period such rate changes are approved by BARDA. For the year ended December 31, 2015, collaboration and licensing revenue primarily consisted of the upfront license fee payment from ContraVir recognized when the Company completed its performance obligations. Research and Development Prepaids and Accruals As part of the process of preparing financial statements, the Company is required to estimate its expenses resulting from its obligation under contracts with vendors and consultants and clinical site agreements in connection with its research and development efforts. The financial terms of these contracts are subject to negotiations which vary contract to contract and may result in payment flows that do not match the periods over which materials or services are provided to the Company under such contracts. The Company’s objective is to reflect the appropriate research and development expenses in its financial statements by matching those expenses with the period in which services and efforts are expended. The Company accounts for these expenses according to the progress of its research and development efforts. The Company determines prepaid and accrual estimates through discussion with applicable personnel and outside service providers as to the progress or state of communication of clinical trials, or other services completed. The Company adjusts its rate of research and development expense recognition if actual results differ from its estimates. The Company makes estimates of its prepaid and accrued expenses as of each balance sheet date in its financial statements based on facts and circumstances known at that time. Although the Company does not expect its estimates to be materially different from amounts actually incurred, its understanding of status and timing of services performed relative to the actual status and timing of services performed may vary and may result in the Company reporting amounts that are too high or too low for any particular period. Through December 31, 2017 , there had been no material adjustments to the Company’s prior period estimates of prepaid and accruals for research and development expenses. The Company’s research and development prepaids and accruals are dependent upon the timely and accurate reporting of contract research organizations and other third-party vendors. Research and Development Expenses Major components of research and development costs include cash compensation, stock based compensation, pre-clinical studies, clinical trial and related clinical manufacturing, drug development, materials and supplies, legal, regulatory compliance, and fees paid to consultants and other entities that conduct certain research and development activities on the Company’s behalf. Research and development costs, including upfront fees and milestones paid to contract research organizations, are expensed as goods as received or services rendered. Costs incurred in connection with clinical trial activities for which the underlying nature of the activities themselves do not directly relate to active research and development, such as costs incurred for market research and focus groups linked to clinical strategy as well as costs to build the Company’s brand, are not included in research and development costs but are reflected as general and administrative costs. Interest Income, Net Interest income, net primarily includes interest earned on short-term and long-term investments, interest incurred on loans payable, the amortization of deferred financing costs related to fees paid to attorneys and other non-lender entities in order to acquire debt, and the amortization of debt discount related to fees paid to the lender in order to acquire debt. Other-than-temporary Impairment of Investment Other-than-temporary impairment of investment includes write-downs in fair value of available-for-sale securities judged to be other-than-temporarily impaired. Income Taxes Deferred tax assets and liabilities are determined based on differences between the financial and tax reporting bases of assets and liabilities and are measured using enacted tax rates and laws that are expected to be in effect when the differences are expected to reverse. Valuation allowances are established when the Company determines that it is more likely than not that some portion of a deferred tax asset will not be realized. The Company has incurred operating losses from April 7, 2000 (inception) through December 31, 2017 , and therefore has not recorded any current provision for income taxes. Additionally, the Company recognizes the tax benefit from an uncertain tax position only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities based on the technical merits of the position. The tax benefit recognized in the financial statements for a particular tax position is based on the largest benefit that is more likely than not to be realized upon settlement. Accordingly, the Company establishes reserves for uncertain tax positions. Share-Based Compensation The Company measures and recognizes compensation expense for all share-based payment awards made to employees and directors, including employee stock options, restricted stock units and the employee stock purchase plan purchase rights, based on estimated fair values. The fair value of employee stock options and employee stock purchase plan purchase rights is estimated on the grant date using the Black-Scholes valuation model. The grant-date fair value for restricted stock units is based upon the market price of the Company's common stock on the date of the grant. The value of the portion of the award that is ultimately expected to vest is recorded as expense over the requisite service periods. For performance-based awards compensation cost is recognized when it is probable that the performance criteria will be met. The Company estimates forfeitures at the time of grant, and revises those estimates in subsequent periods if actual forfeitures differ from its estimates. The Company uses historical data to estimate forfeitures and record share-based compensation expense only for those awards that are expected to vest. To the extent that actual forfeitures differ from the Company’s estimates, the difference is recorded as a cumulative adjustment in the period the estimates were revised. For the years ended December 31, 2017 , 2016 and 2015 , the Company applied a forfeiture rate based on the Company’s historical forfeitures. 401(k) Plan The Company maintains a defined contribution employee retirement plan (“401(k) plan”). In March 2015, the Company began making matching contributions into the 401(k) plan on behalf of participants. For the years ended December 31, 2017 , 2016 and 2015 , the Company recognized expenses for matching contributions of $0.3 million , $0.4 million and $0.3 million , respectively. Basic and Dilutive Net Loss Per Share of Common Stock Basic net loss per share of common stock is computed by dividing net loss by the weighted-average number of shares of common stock outstanding during the period, excluding the dilutive effects of warrants to purchase common stock, non-vested restricted stock, stock options, and employee stock purchase plan purchase rights. Diluted net loss per share of common stock is computed by dividing net loss by the sum of the weighted-average number of shares of common stock outstanding during the period plus the potential dilutive effects of warrants to purchase common stock, non-vested restricted stock, stock options, and employee stock purchase plan purchase rights outstanding during the period calculated in accordance with the treasury stock method, but are excluded if their effect is anti-dilutive. Because the impact of these items is anti-dilutive during the periods of net loss, there was no difference between basic and diluted loss per share of common stock at December 31, 2017 , 2016 and 2015 . Segments The Company operates in only one segment. The chief operating decision-maker, who is the Company’s Chief Executive Officer, and management use cash flows as the primary measure to manage the business and do not segment the business for internal reporting or decision making. Impact of Recently Issued Accounting Standards In May 2014, the FASB issued Accounting Standards Update (ASU) No. 2014-09, “Revenue from Contracts with Customers (Topic 606).” The ASU establishes a principles-based approach for accounting for revenue arising from contracts with customers and supersedes existing revenue recognition guidance. The ASU provides that an entity should apply a five-step approach for recognizing revenue, including (1) identify the contract with a customer; (2) identify the performance obligations in the contract; (3) determine the transaction price; (4) allocate the transaction price to the performance obligations in the contract; and (5) recognize revenue when, or as, the entity satisfies a performance obligation. Also, the entity must provide various disclosures concerning the nature, amount and timing of revenue and cash flows arising from contracts with customers. The FASB has issued several updates to the standard which (1) defer the original effective date to annual periods and interim periods within those annual periods beginning after December 15, 2017, while allowing for early adoption as of January 1, 2017 (ASU 2015-14); (2) clarify the application of the principal versus agent guidance (ASU 2016-08); and (3) clarify the guidance on inconsequential and perfunctory promises and licensing (ASU 2016-10). The Company has evaluated its contract with BARDA and license agreement with ContraVir, and the adoption of the ASU will not have a material impact on its consolidated financial statements. The Company plans to use the full retrospective method of adoption effective January 1, 2018. The Company is implementing changes to its accounting policies, internal controls and disclosures to support the new standard; however, these changes are not anticipated to be significant. In January 2016, the FASB issued ASU No. 2016-01, “Financial Instruments-Overall (Subtopic 825-10)-Recognition and Measurement of Financial Assets and Financial Liabilities.” The new standard enhances reporting for financial instruments. The ASU is effective for financial statements issued for annual periods and interim periods within those annual periods beginning after December 15, 2017. Earlier adoption is permitted for interim and annual reporting periods as of the beginning of the fiscal year of adoption. The Company does not expect the adoption of ASU 2016-01 to have a material impact on its consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842)”, which increases transparency and comparability among companies accounting for lease transactions. The most significant change of this update will require the recognition of lease assets and liabilities on the balance sheet for lessees for operating lease arrangements with lease terms greater than 12 months. This update will require a modified retrospective application which includes a number of optional practical expedients related to the identification and classification of leases commenced before the effective date. This ASU is effective for financial statements issued for annual periods and interim periods within those annual periods, beginning after December 18, 2018. The Company is currently analyzing the impact of the adoption of ASU No. 2016-02 on its consolidated financial statements. Impact of Recently Adopted Accounting Standards In March 2016, the FASB issued ASU No. 2016-09, "Improvements to Employee Share-Based Payment Accounting", which simplifies several aspects of accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The ASU was effective for financial statements issued for annual periods and interim periods within those annual periods, beginning after December 15, 2016, with early adoption permitted. ASU No. 2016-09 became effective for the Company beginning in the first quarter of 2017. The Company elected to continue estimating the number of awards expected to be forfeited and adjusting the estimate when it is no longer probable that the employee will fu

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Investments [Abstract]
Investments	Investments The following tables summarize the Company's short-term and long-term investments (in thousands): December 31, 2017 Amortized Cost Gross Unrealized Gross Unrealized Estimated U.S. Treasury securities $ 210,280 $ — $ (963 ) $ 209,317 Common stock of U.S. corporation 386 — — 386 Total investments $ 210,666 $ — $ (963 ) $ 209,703 December 31, 2016 Amortized Cost Gross Unrealized Gross Unrealized Estimated Certificates of deposit $ 7,445 $ 5 $ — $ 7,450 U.S. Treasury securities 219,415 15 (201 ) 219,229 Common stock of U.S. corporation 1,545 — (259 ) 1,286 Total investments $ 228,405 $ 20 $ (460 ) $ 227,965 The following tables summarize the Company's investments with unrealized losses, aggregated by investment type and the length of time that individual investments have been in a continuous unrealized loss position (in thousands, except number of securities): December 31, 2017 Less than 12 Months Greater than 12 Months Total Fair Value Unrealized Loss Fair Value Unrealized Loss Fair Value Unrealized Loss U.S. Treasury securities $ 170,390 $ (871 ) $ 38,927 $ (92 ) $ 209,317 $ (963 ) Total $ 170,390 $ (871 ) $ 38,927 $ (92 ) $ 209,317 $ (963 ) Number of securities with unrealized losses 39 7 46 December 31, 2016 Less than 12 Months Greater than 12 Months Total Fair Value Unrealized Loss Fair Value Unrealized Loss Fair Value Unrealized Loss U.S. Treasury securities $ 128,204 $ (201 ) $ — $ — $ 128,204 $ (201 ) Preferred stock of U.S. corporation 1,286 (259 ) — — 1,286 $ (259 ) Total $ 129,490 $ (460 ) $ — $ — $ 129,490 $ (460 ) Number of securities with unrealized losses 24 — 24 The following table summarizes the scheduled maturity for the Company's investments at December 31, 2017 (in thousands): December 31, 2017 Maturing in one year or less $ 132,586 Maturing after one year through two years 76,731 Total debt investments $ 209,317 Common stock of U.S. corporation 386 Total investments $ 209,703

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment, net of accumulated depreciation consisted of the following (in thousands): December 31, 2017 2016 Lab equipment $ 2,496 $ 2,419 Leasehold improvements 1,552 1,570 Computer equipment 1,170 1,262 Office furniture and equipment 520 586 Property and equipment 5,738 5,837 Less accumulated depreciation (3,844 ) (2,994 ) Property and equipment, net of accumulated depreciation $ 1,894 $ 2,843

Loan Payable

Loan Payable	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Loan Payable	Loan Payable On January 27, 2012, the Company entered into a Loan and Security Agreement (LSA) with Silicon Valley Bank (SVB) and MidCap Financial SBIC, LP (MidCap) allowing for borrowings up to $15.0 million , split between a first tranche of $3.0 million borrowed at the time of the agreement, and a second tranche of up to $12.0 million that was available to be drawn by December 31, 2012 upon meeting one of three stated financial and/or operational goals. The borrowings under the LSA were collateralized by a security interest in all of the Company's assets, excluding its intellectual property. The first tranche, which was paid in full as of June 30, 2015, had an interest-only period of twelve months followed by a principal and interest amortization period of 30 months with interest being charged at 8.25% per year. The second tranche, which was paid in full as of December 31, 2015, had an interest-only period of six months followed by a principal and interest amortization period of 32 months with interest being charged at 8.25% . There were certain fees in accordance with the LSA which were recorded as discounts or short-term liabilities depending on the nature of the fees and accreted through interest expense over the life of the loans. As of December 31, 2015, the LSA was paid in full and no amounts remain outstanding.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Leases The Company leases its facilities and certain office equipment under long-term non-cancelable operating leases that expire at various dates through 2021. The Company has the following minimum rental payments under non-cancelable operating lease obligations that existed at December 31, 2017 (in thousands): Years Ending December 31, Minimum Rental Payment 2018 $ 735 2019 711 2020 720 2021 182 Total future minimum rental payments $ 2,348 Rent expense under non-cancelable operating leases and other month-to-month equipment rental agreements, including common area maintenance fees, totaled approximately $0.5 million , $0.7 million , and $0.6 million for the years ended December 31, 2017 , 2016 and 2015 , respectively. Sublease The Company subleases 3,537 square feet of its office space under a non-cancelable operating lease that expires February 2021. Total future minimum rentals under the non-cancelable operating sublease as of December 31, 2017 are presented below (in thousands): Years Ending December 31, Minimum Sublease Rentals 2018 $ 75 2019 78 2020 81 2021 14 Total future minimum sublease rentals $ 248 Significance of Revenue Source The Company is the recipient of federal research contract funds from BARDA. Periodic audits are required under the grant and contract agreements and certain costs may be questioned as appropriate under the agreements. Management believes that such amounts in the current year, if any, are not significant. Accordingly, no provision for refundable amounts under the agreements has been made as of December 31, 2017 and 2016 . Claims and Contingencies From time to time, the Company is subject to claims arising in the ordinary course of business. As of December 31, 2017 , the Company has accrued a potential liability of $1 million to a former director related to an indemnification claim.

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit)	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Stockholders' Equity (Deficit)	Stockholders’ Equity (Deficit) Common Stock The Company’s common stock consists of 200 million authorized shares at December 31, 2017 and 2016 , and 47.5 million and 46.5 million shares issued and outstanding at December 31, 2017 and December 31, 2016 , respectively. On June 16, 2015, the Company completed an underwritten public offering of 4,341,250 shares of common stock, including 566,250 shares sold pursuant to the full exercise of an option granted to the underwriters to purchase additional shares of common stock. All of the shares were offered by the Company at a price to the public of $39.75 per share. The net proceeds from this offering, after deducting underwriting discounts and commissions and other offering expenses payable by the Company, were approximately $161.9 million . Shares Reserved for Future Issuance The Company has reserved shares of common stock for future issuances as follows: December 31, 2017 2016 For exercise of common stock warrants 227,794 227,794 For exercise of outstanding common stock options 4,996,661 4,342,466 For delivery upon vesting of outstanding restricted stock units 956,299 946,200 For future equity awards under the 2013 Equity Incentive Plan 1,082,608 662,180 For future purchases under the 2013 Employee Stock Purchase Plan 1,861,472 1,612,759 Total shares of common stock reserved for future issuances 9,124,834 7,791,399 Stock Options In connection with the Company’s IPO, the Company adopted the 2013 Equity Incentive Plan (the 2013 Plan). The 2013 Plan provides for the grant of incentive stock options (ISOs), nonstatutory stock options (NSOs), stock appreciation rights, restricted stock awards, restricted stock unit (RSU) awards, performance-based stock awards, and other forms of equity compensation (collectively, stock awards), all of which may be granted to employees, including officers, non-employee directors and consultants of the Company and its affiliates. Additionally, the 2013 Plan provides for the grant of performance cash awards. ISOs may be granted only to employees. All other awards may be granted to employees, including officers, and to non-employee directors and consultants. Initially, the aggregate number of shares of common stock that may be issued pursuant to stock awards under the 2013 Plan was the sum of (i) 1,408,450 shares, plus (ii) 244,717 shares, which was the number of shares reserved for issuance under the Company’s 2012 Equity Incentive Plan (the 2012 Plan) at the time the 2013 Plan became effective, plus (iii) any shares subject to outstanding stock options or other stock awards that would have otherwise returned to the 2012 Plan (such as upon the expiration or termination of a stock award prior to vesting). Additionally, the number of shares of common stock reserved for issuance under the 2013 Plan will automatically increase on January 1 of each year, beginning on January 1, 2014 and continuing through and including January 1, 2023, by 2.5% of the total number of shares of capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s board of directors. The maximum number of shares that may be issued upon the exercise of ISOs under the 2013 Plan is 2,816,901 shares. Following the effectiveness of the 2013 Plan in April 2013, no further grants were made under the 2012 Plan. At the Company's annual meeting held on June 20, 2014, shareholders approved a change to the annual automatic increase in the number of common shares to be reserved for issuance under the 2013 Plan by changing the percentage increase to 4.0% , or a lesser number of shares determined by the Company's board of directors. The Company estimates the fair value of its share-based awards to employees, directors and consultants using the Black-Scholes option-pricing model. The Black-Scholes model requires the input of highly complex and subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the Company’s limited operating history and historical and implied volatility data, the Company has based its estimates of expected volatility on a blend of Company specific historical data and a group of similar public traded companies. When selecting these public companies on which it has based its expected stock price volatility, the Company selected companies with comparable characteristics to it, including enterprise value, risk profiles, positions within the industry, and with historical share price information sufficient to meet the expected life of its stock options. For employee stock options, the Company uses historical exercise data to estimate the expected life. The risk-free interest rates for the periods within the expected life of the option are based on the U.S. Treasury instrument with a life that is similar to the expected life of the option grant. The Company has never paid, and does not expect to pay, dividends in the foreseeable future. The following table illustrates the assumptions for the Black-Scholes model used in determining the fair value of the stock options granted: Years Ended December 31, 2017 2016 2015 Expected volatility 85.51 % 85.16 % 66.89 % Expected term (in years) 5.9 6.0 6.0 Weighted-average risk-free interest rate 2.02 % 1.70 % 1.53 % Expected dividend yield — % — % — % Weighted-average fair value per option $ 3.71 $ 5.62 $ 25.18 A summary of activity related to the Company’s stock options is as follows: Number of Options Weighted-Average Weighted-Average Total Intrinsic Value Balance, December 31, 2015 2,746,395 $ 28.19 8.41 Granted 2,418,551 7.76 — Exercised (48,441 ) 3.48 — Forfeited (774,039 ) 24.13 — Balance, December 31, 2016 4,342,466 $ 17.81 8.09 Granted 928,816 5.17 — Exercised (38,885 ) 3.98 — Forfeited (235,736 ) 19.10 — Balance, December 31, 2017 4,996,661 $ 15.51 7.59 $ 529,145 Exercisable at December 31, 2017 3,021,179 $ 18.05 7.14 $ 516,430 Vested or expected to vest at December 31, 2017 4,934,708 $ 15.56 7.58 $ 527,433 As of December 31, 2017 , there was approximately $14.1 million of total unrecognized compensation cost related to non-vested share-based compensation arrangements granted under the 2013 Plan. That compensation cost is expected to be recognized over a weighted-average period of approximately 1.64 years . Other information regarding the Company’s stock options is as follows (in thousands, except per share data): Years Ended December 31, 2017 2016 2015 Weighted-average grant-date fair value per share of options granted $ 3.71 $ 5.62 $ 25.18 Total intrinsic value of options exercised $ 48 $ 119 $ 10,139 Total fair value of shares vested $ 11,786 $ 13,330 $ 11,498 The following table summarizes, at December 31, 2017 , by price range: (1) for stock option awards outstanding under the 2013 Plan, the number of stock option awards outstanding, their weighted-average remaining life and their weighted-average exercise price; and (2) for stock option awards exercisable under the 2013 Plan, the number of stock option awards exercisable and their weighted-average exercise price: Outstanding Exercisable Range Number Weighted-Average Remaining Contractual Life (in years) Weighted-Average Exercise Price Number Weighted-Average Exercise Price $1.53 to 7.57 1,324,631 8.10 $ 4.65 631,032 $ 4.11 7.58 to 8.06 1,885,674 8.02 8.06 907,919 8.06 8.07 to 18.75 381,805 6.05 17.73 373,743 17.72 18.76 to 39.17 564,854 6.56 25.39 492,529 25.09 39.18 to 53.74 839,697 7.21 41.70 615,956 41.61 $1.53 to 53.74 4,996,661 7.59 $ 15.51 3,021,179 $ 18.05 Employee Stock Purchase Plan In February 2013, the Company’s board of directors adopted the 2013 Employee Stock Purchase Plan (ESPP), which was subsequently ratified by stockholders and became effective in April 2013. The purpose of the ESPP is to retain the services of new employees and secure the services of new and existing employees while providing incentives for such individuals to exert maximum efforts toward the Company’s success and that of its affiliates. The ESPP initially authorized the issuance of 704,225 shares of common stock pursuant to purchase rights granted to the Company’s employees or to employees of any of its designated affiliates. The number of shares of common stock reserved for issuance will automatically increase on January 1 of each calendar year, from January 1, 2014 through January 1, 2023 by the least of (a) 1% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, (b) 422,535 shares, or (c) a number determined by the Company’s board of directors that is less than (a) and (b). The ESPP is intended to qualify as an “employee stock purchase plan” within the meaning of Section 423 of the Internal Revenue Code of 1986. The common stock reserved for future issuance under the ESPP was automatically increased by an additional 422,535 shares on January 1, 2016 and 2017, bringing the total number of shares of common stock that may be purchased under the ESPP to 1,803,726 and 2,226,261 , respectively. The Company has reserved a total of 2,226,261 shares of common stock to be purchased under the ESPP, of which 1,861,472 and 1,612,759 shares remained available for purchase at December 31, 2017 and 2016 , respectively. Eligible employees may authorize an amount up to 15% of their salary to purchase common stock at the lower of a 15% discount to the beginning price of their offering period or a 15% discount to the ending price of each six -month purchase interval. The ESPP also provides for an automatic reset feature to start participants on a new twenty-four month participation period in the event that the common stock market value on a purchase date is less than the common stock value on the first day of the twenty-four month offering period. The Company issued 173,822 and 108,109 shares of common stock pursuant to the ESPP for the year ended December 31, 2017 and 2016 , respectively. Compensation expense for purchase rights under the ESPP related to the purchase discount and the “look-back” option were determined using a Black-Scholes option pricing model. The following table illustrates the assumptions for the Black-Scholes model used in determining the fair value of the ESPP purchase rights: Years Ended December 31, 2017 2016 2015 Expected volatility 77.18 % 111.57 % 57.77 % Expected term (in years) 0.97 1.37 1.1 Weighted-average risk-free interest rate 0.99 % 0.75 % 0.43 % Expected dividend yield — % — % — % Weighted-average option value per share $ 2.65 $ 3.20 $ 22.10 As of December 31, 2017 , the Company had a liability of $0.2 million representing employees' contributions to the ESPP. Restricted Stock Units For the years ended December 31, 2017 and 2016 , the Company issued RSUs to certain employees which vest based on service criteria. When vested, the RSU represents the right to be issued the number of shares of the Company's common stock that is equal to the number of RSUs granted. The grant date fair value for RSUs is based upon the market price of the Company's common stock on the date of the grant. The fair value is then amortized to compensation expense over the requisite service period or vesting term. For the years ended December 31, 2017 and 2016 , the Company issued 744,450 and 203,400 shares of common stock pursuant to the vesting of RSUs, respectively. In January 2017, the Company also granted performance-based RSUs which, when vested, represent the right to be issued the number of shares of the Company's common stock that is equal to the number of RSUs granted. The grant date fair value for performance-based RSUs is based upon the market price of the Company's common stock on the date of the grant. For the portion of the performance-based RSUs of which the achievement of the performance condition is considered probable, the Company recognizes stock-based compensation expense on the related estimated fair value of such RSUs ratably for each vesting tranche from the service inception date to the end of the requisite service period. For the performance conditions that are not considered probable of achievement at the grant date or upon quarterly re-evaluation, prior to the event actually occurring, the Company begins recognizing the related stock-based compensation expense ratably when the event occurs or when the Company can determine that achievement of the performance condition is probable. In those cases, the Company recognizes the change in estimate at the time it determines the performance condition is probable of achievement (by recognizing stock-based compensation expense as cumulative catch-up adjustment as if the Company had estimated at the grant date that the performance condition would have been achieved) and recognize the remaining compensation cost through the end of the requisite service period. The Company issued no shares of common stock pursuant to the vesting of performance-based RSUs for the year ended December 31, 2017 . A summary of activity related to the Company’s RSUs is as follows: Number of Restricted Weighted-Average Grant-Date Fair Value Balance, December 31, 2016 946,200 $ 4.91 Granted 879,300 5.12 Share issuance (744,450 ) 4.91 Forfeited (124,751 ) 5.09 Balance, December 31, 2017 956,299 $ 5.08 The total unrecognized compensation cost related to the non-vested RSUs as of December 31, 2017 was $3.5 million and will be recognized over a weighted average period of approximately 2.54 years . Warrants In February 2011, the Company issued warrants to purchase an aggregate of 5,501,215 shares of Series F redeemable convertible preferred stock at an exercise price of $2.045 per share. Upon the completion of the Company’s IPO in April 2013, these warrants were converted into warrants to purchase 1,549,628 shares of common stock at an exercise price of $7.26 per share. The warrants were exercisable at any time and expired on February 7, 2018. At December 31, 2017 and 2016 , warrants for the purchase of 227,794 shares of common stock were issued, outstanding and exercisable. Stock-based Compensation For awards with only service conditions and graded-vesting features, the Company recognizes compensation expense on a straight-line basis over the requisite service period. Total stock-based compensation expense was as follows (in thousands): Years Ended December 31, 2017 2016 2015 Income Statement Classification: Research and development expense $ 7,047 $ 7,137 $ 5,578 General and administrative expense 9,063 9,086 7,381 Total stock-based compensation expense $ 16,110 $ 16,223 $ 12,959 Cash received from exercises under all share-based payment arrangements for 2017 , 2016 and 2015 was $0.8 million , $0.6 million and $3.2 million , respectively. There was no actual tax benefit realized for the tax deductions from exercises of the share-based payment arrangements during 2017 , 2016 or 2015 .

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes No income tax expense or benefit has been recorded for the years ended December 31, 2017 , 2016 or 2015 . This is due to the establishment of a valuation allowance against the deferred tax assets generated during those periods. At December 31, 2017 , the Company has concluded that it is more likely than not that the Company may not realize the benefit of its deferred tax assets due to its history of losses. Accordingly, the net deferred tax assets have been fully reserved. A reconciliation of the difference between the benefit for income taxes and income taxes at the statutory U.S. federal income tax rate is as follows for the years ended December 31, 2017 , 2016 , and 2015 (in thousands, except percentages): 2017 2016 2015 Amount % of Pretax Amount % of Pretax Amount % of Pretax Income tax benefit at statutory rate $ (24,134 ) 34.0 % $ (25,973 ) 34.0 % $ (39,907 ) 34.0 % State income taxes (1,090 ) 1.5 % (1,544 ) 2.0 % (2,176 ) 1.9 % Research and development credits (2,039 ) 2.9 % (2,691 ) 3.5 % (5,698 ) 4.9 % Foreign rate differential 60 (0.1 )% (2 ) — % 2 — % Permanent items 1,646 (2.3 )% 2,537 (3.3 )% 3,687 (3.1 )% Provision to return adjustments 1,212 (1.7 )% 259 (0.3 )% (426 ) 0.2 % Effect of change in federal tax rate 57,950 (81.6 )% — — % — — % Effect of change in state tax rate 193 (0.3 )% 1,585 (2.1 )% 932 (0.8 )% Removal of excess tax benefit (12,930 ) 18.2 % — — % — — % Increase in unrecognized tax benefits 403 (0.6 )% 444 (0.6 )% 950 (0.8 )% Change in valuation allowance (21,271 ) 30.0 % 25,385 (33.2 )% 42,636 (36.3 )% Net benefit $ — — % $ — — % $ — — % The components of deferred tax assets and liabilities at December 31, 2017 and 2016 were as follows (in thousands): December 31, 2017 2016 Deferred tax assets: Domestic net operating loss carryforwards $ 92,020 $ 114,111 Foreign net operating loss carryforwards 61 — Research and development expenses 763 813 Capitalized Section 174 expenses 28 48 Research and development credits 12,437 10,907 Accrued bonuses 777 1,006 Share-based compensation 6,156 7,214 Other 983 460 Total gross deferred tax assets 113,225 134,559 Valuation allowance (113,225 ) (134,496 ) Total deferred tax assets — 63 Deferred tax liabilities: Other — (63 ) Total deferred tax liabilities — (63 ) Total deferred tax assets and liabilities, net $ — $ — At December 31, 2017 , the Company had net operating loss carryforwards for federal, state, and foreign tax purposes of approximately $408.1 million , $319.9 million and $0.4 million , respectively. At December 31, 2016 , the Company had net operating loss carryforwards for federal and state tax purposes of approximately $356.1 million and $287.2 million , respectively. The federal losses begin to expire in 2020 and the state losses begin to expire in 2018 . The foreign losses do not expire. In addition, the Company has tax credit carryforwards for federal tax purposes of approximately $16.6 million as of December 31, 2017 , which begin to expire in 2022 . The future utilization of net operating loss and tax credit carryforwards may be limited due to changes in ownership. Management has recorded a valuation allowance for all of the deferred tax assets due to the uncertainty of future taxable income. The Company incorporated a subsidiary in the United Kingdom in 2014. However, the subsidiary has had minimal activity since inception. The subsidiary recorded a net loss as of December 31, 2017 and a small amount of net income as of December 31, 2016 and as such, has no undistributed earnings. In general, if the Company experiences a greater than 50% aggregate change in ownership of certain significant stockholders over a three -year period (a Section 382 ownership change), utilization of its pre-change net operating loss carryforwards is subject to an annual limitation under Section 382 of the Internal Revenue Code (and similar state laws). The annual limitation generally is determined by multiplying the value of the Company’s stock at the time of such ownership change (subject to certain adjustments) by the applicable long-term tax-exempt rate. Such limitations may result in expiration of a portion of the net operating loss carryforwards before utilization and may be substantial. The ability of the Company to use its net operating loss carryforwards may be limited or lost if the Company experiences a Section 382 ownership change in connection with offerings or as a result of future changes in its stock ownership. Losses from a specific period may be subject to multiple limitations, and would generally be limited by the lowest of those limitations. The Company has determined that a Section 382 ownership change occurred in 2002, and as such, losses incurred prior to that date are subject to an annual limitation of at least $64,000 . Additionally, the Company has determined that a Section 382 ownership change occurred in 2007, and as such, losses incurred prior to that date are subject to an annual limitation of at least $762,000 . The Company evaluated Section 382 ownership changes subsequent to 2007 through September 30, 2015 and concluded that a Section 382 ownership change occurred in 2013 as a result of the initial public offering. As such, losses incurred prior to that date are subject to an annual limitation of at least $6.7 million . As of December 31, 2017 , the Company has adopted ASU 2016-09 which is effective for public companies for annual periods beginning after December 15, 2016. The ASU requires all excess tax benefits and tax deficiencies to be recognized as income tax expense or benefit in the income statement in the year in which they occur. As such, the Company has grossed up its net operation loss deferred tax asset to include all excess tax benefits as of December 31, 2017 . The Company has determined that there may be a future limitation on the Company’s ability to utilize its entire federal R&D credit carryover. Therefore, the Company recognized an uncertain tax benefit associated with the federal R&D credit carryover during the years ended December 31, 2017 and 2016 , as follows (in thousands): Balance at December 31, 2015 $ 1,956 Increases related to 2016 444 Increases related to prior periods — Balance at December 31, 2016 2,400 Increases related to 2017 403 Increases related to prior periods 473 Balance at December 31, 2017 $ 3,276 The change in the amount of the unrecognized tax benefits accrued for prior years is due to the change in the federal tax rate and is reflected as a component of such in the statutory rate reconciliation. The Company has determined that it had no other material uncertain tax benefits for the year ended December 31, 2017 . As of January 1, 2018, due to the carry forward of unutilized net operating losses and research and development credits, the Company is subject to U.S. Federal and state income tax examinations for the tax years 2000 through 2017 . The Company recognizes accrued interest related to unrecognized tax benefits in interest expense and penalties in operating expense. No amounts were accrued for the payment of interest and penalties at December 31, 2017 . On December 22, 2017, the Tax Cuts and Jobs Act was enacted into law, which reduced the federal corporate income tax rate to 21% for tax years beginning after December 31, 2017 . As a result of the new enacted tax rate, the Company adjusted its deferred tax assets as of December 31, 2017 by applying the new 21% rate, which resulted in a decrease to the deferred tax assets and a corresponding decrease to the valuation allowance of approximately $58 million . The Tax Legislation also implements a territorial tax system. Under the territorial tax system, in general, the Company's foreign earnings will no longer be subject to tax in the U.S. As part of transition to the territorial tax system the Tax Legislation includes a mandatory deemed repatriation of all undistributed foreign earnings that are subject to a U.S. income tax. The Company estimates that the deemed repatriation will not result in an additional U.S. income tax liability as it has no undistributed foreign earnings. The SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (SAB 118) which will allow the Company to record provisional amounts during a measurement period which is similar to the measurement period used when accounting for business combinations. Provisional amounts have been recorded related to deferred taxes for stock compensation and the deferred rate change. The Company will continue to assess the impact of the recently enacted tax law on its business and consolidated financial statements.

Significant Agreements

Significant Agreements	12 Months Ended
	Dec. 31, 2017
Revenue Recognition [Abstract]
Significant Agreements	Significant Agreements The Regents of the University of California In May 2002, the Company entered into a license agreement with The Regents of the University of California (UC) under which the Company obtained an exclusive, worldwide license to UC’s patent rights in certain inventions (the UC Patent Rights) related to lipid-conjugated antiviral compounds and their use, including certain patents relating to brincidofovir. The license agreement was amended in September 2002 in order to expand the scope of the license and again in December 2010 in order to modify certain financial terms. The agreement was amended a third time in September 2011 to add additional patents related to certain metabolically stable lipid-conjugate compounds. A fourth amendment was executed in July 2012 to alter the rights and obligations of the parties in light of the Company’s current business plans. Under the license agreement, the Company is permitted to research, develop, manufacture and commercialize products utilizing the UC Patent Rights for all human and veterinary uses, and to sublicense such rights. UC retained the right, on behalf of itself and other non-profit institutions, to use the UC Patent Rights for educational and research purposes and to publish information about the UC Patent Rights. In consideration for the rights granted under the license agreement, the Company has issued UC an aggregate of 64,788 shares of common stock. As additional consideration, the Company is required to pay certain cash milestone payments in connection with the development and commercialization of compounds that are covered by the UC Patent Rights, plus certain annual fees to maintain such patents until the Company commercializes a product utilizing UC Patent Rights. In connection with the development and commercialization of brincidofovir and CMX157, the Company could be required to pay UC up to an aggregate of $3.4 million in milestone payments, assuming the achievement of all applicable milestone events under the license agreement. In addition, upon commercialization of any product utilizing the UC Patent Rights (which would include the commercialization of brincidofovir), the Company will be required to pay low single digit royalties on net sales of such product. The license agreement requires that we diligently develop, manufacture and commercialize compounds that are covered by the UC Patent Rights, and we have agreed to meet certain development and commercialization milestones. UC may, at its option, either terminate the license agreement or change the license granted from an exclusive license to a non-exclusive license if we fail to meet such development and commercialization milestones. We are currently in compliance with these milestone requirements. In the event the Company sublicenses a UC Patent Right (including UC Patent Rights relating to brincidofovir or CMX157) the Company is obligated to pay to UC a fee, which amount will vary depending upon the amount of any payments the Company receives and the clinical development stage of the compound being sublicensed, but which could be up to approximately 50% of the sublicense fee in certain circumstances. With respect to brincidofovir, the fee payable to UC will not exceed 5% of the sublicense fee. In addition, the Company will also be required to pay to UC a low single digit sublicense royalty on net sales of products that use the sublicensed UC Patent Rights, but in no event will the Company be required to pay more than 50% of the royalties it receives in connection with the relevant sublicense. Any such royalty payment will be reduced by other payments the Company is required to make to third parties until a minimum royalty has been reached. Biomedical Advanced Research and Development Authority (BARDA) In February 2011, the Company entered into a contract with BARDA for the advanced development of brincidofovir as a medical countermeasure in the event of a smallpox release. Under the contract, BARDA will reimburse the Company, plus pay a fixed fee, for the research and development of brincidofovir as a broad-spectrum therapeutic antiviral for the treatment of smallpox infections. The contract consists of an initial performance period, referred to as the base performance segment, plus up to four extension periods, referred to as option segments, each of which may be exercised at BARDA’s sole discretion. The Company must complete the agreed upon milestones and deliverables in each discrete work segment before the next option segment is eligible to be exercised. Under the contract as currently in effect, the Company may receive up to $75.8 million in expense reimbursement and $5.3 million in fees. The Company is currently performing under the second and third option segments of the contract during which the Company may receive up to a total of $21.6 million and $11.6 million in expense reimbursement and fees, respectively. The second and third option segments are scheduled to end on September 30, 2018. As of December 31, 2017 , the Company has recognized revenue in aggregate of $56.1 million with respect to the base performance segment and the first three extension periods. ContraVir Pharmaceuticals On December 17, 2014, the Company entered into a license agreement with ContraVir Pharmaceuticals (NASDAQ: CTRV) for the development and commercialization of CMX157 for certain antiviral indications. Under the terms of the agreement, ContraVir has sole responsibility with respect to the control of the development and commercialization of CMX157. In exchange for the license to CMX157 rights, the Company received an upfront payment consisting of 120,000 shares of ContraVir Series B Convertible Preferred Stock with a stated value of $1.2 million . In addition, the Company is eligible to receive up to approximately $20 million in clinical, regulatory and initial commercial milestones in the United States and Europe, as well as royalties and additional milestones based on commercial sales in those territories. Either party may terminate the license agreement upon the occurrence of a material breach by the other party (subject to standard cure periods), or upon certain events involving the bankruptcy or insolvency of the other party. ContraVir may also terminate the license agreement without cause on a country-by-country basis upon sixty days’ prior written notice. The upfront payment of 120,000 shares of ContraVir Series B Convertible Preferred Stock was valued at $1.5 million at the time of the agreement. The Company recorded this amount as a long-term investment and deferred revenue, which is included in accrued liabilities in the Consolidated Balance Sheets. Upon completion of the transfer of the IND and technical know-how related to CMX157 in April 2015, the Company recognized the $1.5 million upfront payment as revenue. In September 2016, the Company converted its shares of ContraVir Series B convertible preferred stock into 1,071,429 shares of ContraVir common stock. As of December 31, 2017 and 2016 , the fair value of the investment was recorded as a short-term investment of $0.4 million and $1.3 million , respectively. University of Michigan In 2006, the Company entered into a license agreement with The Regents of the University of Michigan (UM) under which the Company obtained an exclusive, worldwide license to UM’s patent rights in certain inventions (UM Patent Rights) related to certain compounds originally synthesized at UM. Under the license agreement, the Company is permitted to research, develop, manufacture and commercialize products utilizing the UM Patent Rights, and to sublicense such rights subject to certain sublicensing fees and royalty payments. In consideration for the rights granted to the Company, under the license agreement as amended in December 2016, the Company paid UM $50,000 in fees in 2016 and in January 2017 issued UM an aggregate of 33,058 shares of its common stock. In connection with the Company's commercialization or sublicensing of certain products covered by the license agreement, including CMX521, the Company could be required to pay royalties on net sales of such products ranging from 0.25% to 2% . Beginning in 2024, the Company is also subject to certain minimum annual royalty payments. The UM license agreement requires that the Company uses commercially reasonable efforts to develop and make commercially available licensed products as soon as practicable. Specifically, the Company has agreed to make the first commercial sale of a licensed product by June of 2026. UM may terminate the license agreement if the Company materially breaches the license agreement. The Company is currently in compliance with its milestone requirements.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Selected Quarterly Financial Data (Unaudited)	Selected Quarterly Financial Data (Unaudited) The following financial information reflects all normal recurring adjustments, which are, in the opinion of management, necessary for a fair statement of the results of the interim periods. Summarized quarterly data for 2017 and 2016 are as follows (in thousands, except share and per share data): 2017 Quarters Fourth Third Second First Total revenues $ 1,844 $ 897 $ 675 $ 1,078 Operating loss (18,687 ) (17,910 ) (17,245 ) (18,260 ) Net loss (19,238 ) (17,312 ) (16,680 ) (17,754 ) Net loss per share, basic and diluted $ (0.41 ) $ (0.37 ) $ (0.36 ) $ (0.38 ) Weighted-average shares outstanding, basic and diluted 47,341,271 47,065,756 46,863,753 46,573,394 2016 Quarters Fourth Third Second First Total revenues $ 1,980 $ 653 $ 1,841 $ 1,228 Operating loss (15,373 ) (17,422 ) (18,525 ) (26,632 ) Net loss (14,957 ) (17,025 ) (18,148 ) (26,260 ) Net loss per share, basic and diluted $ (0.32 ) $ (0.37 ) $ (0.39 ) $ (0.57 ) Weighted-average shares outstanding, basic and diluted 46,431,809 46,236,749 46,214,086 46,184,134 Net loss per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per share calculations will not necessarily equal the annual per share calculation. Diluted weighted-average shares outstanding are identical to basic weighted-average shares outstanding and diluted net loss per share is identical to basic net loss per share for all quarters of 2017 and 2016 .

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events The Company has evaluated subsequent events through the issuance date of these financial statements to ensure that this filing includes appropriate disclosure of events both recognized in the financial statements as of December 31, 2017 , and events which occurred subsequently but were not recognized in the financial statements.

The Business and Summary of S18

The Business and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The consolidated financial statements include the accounts of the Company, and its wholly owned subsidiaries. The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (GAAP). The preparation of the Company’s consolidated financial statements requires estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and the disclosure of contingent assets and liabilities in the consolidated financial statements and accompanying notes. Although these estimates are based on knowledge of current events and actions the Company may undertake in the future, actual results may ultimately differ from these estimates and assumptions.
Reclassifications	Reclassifications Certain prior period amounts in the accompanying consolidated financial statements have been reclassified to conform to the current year presentation. These reclassifications had no effect on previously reported net income or stockholders' equity (deficit).
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers any highly liquid instrument with an original maturity of three months or less at acquisition to be a cash equivalent. Cash equivalents consist of money market funds and commercial paper.
Investments and Other-than-temporary Impairment of Investment	Investments Investments consist primarily of brokered certificates of deposit, U.S. Treasury securities and stock of a U.S. corporation. The Company invests in high-credit quality investments in accordance with its investment policy which minimizes the probability of loss. Available-for-sale securities are carried at fair value as determined by quoted market prices, with the unrealized gains and losses, net of tax, reported as a separate component of stockholders equity. Realized gains and losses are determined using the specific identification method and transactions are recorded on a settlement date basis in interest income or expense, net. Investments with original maturities beyond three months at the date of purchase and which mature on, or less than twelve months from, the balance sheet date are classified as short-term. Investments with a maturity beyond twelve months from the balance sheet date are classified as long-term. The Company periodically reviews available-for-sale securities for other-than-temporary declines in fair value below the cost basis and whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. The Company evaluates, among other things, the duration and extent to which the fair value of a security is less than its cost; the financial condition of the issuer and any changes thereto; and the Company’s intent to sell, or whether it will more likely than not be required to sell, the security before recovery of its amortized cost basis. The Company does not intend to sell, and is not likely to be required to sell, the available-for-sale securities in an unrealized loss position before recovery of the amortized cost bases of the securities, which may be maturity. Any such declines in value judged to be other-than-temporary on available-for-sale securities are reported in other-than-temporary impairment of investment. For the year ended December 31, 2017, the Company determined the decline in value for its investment in ContraVir Pharmaceuticals common stock to be other-than temporary. As such, during the fourth quarter of 2017, the Company reclassified a loss of $1.2 million from accumulated other comprehensive loss, net in the Consolidated Balance Sheets to other-than-temporary impairment of investment in the Consolidated Statements of Operations and Comprehensive Loss. There were no such declines in value for the years ended December 31, 2016 and 2015 . The Company recognizes interest income on an accrual basis in interest income, net in the Consolidated Statements of Operations and Comprehensive Loss. Other-than-temporary Impairment of Investment Other-than-temporary impairment of investment includes write-downs in fair value of available-for-sale securities judged to be other-than-temporarily impaired.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk consist of cash, cash equivalents, short-term investments, long-term investments and accounts receivable. The Company is exposed to credit risk, subject to federal deposit insurance, in the event of default by the financial institutions holding its cash and cash equivalents to the extent of amounts recorded on the balance sheets. Accounts receivable represent amounts due from an agency of the federal government.
Accounts Receivable	Accounts Receivable Accounts receivable at December 31, 2017 and December 31, 2016 consisted of amounts billed under the Company’s contract with the Biomedical Advanced Research and Development Authority (BARDA). Receivables under the BARDA contract are recorded as qualifying research activities are conducted and invoices from the Company’s vendors are received. The Company carries its accounts receivable at cost less an allowance for doubtful accounts. On a periodic basis, the Company evaluates its accounts receivable and establishes an allowance based on its history of collections and write-offs and the current status of all receivables. The Company does not accrue interest on trade receivables. If accounts become uncollectible, they will be written off through a charge to the allowance for doubtful accounts. The Company has not recorded a charge to allowance for doubtful accounts as management believes all receivables are fully collectible.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The carrying amounts of certain financial instruments, including accounts receivable, accounts payable and accrued expenses approximate their fair values due to the short-term nature of such instruments. For assets and liabilities recorded at fair value, it is the Company’s policy to maximize the use of observable inputs and minimize the use of unobservable inputs when developing fair value measurements, in accordance with the fair value hierarchy. Fair value measurements for assets and liabilities where there exists limited or no observable market data are based primarily upon estimates and are often calculated based on the economic and competitive environment, the characteristics of the asset or liability and other factors. Therefore, fair value measurements cannot be determined with precision and may not be realized in an actual sale or immediate settlement of the asset or liability. Additionally, there may be inherent weaknesses in any calculation technique and changes in the underlying assumptions used, including discount rates and estimates of future cash flows, could significantly affect the calculated current or future fair values. The Company utilizes fair value measurements to record fair value adjustments to certain assets and liabilities and to determine fair value disclosures. The Company groups assets and liabilities at fair value in three levels, based on the markets in which the assets and liabilities are traded and the reliability of the assumptions used to determine fair value. An adjustment to the pricing method used within either Level 1 or Level 2 inputs could generate a fair value measurement that effectively falls in a lower level in the hierarchy. These levels are: • Level 1 — Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access. • Level 2 — Valuations based on quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and models for which all significant inputs are observable, either directly or indirectly. • Level 3 — Valuations based on inputs that are unobservable and significant to the overall fair value measurement. The determination of where an asset or liability falls in the hierarchy requires significant judgment. The Company evaluates hierarchy disclosures and, based on various factors, it is possible that an asset or liability may be classified differently from period to period. However, the Company expects that changes in classification between levels will be rare. There were no transfers between Level 1 and Level 2 or transfers to or from Level 3 during the year ended December 31, 2017 . When the 18 month restriction on selling the Company's investment in ContraVir Pharmaceuticals ended on June 17, 2016, the investment was transferred from Level 3 to Level 2 as the fair value was based on quoted prices for similar assets and there were no significant unobservable inputs. The Company's investment in ContraVir Pharmaceuticals transferred from Level 2 to Level 1 when the Company converted its investment in Series B Preferred shares into common stock in September 2016. At December 31, 2017 and 2016 , the Company had cash equivalents, consisting of money market funds, and short-term and long-term investments consisting of U.S. Treasury securities, whose value is based on using quoted market prices. Accordingly, these securities are classified as Level 1. At December 31, 2017 , the Company had cash equivalents, consisting of commercial paper, and at December 31, 2016 , the Company had cash equivalents consisting of commercial paper, and short-term investments comprised of brokered certificates of deposits, for which quoted prices are not available that are valued using independent pricing models or other model-based valuation techniques such as the present value of future cash flows, adjusted for the security’s credit rating, prepayment assumptions and other factors such as credit loss assumptions. Accordingly, these securities are classified as Level 2. At December 31, 2015, the Company's preferred stock investment in ContraVir Pharmaceuticals was categorized as Level 3 as there were significant unobservable inputs. The valuation of the investment at December 31, 2015 was calculated on an as if converted to common share basis with a discount for lack of marketability applied due to the 18 month restriction from the date of the investment on selling the converted common shares, which ended on June 17, 2016. An option pricing model was used to determine the discount for lack of marketability of 10% at December 31, 2015. The key unobservable inputs used in the option pricing model at December 31, 2015 were (i) exercise price - $1.54 , (ii) dividend yield - 0% , (iii) expected holding period - 0.46 years , (iv) risk-free rate - 0.44% , and (v) volatility - 75% . On September 30, 2016, the Company converted its preferred stock investment in ContraVir into 1,071,429 shares of ContraVir common stock, which was categorized as a Level 1 asset and valued based on ContraVir's common stock value. The Company evaluates, among other things, the duration and extent to which the fair value of a security is less than its cost; the financial condition of the issuer and any changes thereto; and the Company’s intent to sell, or whether it will more likely than not be required to sell, the security before recovery of its market value. For the years ended December 31, 2016 and 2015 , the Company did not intend to sell, and was not more likely than not to be required to sell, its investment in ContraVir before recovery of its market value, therefore the changes in the fair value of ContraVir common shares was recorded to unrealized (loss) gain on investments, net in the Consolidated Statements of Operations and Comprehensive Loss. There was no material re-measurement to fair value of financial assets and liabilities that are not measured at fair value on a recurring basis.
Property and Equipment	Property and Equipment Property and equipment are stated at cost, less accumulated depreciation. Depreciation is determined on a straight-line basis over the estimated useful lives of the assets, which generally range from three to five years . Leasehold improvements are amortized over the shorter of the useful life of the asset or the term of the related lease . Maintenance and repairs are charged against expense as incurred.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company evaluates long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. If the estimated future cash flows (undiscounted and without interest charges) from the use of an asset are less than the carrying value, a write-down would be recorded to reduce the related asset to its estimated fair value.
Deferred Lease Obligation	Deferred Lease Obligations The Company recognizes rent expense on a straight-line basis over the non-cancelable term of its operating lease and records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. The Company also records landlord-funded lease incentives, such as reimbursable leasehold improvements, as a deferred rent liability, which is amortized as a reduction of rent expense over the non-cancelable term of its operating lease.
Revenue Recognition	Revenue Recognition The Company’s revenues generally consist of (i) contract and grant revenue – revenue generated under federal contracts and other awarded grants, and (ii) collaboration and licensing revenue – revenue related to non-refundable upfront fees, royalties and milestone payments earned under license agreements. Revenue is recognized in accordance with the criteria outlined in the Securities and Exchange Commission (SEC)’s Topic 13 and Accounting Standards Codification (ASC) 605-25 and by the Financial Accounting Standards Board. Following these accounting pronouncements, revenue is recognized when all four of the following criteria are met: (i) persuasive evidence of an arrangement exists; (ii) delivery of the products and/or services has occurred and risk of loss has passed; (iii) the selling price is fixed or determinable; and (iv) collectability is reasonably assured. For arrangements that involve the delivery of more than one element, each product, service and/or right to use assets is evaluated to determine whether it qualifies as a separate unit of accounting. This determination is based on whether the deliverable has “stand-alone value” to the customer. The consideration that is fixed or determinable is then allocated to each separate unit of accounting based on the relative selling prices of each deliverable. The consideration allocated to each unit of accounting is recognized as the related goods and services are delivered, limited to the consideration that is not contingent upon future deliverables. If the arrangement constitutes a single unit of accounting, the revenue recognition policy must be determined for the entire arrangement and the consideration received is recognized over the period of inception through the date the last deliverable within the single unit of accounting is expected to be delivered. Revisions to the estimated period of recognition are reflected in revenue prospectively. Non-refundable upfront fees are recorded as deferred revenue and recognized into revenue as license fees from collaborations on a straight-line basis over the estimated period of the Company’s substantive performance obligations. If the Company does not have substantive performance obligations, the Company recognizes non-refundable upfront fees into revenue through the date the deliverable is satisfied. Analyzing the arrangement to identify deliverables requires the use of judgment and each deliverable may be an obligation to deliver services, a right or license to use an asset, or another performance obligation. Milestone payments are recognized when earned, provided that (i) the milestone event is substantive; (ii) there is no ongoing performance obligation related to the achievement of the milestone earned; and (iii) it would result in additional payments. Milestone payments are considered substantive if all of the following conditions are met: the milestone payment is non-refundable; achievement of the milestone was not reasonably assured at the inception of the arrangement; substantive effort is involved to achieve the milestone; and the amount of the milestone appears reasonable in relation to the effort expended, the other milestones in the arrangement; and the related risk associated with the achievement of the milestone. Contingent based event payments the Company may receive under a license or collaboration agreement will be recognized when received. For the years ended December 31, 2017 , 2016 and 2015 , contract and grant revenue consisted only of revenue from the BARDA contract as there was no grant revenue. The Company recognizes contract and grant revenue as qualifying research activities are conducted based on invoices received from the Company’s vendors. Changes in fringe and indirect rates are recognized as a change in estimate in the period such rate changes are approved by BARDA. For the year ended December 31, 2015, collaboration and licensing revenue primarily consisted of the upfront license fee payment from ContraVir recognized when the Company completed its performance obligations.
Research and Development Prepaids and Accruals	Research and Development Prepaids and Accruals As part of the process of preparing financial statements, the Company is required to estimate its expenses resulting from its obligation under contracts with vendors and consultants and clinical site agreements in connection with its research and development efforts. The financial terms of these contracts are subject to negotiations which vary contract to contract and may result in payment flows that do not match the periods over which materials or services are provided to the Company under such contracts. The Company’s objective is to reflect the appropriate research and development expenses in its financial statements by matching those expenses with the period in which services and efforts are expended. The Company accounts for these expenses according to the progress of its research and development efforts. The Company determines prepaid and accrual estimates through discussion with applicable personnel and outside service providers as to the progress or state of communication of clinical trials, or other services completed. The Company adjusts its rate of research and development expense recognition if actual results differ from its estimates. The Company makes estimates of its prepaid and accrued expenses as of each balance sheet date in its financial statements based on facts and circumstances known at that time. Although the Company does not expect its estimates to be materially different from amounts actually incurred, its understanding of status and timing of services performed relative to the actual status and timing of services performed may vary and may result in the Company reporting amounts that are too high or too low for any particular period. Through December 31, 2017 , there had been no material adjustments to the Company’s prior period estimates of prepaid and accruals for research and development expenses. The Company’s research and development prepaids and accruals are dependent upon the timely and accurate reporting of contract research organizations and other third-party vendors.
Research and Development Expenses	Research and Development Expenses Major components of research and development costs include cash compensation, stock based compensation, pre-clinical studies, clinical trial and related clinical manufacturing, drug development, materials and supplies, legal, regulatory compliance, and fees paid to consultants and other entities that conduct certain research and development activities on the Company’s behalf. Research and development costs, including upfront fees and milestones paid to contract research organizations, are expensed as goods as received or services rendered. Costs incurred in connection with clinical trial activities for which the underlying nature of the activities themselves do not directly relate to active research and development, such as costs incurred for market research and focus groups linked to clinical strategy as well as costs to build the Company’s brand, are not included in research and development costs but are reflected as general and administrative costs.
Interest Income, Net	Interest Income, Net Interest income, net primarily includes interest earned on short-term and long-term investments, interest incurred on loans payable, the amortization of deferred financing costs related to fees paid to attorneys and other non-lender entities in order to acquire debt, and the amortization of debt discount related to fees paid to the lender in order to acquire debt.
Income Taxes	Income Taxes Deferred tax assets and liabilities are determined based on differences between the financial and tax reporting bases of assets and liabilities and are measured using enacted tax rates and laws that are expected to be in effect when the differences are expected to reverse. Valuation allowances are established when the Company determines that it is more likely than not that some portion of a deferred tax asset will not be realized. The Company has incurred operating losses from April 7, 2000 (inception) through December 31, 2017 , and therefore has not recorded any current provision for income taxes. Additionally, the Company recognizes the tax benefit from an uncertain tax position only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities based on the technical merits of the position. The tax benefit recognized in the financial statements for a particular tax position is based on the largest benefit that is more likely than not to be realized upon settlement. Accordingly, the Company establishes reserves for uncertain tax positions.
Share-Based Compensation	Share-Based Compensation The Company measures and recognizes compensation expense for all share-based payment awards made to employees and directors, including employee stock options, restricted stock units and the employee stock purchase plan purchase rights, based on estimated fair values. The fair value of employee stock options and employee stock purchase plan purchase rights is estimated on the grant date using the Black-Scholes valuation model. The grant-date fair value for restricted stock units is based upon the market price of the Company's common stock on the date of the grant. The value of the portion of the award that is ultimately expected to vest is recorded as expense over the requisite service periods. For performance-based awards compensation cost is recognized when it is probable that the performance criteria will be met. The Company estimates forfeitures at the time of grant, and revises those estimates in subsequent periods if actual forfeitures differ from its estimates. The Company uses historical data to estimate forfeitures and record share-based compensation expense only for those awards that are expected to vest. To the extent that actual forfeitures differ from the Company’s estimates, the difference is recorded as a cumulative adjustment in the period the estimates were revised. For the years ended December 31, 2017 , 2016 and 2015 , the Company applied a forfeiture rate based on the Company’s historical forfeitures.
401(k) Plan	401(k) Plan The Company maintains a defined contribution employee retirement plan (“401(k) plan”). In March 2015, the Company began making matching contributions into the 401(k) plan on behalf of participants.
Basic and Dilutive Net Loss Per Share of Common Stock	Basic and Dilutive Net Loss Per Share of Common Stock Basic net loss per share of common stock is computed by dividing net loss by the weighted-average number of shares of common stock outstanding during the period, excluding the dilutive effects of warrants to purchase common stock, non-vested restricted stock, stock options, and employee stock purchase plan purchase rights. Diluted net loss per share of common stock is computed by dividing net loss by the sum of the weighted-average number of shares of common stock outstanding during the period plus the potential dilutive effects of warrants to purchase common stock, non-vested restricted stock, stock options, and employee stock purchase plan purchase rights outstanding during the period calculated in accordance with the treasury stock method, but are excluded if their effect is anti-dilutive. Because the impact of these items is anti-dilutive during the periods of net loss, there was no difference between basic and diluted loss per share of common stock at December 31, 2017 , 2016 and 2015 .
Segments	Segments The Company operates in only one segment. The chief operating decision-maker, who is the Company’s Chief Executive Officer, and management use cash flows as the primary measure to manage the business and do not segment the business for internal reporting or decision making.
Impact of Recently Issued Accounting Standards and Impact of Recently Adopted Accounting Standards	Impact of Recently Issued Accounting Standards In May 2014, the FASB issued Accounting Standards Update (ASU) No. 2014-09, “Revenue from Contracts with Customers (Topic 606).” The ASU establishes a principles-based approach for accounting for revenue arising from contracts with customers and supersedes existing revenue recognition guidance. The ASU provides that an entity should apply a five-step approach for recognizing revenue, including (1) identify the contract with a customer; (2) identify the performance obligations in the contract; (3) determine the transaction price; (4) allocate the transaction price to the performance obligations in the contract; and (5) recognize revenue when, or as, the entity satisfies a performance obligation. Also, the entity must provide various disclosures concerning the nature, amount and timing of revenue and cash flows arising from contracts with customers. The FASB has issued several updates to the standard which (1) defer the original effective date to annual periods and interim periods within those annual periods beginning after December 15, 2017, while allowing for early adoption as of January 1, 2017 (ASU 2015-14); (2) clarify the application of the principal versus agent guidance (ASU 2016-08); and (3) clarify the guidance on inconsequential and perfunctory promises and licensing (ASU 2016-10). The Company has evaluated its contract with BARDA and license agreement with ContraVir, and the adoption of the ASU will not have a material impact on its consolidated financial statements. The Company plans to use the full retrospective method of adoption effective January 1, 2018. The Company is implementing changes to its accounting policies, internal controls and disclosures to support the new standard; however, these changes are not anticipated to be significant. In January 2016, the FASB issued ASU No. 2016-01, “Financial Instruments-Overall (Subtopic 825-10)-Recognition and Measurement of Financial Assets and Financial Liabilities.” The new standard enhances reporting for financial instruments. The ASU is effective for financial statements issued for annual periods and interim periods within those annual periods beginning after December 15, 2017. Earlier adoption is permitted for interim and annual reporting periods as of the beginning of the fiscal year of adoption. The Company does not expect the adoption of ASU 2016-01 to have a material impact on its consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842)”, which increases transparency and comparability among companies accounting for lease transactions. The most significant change of this update will require the recognition of lease assets and liabilities on the balance sheet for lessees for operating lease arrangements with lease terms greater than 12 months. This update will require a modified retrospective application which includes a number of optional practical expedients related to the identification and classification of leases commenced before the effective date. This ASU is effective for financial statements issued for annual periods and interim periods within those annual periods, beginning after December 18, 2018. The Company is currently analyzing the impact of the adoption of ASU No. 2016-02 on its consolidated financial statements. Impact of Recently Adopted Accounting Standards In March 2016, the FASB issued ASU No. 2016-09, "Improvements to Employee Share-Based Payment Accounting", which simplifies several aspects of accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The ASU was effective for financial statements issued for annual periods and interim periods within those annual periods, beginning after December 15, 2016, with early adoption permitted. ASU No. 2016-09 became effective for the Company beginning in the first quarter of 2017. The Company elected to continue estimating the number of awards expected to be forfeited and adjusting the estimate when it is no longer probable that the employee will fulfill the service condition. The Company's adoption of ASU No. 2016-09 did not have a material impact on its consolidated financial statements.

The Business and Summary of S19

The Business and Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Assets and Liabilities Measured at Fair Value on a Recurring Basis	Below is a table that presents information about certain assets measured at fair value on a recurring basis (in thousands): Fair Value Measurements December 31, 2017 Total Quoted Prices in Significant Other Significant Cash equivalents Money market funds $ 10,816 $ 10,816 $ — $ — Commercial paper 3,995 — 3,995 — Total cash equivalents 14,811 10,816 3,995 — Short-term investments U.S. Treasury securities 132,586 132,586 — — Common stock of U.S. corporation 386 386 — — Total short-term investments 132,972 132,972 — — Long-term investments U.S. Treasury securities 76,731 76,731 — — Total long-term investments 76,731 76,731 — — Total assets $ 224,514 $ 220,519 $ 3,995 $ — Fair Value Measurements December 31, 2016 Total Quoted Prices in Significant Other Significant Cash equivalents Money market funds $ 15,733 $ 15,733 $ — $ — Commercial paper 35,097 — 35,097 — Total cash equivalents 50,830 15,733 35,097 — Short-term investments Certificates of deposit 7,450 — 7,450 — U.S. Treasury securities 171,822 171,822 — — Common stock of U.S. corporation 1,286 1,286 — — Total short-term investments 180,558 173,108 7,450 — Long-term investments U.S. Treasury securities 47,407 47,407 — — Total long-term investments 47,407 47,407 — — Total assets $ 278,795 $ 236,248 $ 42,547 $ —
Reconciliation of Assets and Liabilities Measured at Fair Value on a Recurring Basis	Below is a table that presents a reconciliation of the beginning and ending balances of assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) (in thousands): Fair Value Measurements (Level 3) Preferred stock of U.S. corporation: Fair value at December 31, 2015 1,485 Fair value decrease recorded in other comprehensive loss (371 ) Fair value transferred to Level 2 (1,114 ) Fair value at December 31, 2016 $ —
Schedule of Prepaid and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2017 2016 Prepaid research and development expenses $ 1,138 $ 843 Interest receivable 601 772 Prepaid insurance 481 389 Other prepaid expenses and current assets 1,111 841 Total prepaid expenses and other current assets $ 3,331 $ 2,845
Schedule of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): December 31, 2017 2016 Accrued compensation $ 3,678 $ 2,906 Accrued research and development expenses 3,384 2,257 Accrued indemnification claim 1,000 — Other accrued liabilities 1,322 1,052 Total accrued liabilities $ 9,384 $ 6,215

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investments [Abstract]
Summary of Short-term and Long-term Investments	The following tables summarize the Company's short-term and long-term investments (in thousands): December 31, 2017 Amortized Cost Gross Unrealized Gross Unrealized Estimated U.S. Treasury securities $ 210,280 $ — $ (963 ) $ 209,317 Common stock of U.S. corporation 386 — — 386 Total investments $ 210,666 $ — $ (963 ) $ 209,703 December 31, 2016 Amortized Cost Gross Unrealized Gross Unrealized Estimated Certificates of deposit $ 7,445 $ 5 $ — $ 7,450 U.S. Treasury securities 219,415 15 (201 ) 219,229 Common stock of U.S. corporation 1,545 — (259 ) 1,286 Total investments $ 228,405 $ 20 $ (460 ) $ 227,965
Summary of Investments with Unrealized Losses, Aggregated by Investment Type and the Length of Time	The following tables summarize the Company's investments with unrealized losses, aggregated by investment type and the length of time that individual investments have been in a continuous unrealized loss position (in thousands, except number of securities): December 31, 2017 Less than 12 Months Greater than 12 Months Total Fair Value Unrealized Loss Fair Value Unrealized Loss Fair Value Unrealized Loss U.S. Treasury securities $ 170,390 $ (871 ) $ 38,927 $ (92 ) $ 209,317 $ (963 ) Total $ 170,390 $ (871 ) $ 38,927 $ (92 ) $ 209,317 $ (963 ) Number of securities with unrealized losses 39 7 46 December 31, 2016 Less than 12 Months Greater than 12 Months Total Fair Value Unrealized Loss Fair Value Unrealized Loss Fair Value Unrealized Loss U.S. Treasury securities $ 128,204 $ (201 ) $ — $ — $ 128,204 $ (201 ) Preferred stock of U.S. corporation 1,286 (259 ) — — 1,286 $ (259 ) Total $ 129,490 $ (460 ) $ — $ — $ 129,490 $ (460 ) Number of securities with unrealized losses 24 — 24
Summary of the scheduled maturity of Company investments	The following table summarizes the scheduled maturity for the Company's investments at December 31, 2017 (in thousands): December 31, 2017 Maturing in one year or less $ 132,586 Maturing after one year through two years 76,731 Total debt investments $ 209,317 Common stock of U.S. corporation 386 Total investments $ 209,703

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Summary of Property and Equipment, net	Property and equipment, net of accumulated depreciation consisted of the following (in thousands): December 31, 2017 2016 Lab equipment $ 2,496 $ 2,419 Leasehold improvements 1,552 1,570 Computer equipment 1,170 1,262 Office furniture and equipment 520 586 Property and equipment 5,738 5,837 Less accumulated depreciation (3,844 ) (2,994 ) Property and equipment, net of accumulated depreciation $ 1,894 $ 2,843

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Rental Payments for Operating Leases	Total future minimum rentals under the non-cancelable operating sublease as of December 31, 2017 are presented below (in thousands): Years Ending December 31, Minimum Sublease Rentals 2018 $ 75 2019 78 2020 81 2021 14 Total future minimum sublease rentals $ 248 The Company has the following minimum rental payments under non-cancelable operating lease obligations that existed at December 31, 2017 (in thousands): Years Ending December 31, Minimum Rental Payment 2018 $ 735 2019 711 2020 720 2021 182 Total future minimum rental payments $ 2,348

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit) (Tables)	12 Months Ended
	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Schedule of Shares Reserved for Future Issuance	The Company has reserved shares of common stock for future issuances as follows: December 31, 2017 2016 For exercise of common stock warrants 227,794 227,794 For exercise of outstanding common stock options 4,996,661 4,342,466 For delivery upon vesting of outstanding restricted stock units 956,299 946,200 For future equity awards under the 2013 Equity Incentive Plan 1,082,608 662,180 For future purchases under the 2013 Employee Stock Purchase Plan 1,861,472 1,612,759 Total shares of common stock reserved for future issuances 9,124,834 7,791,399
Schedule of Fair Value Assumptions, Stock Options	The following table illustrates the assumptions for the Black-Scholes model used in determining the fair value of the stock options granted: Years Ended December 31, 2017 2016 2015 Expected volatility 85.51 % 85.16 % 66.89 % Expected term (in years) 5.9 6.0 6.0 Weighted-average risk-free interest rate 2.02 % 1.70 % 1.53 % Expected dividend yield — % — % — % Weighted-average fair value per option $ 3.71 $ 5.62 $ 25.18
Summary of Activity Related to Stock Options	A summary of activity related to the Company’s stock options is as follows: Number of Options Weighted-Average Weighted-Average Total Intrinsic Value Balance, December 31, 2015 2,746,395 $ 28.19 8.41 Granted 2,418,551 7.76 — Exercised (48,441 ) 3.48 — Forfeited (774,039 ) 24.13 — Balance, December 31, 2016 4,342,466 $ 17.81 8.09 Granted 928,816 5.17 — Exercised (38,885 ) 3.98 — Forfeited (235,736 ) 19.10 — Balance, December 31, 2017 4,996,661 $ 15.51 7.59 $ 529,145 Exercisable at December 31, 2017 3,021,179 $ 18.05 7.14 $ 516,430 Vested or expected to vest at December 31, 2017 4,934,708 $ 15.56 7.58 $ 527,433
Schedule of Other Information Regarding Stock Options	Other information regarding the Company’s stock options is as follows (in thousands, except per share data): Years Ended December 31, 2017 2016 2015 Weighted-average grant-date fair value per share of options granted $ 3.71 $ 5.62 $ 25.18 Total intrinsic value of options exercised $ 48 $ 119 $ 10,139 Total fair value of shares vested $ 11,786 $ 13,330 $ 11,498
Schedule of Share-based Compensation, Shares Authorized	The following table summarizes, at December 31, 2017 , by price range: (1) for stock option awards outstanding under the 2013 Plan, the number of stock option awards outstanding, their weighted-average remaining life and their weighted-average exercise price; and (2) for stock option awards exercisable under the 2013 Plan, the number of stock option awards exercisable and their weighted-average exercise price: Outstanding Exercisable Range Number Weighted-Average Remaining Contractual Life (in years) Weighted-Average Exercise Price Number Weighted-Average Exercise Price $1.53 to 7.57 1,324,631 8.10 $ 4.65 631,032 $ 4.11 7.58 to 8.06 1,885,674 8.02 8.06 907,919 8.06 8.07 to 18.75 381,805 6.05 17.73 373,743 17.72 18.76 to 39.17 564,854 6.56 25.39 492,529 25.09 39.18 to 53.74 839,697 7.21 41.70 615,956 41.61 $1.53 to 53.74 4,996,661 7.59 $ 15.51 3,021,179 $ 18.05
Schedule of Fair Value Assumptions, Employee Stock Purchase Plan	The following table illustrates the assumptions for the Black-Scholes model used in determining the fair value of the ESPP purchase rights: Years Ended December 31, 2017 2016 2015 Expected volatility 77.18 % 111.57 % 57.77 % Expected term (in years) 0.97 1.37 1.1 Weighted-average risk-free interest rate 0.99 % 0.75 % 0.43 % Expected dividend yield — % — % — % Weighted-average option value per share $ 2.65 $ 3.20 $ 22.10
Summary of Activity Related to Restricted Stock Units	A summary of activity related to the Company’s RSUs is as follows: Number of Restricted Weighted-Average Grant-Date Fair Value Balance, December 31, 2016 946,200 $ 4.91 Granted 879,300 5.12 Share issuance (744,450 ) 4.91 Forfeited (124,751 ) 5.09 Balance, December 31, 2017 956,299 $ 5.08
Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs	For awards with only service conditions and graded-vesting features, the Company recognizes compensation expense on a straight-line basis over the requisite service period. Total stock-based compensation expense was as follows (in thousands): Years Ended December 31, 2017 2016 2015 Income Statement Classification: Research and development expense $ 7,047 $ 7,137 $ 5,578 General and administrative expense 9,063 9,086 7,381 Total stock-based compensation expense $ 16,110 $ 16,223 $ 12,959

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Reconciliation of the Difference between the Benefit for Income Taxes and Income Taxes at the Statutory U.S. Federal Income Tax Rate	A reconciliation of the difference between the benefit for income taxes and income taxes at the statutory U.S. federal income tax rate is as follows for the years ended December 31, 2017 , 2016 , and 2015 (in thousands, except percentages): 2017 2016 2015 Amount % of Pretax Amount % of Pretax Amount % of Pretax Income tax benefit at statutory rate $ (24,134 ) 34.0 % $ (25,973 ) 34.0 % $ (39,907 ) 34.0 % State income taxes (1,090 ) 1.5 % (1,544 ) 2.0 % (2,176 ) 1.9 % Research and development credits (2,039 ) 2.9 % (2,691 ) 3.5 % (5,698 ) 4.9 % Foreign rate differential 60 (0.1 )% (2 ) — % 2 — % Permanent items 1,646 (2.3 )% 2,537 (3.3 )% 3,687 (3.1 )% Provision to return adjustments 1,212 (1.7 )% 259 (0.3 )% (426 ) 0.2 % Effect of change in federal tax rate 57,950 (81.6 )% — — % — — % Effect of change in state tax rate 193 (0.3 )% 1,585 (2.1 )% 932 (0.8 )% Removal of excess tax benefit (12,930 ) 18.2 % — — % — — % Increase in unrecognized tax benefits 403 (0.6 )% 444 (0.6 )% 950 (0.8 )% Change in valuation allowance (21,271 ) 30.0 % 25,385 (33.2 )% 42,636 (36.3 )% Net benefit $ — — % $ — — % $ — — %
Components of Deferred Tax Assets and Liabilities	The components of deferred tax assets and liabilities at December 31, 2017 and 2016 were as follows (in thousands): December 31, 2017 2016 Deferred tax assets: Domestic net operating loss carryforwards $ 92,020 $ 114,111 Foreign net operating loss carryforwards 61 — Research and development expenses 763 813 Capitalized Section 174 expenses 28 48 Research and development credits 12,437 10,907 Accrued bonuses 777 1,006 Share-based compensation 6,156 7,214 Other 983 460 Total gross deferred tax assets 113,225 134,559 Valuation allowance (113,225 ) (134,496 ) Total deferred tax assets — 63 Deferred tax liabilities: Other — (63 ) Total deferred tax liabilities — (63 ) Total deferred tax assets and liabilities, net $ — $ —
Summary of Uncertain Tax Benefit	Therefore, the Company recognized an uncertain tax benefit associated with the federal R&D credit carryover during the years ended December 31, 2017 and 2016 , as follows (in thousands): Balance at December 31, 2015 $ 1,956 Increases related to 2016 444 Increases related to prior periods — Balance at December 31, 2016 2,400 Increases related to 2017 403 Increases related to prior periods 473 Balance at December 31, 2017 $ 3,276

Selected Quarterly Financial 25

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Schedule of Quarterly Financial Information	The following financial information reflects all normal recurring adjustments, which are, in the opinion of management, necessary for a fair statement of the results of the interim periods. Summarized quarterly data for 2017 and 2016 are as follows (in thousands, except share and per share data): 2017 Quarters Fourth Third Second First Total revenues $ 1,844 $ 897 $ 675 $ 1,078 Operating loss (18,687 ) (17,910 ) (17,245 ) (18,260 ) Net loss (19,238 ) (17,312 ) (16,680 ) (17,754 ) Net loss per share, basic and diluted $ (0.41 ) $ (0.37 ) $ (0.36 ) $ (0.38 ) Weighted-average shares outstanding, basic and diluted 47,341,271 47,065,756 46,863,753 46,573,394 2016 Quarters Fourth Third Second First Total revenues $ 1,980 $ 653 $ 1,841 $ 1,228 Operating loss (15,373 ) (17,422 ) (18,525 ) (26,632 ) Net loss (14,957 ) (17,025 ) (18,148 ) (26,260 ) Net loss per share, basic and diluted $ (0.32 ) $ (0.37 ) $ (0.39 ) $ (0.57 ) Weighted-average shares outstanding, basic and diluted 46,431,809 46,236,749 46,214,086 46,184,134

The Business and Summary of S26

The Business and Summary of Significant Accounting Policies (Narrative) (Details)	6 Months Ended	12 Months Ended
	Jun. 17, 2016	Dec. 31, 2017USD ($)segment	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)$ / shares	Sep. 30, 2016shares
Class of Stock [Line Items]
Other than temporary impairment losses, investments		$ 1,160,000	$ 0	$ 0
Accounts Receivable [Abstract]
Allowance for doubtful accounts		$ 0	0
Fair Value of Financial Instruments
Redemption restriction period, entities that calculate net asset value per share	18 months
Fair value inputs, discount for lack of marketability				10.00%
Fair value assumptions, exercise price (in USD per share) | $ / shares				$ 1.54
Fair value assumptions, expected dividend rate				0.00%
Fair value assumptions, expected term				168 days
Fair value assumptions, risk free interest rate				0.44%
Fair value assumptions, expected volatility rate				75.00%
Property and Equipment
Property and equipment, useful life, leasehold improvements		amortized over the shorter of the useful life of the asset or the term of the related lease
Impairment of long-lived assets		$ 0
Income tax expense (benefit)		0	0	$ 0
Defined Contribution Plan [Abstract]
Defined contribution plan, cost recognized		$ 300,000	400,000	300,000
Segments
Number of operating segments | segment		1
Minimum
Property and Equipment
Property and equipment, useful life		3 years
Maximum
Property and Equipment
Property and equipment, useful life		5 years
Fair Value, Measurements, Nonrecurring
Fair Value of Financial Instruments
Fair value adjustments, assets		$ 0
Fair value adjustments, liabilities		0
BARDA
Property and Equipment
Revenue from grants		0	$ 0	$ 0
Research and development expense
Property and Equipment
Prior period reclassification adjustments		$ 0
Common Stock | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value of Financial Instruments
Convertible preferred stock, shares issued upon conversion (in shares) | shares					1,071,429

The Business and Summary of S27

The Business and Summary of Significant Accounting Policies (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale	$ 132,972	$ 180,558
Long-term investments	76,731	47,407
Fair value, measurements, recurring
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	14,811	50,830
Short-term investments, available-for-sale	132,972	180,558
Long-term investments	76,731	47,407
Total assets	224,514	278,795
Fair value, measurements, recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	10,816	15,733
Short-term investments, available-for-sale	132,972	173,108
Long-term investments	76,731	47,407
Total assets	220,519	236,248
Fair value, measurements, recurring | Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	3,995	35,097
Short-term investments, available-for-sale	0	7,450
Long-term investments	0	0
Total assets	3,995	42,547
Fair value, measurements, recurring | Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	0	0
Short-term investments, available-for-sale	0	0
Long-term investments	0	0
Total assets	0	0
Fair value, measurements, recurring | Money market funds
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	10,816	15,733
Fair value, measurements, recurring | Money market funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	10,816	15,733
Fair value, measurements, recurring | Money market funds | Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	0	0
Fair value, measurements, recurring | Money market funds | Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	0	0
Fair value, measurements, recurring | Commercial paper
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	3,995	35,097
Fair value, measurements, recurring | Commercial paper | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	0	0
Fair value, measurements, recurring | Commercial paper | Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	3,995	35,097
Fair value, measurements, recurring | Commercial paper | Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	0	0
Fair value, measurements, recurring | Certificates of deposit
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale		7,450
Fair value, measurements, recurring | Certificates of deposit | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale		0
Fair value, measurements, recurring | Certificates of deposit | Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale		7,450
Fair value, measurements, recurring | Certificates of deposit | Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale		0
Fair value, measurements, recurring | U.S. Treasury securities
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale	132,586	171,822
Long-term investments	76,731	47,407
Fair value, measurements, recurring | U.S. Treasury securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale	132,586	171,822
Long-term investments	76,731	47,407
Fair value, measurements, recurring | U.S. Treasury securities | Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale	0	0
Long-term investments	0	0
Fair value, measurements, recurring | U.S. Treasury securities | Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale	0	0
Long-term investments	0	0
Fair value, measurements, recurring | Common Stock
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale	386	1,286
Fair value, measurements, recurring | Common Stock | Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale	386	1,286
Fair value, measurements, recurring | Common Stock | Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale	0	0
Fair value, measurements, recurring | Common Stock | Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments, available-for-sale	$ 0	$ 0

The Business and Summary of S28

The Business and Summary of Significant Accounting Policies (Reconciliation of Liabilities Measured at Fair Value on a Recurring Basis) (Details) - Preferred stock of U.S. corporation - Significant Unobservable Inputs (Level 3) $ in Thousands	12 Months Ended
	Dec. 31, 2016USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning of period	$ 1,485
Fair value decrease recorded in other comprehensive loss	(371)
Fair value transferred to Level 2	(1,114)
Ending of period	$ 0

The Business and Summary of S29

The Business and Summary of Significant Accounting Policies (Schedule of Prepaid and Other Current Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Prepaid research and development expenses	$ 1,138	$ 843
Interest receivable	601	772
Prepaid insurance	481	389
Other prepaid expenses and current assets	1,111	841
Total prepaid expenses and other current assets	$ 3,331	$ 2,845

The Business and Summary of S30

The Business and Summary of Significant Accounting Policies (Schedule of Accrued Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Accrued compensation	$ 3,678	$ 2,906
Accrued research and development expenses	3,384	2,257
Accrued indemnification claim	1,000	0
Other accrued liabilities	1,322	1,052
Total accrued liabilities	$ 9,384	$ 6,215

Investments - Summary of Availa

Investments - Summary of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Amortized Cost	$ 210,666	$ 228,405
Gross Unrealized Gains	0	20
Gross Unrealized Losses	(963)	(460)
Estimated Fair Value	209,703	227,965
Certificates of deposit
Schedule of Investments [Line Items]
Amortized Cost		7,445
Gross Unrealized Gains		5
Gross Unrealized Losses		0
Estimated Fair Value		7,450
U.S. Treasury securities
Schedule of Investments [Line Items]
Amortized Cost	210,280	219,415
Gross Unrealized Gains	0	15
Gross Unrealized Losses	(963)	(201)
Estimated Fair Value	209,317	219,229
Common Stock
Schedule of Investments [Line Items]
Amortized Cost	386	1,545
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(259)
Estimated Fair Value	$ 386	$ 1,286

Investments - Summary of Invest

Investments - Summary of Investments with Unrealized Losses, Aggregated by Investment Type and the Length of Time (Details) $ in Thousands	Dec. 31, 2017USD ($)security	Dec. 31, 2016USD ($)security
Schedule of Investments [Line Items]
Fair Value, Less than 12 Months	$ 170,390	$ 129,490
Unrealized Loss, Less than 12 Months	(871)	(460)
Fair Value, Greater than 12 Months	38,927	0
Unrealized Loss, Greater than 12 Months	(92)	0
Fair Value, Total	209,317	129,490
Unrealized Loss, Total	$ (963)	$ (460)
Number of securities with unrealized losses, Less than 12 Months | security	39	24
Number of securities with unrealized losses, Greater than 12 Months | security	7	0
Number of securities with unrealized losses, Total | security	46	24
U.S. Treasury securities
Schedule of Investments [Line Items]
Fair Value, Less than 12 Months	$ 170,390	$ 128,204
Unrealized Loss, Less than 12 Months	(871)	(201)
Fair Value, Greater than 12 Months	38,927	0
Unrealized Loss, Greater than 12 Months	(92)	0
Fair Value, Total	209,317	128,204
Unrealized Loss, Total	$ (963)	(201)
Preferred stock of U.S. corporation
Schedule of Investments [Line Items]
Fair Value, Less than 12 Months		1,286
Unrealized Loss, Less than 12 Months		(259)
Fair Value, Greater than 12 Months		0
Unrealized Loss, Greater than 12 Months		0
Fair Value, Total		1,286
Unrealized Loss, Total		$ (259)

Investments - Available for Sal

Investments - Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Maturing in one year or less	$ 132,586
Maturing after one year through two years	76,731
Total debt investments	209,317
Total investments	209,703	$ 227,965
Common Stock
Schedule of Investments [Line Items]
Total investments	$ 386	$ 1,286

Property and Equipment (Details

Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 5,738	$ 5,837
Less accumulated depreciation	(3,844)	(2,994)
Property and equipment, net of accumulated depreciation	1,894	2,843
Lab equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	2,496	2,419
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,552	1,570
Computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,170	1,262
Office furniture and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 520	$ 586

Loan Payable (Narrative) (Detai

Loan Payable (Narrative) (Details) - USD ($)	Jan. 27, 2012	Dec. 31, 2016
Line of Credit Facility [Line Items]
Loan and security agreement, maximum capacity	$ 15,000,000
Loans payable, current		$ 0
First tranche
Line of Credit Facility [Line Items]
Loan and security agreement, maximum capacity	$ 3,000,000
Interest only period	12 months
Principal and interest amortization period	30 months
Loan and security agreement, interest rate	8.25%
Second tranche
Line of Credit Facility [Line Items]
Loan and security agreement, maximum capacity	$ 12,000,000
Interest only period	6 months
Principal and interest amortization period	32 months
Loan and security agreement, interest rate	8.25%

Commitments and Contingencies36

Commitments and Contingencies (Schedule of Future Minimum Rental Payments for Operating Leases) (Details) $ in Thousands	Dec. 31, 2017USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Minimum Rental Payment, 2018	$ 735
Minimum Rental Payment, 2019	711
Minimum Rental Payment, 2020	720
Minimum Rental Payment, 2021	182
Total future minimum rental payments	$ 2,348

Commitments and Contingencies37

Commitments and Contingencies (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017USD ($)ft²	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Commitments and Contingencies Disclosure [Abstract]
Operating leases, rent expense	$ 500,000	$ 700,000	$ 600,000
Area of sublease property (in sqft) | ft²	3,537
Provision for refundable amounts under agreements	$ 0	0
Accrued indemnification claim	$ 1,000,000	$ 0

Commitments and Contingencies38

Commitments and Contingencies (Schedule of Future Minimum Sublease Rental Payments for Operating Leases) (Details) (Details) $ in Thousands	Dec. 31, 2017USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Minimum Sublease Rental Payment, 2018	$ 75
Minimum Sublease Rental Payment, 2019	78
Minimum Sublease Rental Payment, 2020	81
Minimum Sublease Rental Payment, 2021	14
Total future minimum sublease rental payments	$ 248

Stockholders' Equity (Deficit39

Stockholders' Equity (Deficit) (Narrative) (Details) - USD ($)	Jan. 01, 2017	Jan. 01, 2016	Jun. 16, 2015	Jun. 20, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Apr. 30, 2013	Feb. 28, 2013	Feb. 28, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized (in shares)					200,000,000	200,000,000		200,000,000
Common stock, shares, issued (in shares)					47,505,532	46,522,475		47,505,532
Common stock, shares, outstanding (in shares)					47,505,532	46,522,475		47,505,532
Proceeds from public offerings, net of offering costs					$ 0	$ 0	$ 161,879,000
Shares reserved for future issuance (in shares)					9,124,834	7,791,399		9,124,834
Employees' contribution liabilities, ESPP					$ 9,384,000	$ 6,215,000		$ 9,384,000
Stock issued during period, shares, restricted stock award, gross (in shares)					744,450	203,400
Warrants to purchase common stock (in shares)					227,794	227,794		227,794
Warrants, outstanding (in shares)					227,794	227,794		227,794
Proceeds from exercise of all share-based payment arrangements					$ 800,000	$ 600,000	3,200,000
Tax Benefit from share-based payment arrangements					$ 0	$ 0	$ 0
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award, gross (in shares)					0
Restricted stock units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, granted (in shares)					879,300
Unrecognized compensation costs, period of recognition					2 years 197 days
Shares reserved for future issuance (in shares)					956,299	946,200		956,299
Unrecognized compensation cost					$ 3,500,000			$ 3,500,000
The 2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized to be granted (in shares)									1,408,450
Share-based compensation, equity increase percentage				4.00%	2.50%
Share-based compensation, nonvested awards, compensation not yet recognized, stock options					$ 14,100,000			$ 14,100,000
Unrecognized compensation costs, period of recognition					1 year 234 days
Shares reserved for future issuance (in shares)					1,082,608	662,180		1,082,608
The 2013 Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized to be granted (in shares)					2,816,901			2,816,901
The 2012 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized to be granted (in shares)									244,717
Share-based compensation, granted (in shares)								0
The 2013 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized to be granted (in shares)										704,225
Share-based compensation, equity increase percentage					1.00%
Number of shares authorized, increase amount (in shares)					422,535
Number of additional shares authorized (in shares)	422,535	422,535,000
Number of shares available for grant (in shares)	2,226,261	1,803,726			2,226,261			2,226,261
Shares reserved for future issuance (in shares)					1,861,472	1,612,759		1,861,472
Share-based compensation, maximum employee subscription rate					15.00%			15.00%
Discount from market price, entry date					15.00%
Discount from market price, purchase date					15.00%
ESPP purchase period					6 months
Employee stock purchase plan, automatic reset participation period					24 months
Shares issued during period, employee stock purchase plans (in shares)					173,822	108,109
Employees' contribution liabilities, ESPP					$ 200,000			$ 200,000
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New issues of stock during period (in shares)			4,341,250
Offering price per share (in USD per share)			$ 39.75
Proceeds from public offerings, net of offering costs			$ 161,900,000
Warrants to purchase common stock (in shares)									1,549,628
Common Stock | IPO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrant strike price (in USD per share)									$ 7.26
Common Stock | Over-allotment option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New issues of stock during period (in shares)			566,250
Series F
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants to purchase common stock (in shares)											5,501,215
Warrant strike price (in USD per share)											$ 2.045

Stockholders' Equity (Deficit40

Stockholders' Equity (Deficit) (Schedule of Shares Reserved for Future Issuance) (Details) - shares	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	9,124,834	7,791,399
The 2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	1,082,608	662,180
The 2013 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	1,861,472	1,612,759
Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	4,996,661	4,342,466
Restricted stock units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	956,299	946,200
Warrant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	227,794	227,794

Stockholders' Equity (Deficit41

Stockholders' Equity (Deficit) (Assumption Used to Determine the Fair Value of Options Granted) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average fair value per option (in USD per share)	$ 3.71	$ 5.62	$ 25.18
Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	85.51%	85.16%	66.89%
Expected term (in years)	5 years 329 days	6 years	6 years
Weighted-average risk-free interest rate	2.02%	1.70%	1.53%
Expected dividend yield	0.00%	0.00%	0.00%
Weighted-average fair value per option (in USD per share)	$ 3.71	$ 5.62	$ 25.18

Stockholders' Equity (Deficit42

Stockholders' Equity (Deficit) (Summary of Activity Related to Stock Options) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of options outstanding, Beginning balance	4,342,466	2,746,395
Number of options outstanding, Granted	928,816	2,418,551
Number of options outstanding, Exercised	(38,885)	(48,441)
Number of options outstanding, Forfeited	(235,736)	(774,039)
Number of options outstanding, Ending balance	4,996,661	4,342,466	2,746,395
Number of options outstanding, Exercisable	3,021,179
Number of options outstanding, Vested and expected to vest	4,934,708
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average exercise price, Beginning balance (in USD per share)	$ 17.81	$ 28.19
Weighted-average exercise price, Granted (in USD per share)	5.17	7.76
Weighted-average exercise price, Exercised (in USD per share)	3.98	3.48
Weighted-average exercise price, Forfeited (in USD per share)	19.10	24.13
Weighted-average exercise price, Ending balance (in USD per share)	15.51	$ 17.81	$ 28.19
Weighted-average exercise price, Exercisable (in USD per share)	18.05
Weighted-average exercise price, Vested or expected to vest (in USD per share)	$ 15.56
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted-average remaining contractual life (in years)	7 years 215 days	8 years 1 month 2 days	8 years 4 months 28 days
Weighted-average remaining contractual life (in years), Exercisable	7 years 51 days
Weighted-average remaining contractual life (in years), Vested or expected to vest	7 years 212 days
Aggregate intrinsic value of options outstanding	$ 529,145
Aggregate intrinsic value of options exercisable	516,430
Aggregate intrinsic value of options vested or expected to vest	$ 527,433

Stockholders' Equity (Deficit43

Stockholders' Equity (Deficit) (Schedule of other information regarding stock options) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stockholders' Equity Note [Abstract]
Weighted-average fair value per option (in USD per share)	$ 3.71	$ 5.62	$ 25.18
Total intrinsic value of options exercised	$ 48	$ 119	$ 10,139
Total fair value of shares vested	$ 11,786	$ 13,330	$ 11,498

Stockholders' Equity (Deficit44

Stockholders' Equity (Deficit) (Schedule of Share-based Compensation, Shares Authorized) (Details)	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Exercise price range, $1.53 to 7.57
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options (in shares) | shares	1,324,631
Share-based compensation, outstanding options, weighted average remaining contractual term	8 years 37 days
Share-based compensation, outstanding options, weighted average exercise price (in USD per share)	$ 4.65
Share-based compensation, number of exercisable options (in shares) | shares	631,032
Share-based compensation, exercisable options, weighted average exercise price (in USD per share)	$ 4.11
Exercise Price Range, $7.58 to 8.06
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options (in shares) | shares	1,885,674
Share-based compensation, outstanding options, weighted average remaining contractual term	8 years 7 days
Share-based compensation, outstanding options, weighted average exercise price (in USD per share)	$ 8.06
Share-based compensation, number of exercisable options (in shares) | shares	907,919
Share-based compensation, exercisable options, weighted average exercise price (in USD per share)	$ 8.06
Exercise Price Range, $8.07 to 18.75
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options (in shares) | shares	381,805
Share-based compensation, outstanding options, weighted average remaining contractual term	6 years 18 days
Share-based compensation, outstanding options, weighted average exercise price (in USD per share)	$ 17.73
Share-based compensation, number of exercisable options (in shares) | shares	373,743
Share-based compensation, exercisable options, weighted average exercise price (in USD per share)	$ 17.72
Exercise Price Range, $18.76 to 39.17
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options (in shares) | shares	564,854
Share-based compensation, outstanding options, weighted average remaining contractual term	6 years 204 days
Share-based compensation, outstanding options, weighted average exercise price (in USD per share)	$ 25.39
Share-based compensation, number of exercisable options (in shares) | shares	492,529
Share-based compensation, exercisable options, weighted average exercise price (in USD per share)	$ 25.09
Exercise Price Range, $39.18 to 53.74
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options (in shares) | shares	839,697
Share-based compensation, outstanding options, weighted average remaining contractual term	7 years 77 days
Share-based compensation, outstanding options, weighted average exercise price (in USD per share)	$ 41.70
Share-based compensation, number of exercisable options (in shares) | shares	615,956
Share-based compensation, exercisable options, weighted average exercise price (in USD per share)	$ 41.61
Exercise Price Range, $1.53 to 53.74
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options (in shares) | shares	4,996,661
Share-based compensation, outstanding options, weighted average remaining contractual term	7 years 215 days
Share-based compensation, outstanding options, weighted average exercise price (in USD per share)	$ 15.51
Share-based compensation, number of exercisable options (in shares) | shares	3,021,179
Share-based compensation, exercisable options, weighted average exercise price (in USD per share)	$ 18.05
Minimum | Exercise price range, $1.53 to 7.57
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit (in USD per share)	1.53
Minimum | Exercise Price Range, $7.58 to 8.06
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit (in USD per share)	7.58
Minimum | Exercise Price Range, $8.07 to 18.75
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit (in USD per share)	8.07
Minimum | Exercise Price Range, $18.76 to 39.17
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit (in USD per share)	18.76
Minimum | Exercise Price Range, $39.18 to 53.74
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit (in USD per share)	39.18
Minimum | Exercise Price Range, $1.53 to 53.74
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit (in USD per share)	1.53
Maximum | Exercise price range, $1.53 to 7.57
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit (in USD per share)	7.57
Maximum | Exercise Price Range, $7.58 to 8.06
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit (in USD per share)	8.06
Maximum | Exercise Price Range, $8.07 to 18.75
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit (in USD per share)	18.75
Maximum | Exercise Price Range, $18.76 to 39.17
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit (in USD per share)	39.17
Maximum | Exercise Price Range, $39.18 to 53.74
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit (in USD per share)	53.74
Maximum | Exercise Price Range, $1.53 to 53.74
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit (in USD per share)	$ 53.74

Stockholders' Equity (Deficit45

Stockholders' Equity (Deficit) (Schedule of Share-Based Compensation) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average fair value per option (in USD per share), ESPP purchase rights	$ 3.71	$ 5.62	$ 25.18
The 2013 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility, ESPP purchase rights	77.18%	111.57%	57.77%
Expected term (in years), ESPP purchase rights	355 days	1 year 135 days	1 year 1 month 24 days
Weighted-average risk-free interest rate, ESPP purchase rights	0.99%	0.75%	0.43%
Expected dividend yield, ESPP purchase rights	0.00%	0.00%	0.00%
Weighted-average fair value per option (in USD per share), ESPP purchase rights	$ 2.65	$ 3.20	$ 22.10

Stockholders' Equity (Deficit46

Stockholders' Equity (Deficit) (Summary of Activity Related to Restricted Stock Units) (Details) - Restricted stock units (RSUs)	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Number of Restricted Stock Units Outstanding
Beginning Balance (in shares) | shares	946,200
Granted (in shares) | shares	879,300
Shares issuance (in shares) | shares	(744,450)
Forfeited (in shares) | shares	(124,751)
Ending Balance (in shares) | shares	956,299
Weighted-Average Grant-Date Fair Value
Beginning Balance (in USD per share) | $ / shares	$ 4.91
Granted (in USD per share) | $ / shares	5.12
Shares issuance (in USD per share) | $ / shares	4.91
Forfeited (in USD per share) | $ / shares	5.09
Ending Balance (in USD per share) | $ / shares	$ 5.08

Stockholders' Equity (Deficit47

Stockholders' Equity (Deficit) (Schedule of Allocation of Recognized Period Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 16,110	$ 16,223	$ 12,959
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	7,047	7,137	5,578
General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 9,063	$ 9,086	$ 7,381

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended					93 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2007	Dec. 31, 2002	Sep. 30, 2015
Operating Loss Carryforwards [Line Items]
Income tax expense (benefit)	$ 0	$ 0	$ 0
Undistributed earnings of foreign subsidiary	0
Loss limitation ownership change benchmark amount				$ 762,000	$ 64,000
Annual limitation of operating loss carryforwards						$ 6,700,000
Penalties and interest accrued	0
Effect of change in federal tax rate	57,950,000	0	$ 0
Domestic tax authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	408,100,000	356,100,000
Tax credit carryforward	16,600,000
State and local jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	319,900,000	$ 287,200,000
Foreign tax authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 400,000

Income Taxes (Reconciliation of

Income Taxes (Reconciliation of the Difference between the Benefit for Income Taxes and Income Taxes at the Statutory U.S. Federal Income Tax Rate) (Alternate) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income tax benefit at statutory rate	$ (24,134,000)	$ (25,973,000)	$ (39,907,000)
State income taxes	(1,090,000)	(1,544,000)	(2,176,000)
Research and development credits	(2,039,000)	(2,691,000)	(5,698,000)
Foreign rate differential	60,000	(2,000)	2,000
Permanent items	1,646,000	2,537,000	3,687,000
Provision to return adjustments	1,212,000	259,000	(426,000)
Effect of change in federal tax rate	57,950,000	0	0
Effect of change in state tax rate	193,000	1,585,000	932,000
Removal of excess tax benefit	(12,930,000)	0	0
Increase in unrecognized tax benefits	403,000	444,000	950,000
Change in valuation allowance	(21,271,000)	25,385,000	42,636,000
Net benefit	$ 0	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax benefit at statutory rate, tax rate	34.00%	34.00%	34.00%
State income taxes, tax rate	1.50%	2.00%	1.90%
Research and development credits, tax rate	2.90%	3.50%	4.90%
Foreign rate differential, tax rate	(0.10%)	0.00%	0.00%
Permanent items, tax rate	(2.30%)	(3.30%)	(3.10%)
Provision to return adjustments, tax rate	(1.70%)	(0.30%)	0.20%
Effect of change in federal tax rate, tax rate	(81.60%)	0.00%	0.00%
Effect of change in state tax rate, tax rate	(0.30%)	(2.10%)	(0.80%)
Removal of excess tax benefit, tax rate	18.20%	(0.00%)	(0.00%)
Increase in unrecognized tax benefits, tax rate	(0.60%)	(0.60%)	(0.80%)
Change in valuation allowance, tax rate	30.00%	(33.20%)	(36.30%)
Net benefit, tax rate	0.00%	0.00%	0.00%

Income Taxes (Components of Def

Income Taxes (Components of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Domestic net operating loss carryforwards	$ 92,020	$ 114,111
Foreign net operating loss carryforwards	61	0
Research and development expenses	763	813
Capitalized Section 174 expenses	28	48
Research and development credits	12,437	10,907
Accrued bonuses	777	1,006
Share-based compensation	6,156	7,214
Other	983	460
Total gross deferred tax assets	113,225	134,559
Valuation allowance	(113,225)	(134,496)
Total deferred tax assets	0	63
Deferred tax liabilities:
Other	0	(63)
Total deferred tax liabilities	0	(63)
Total deferred tax assets and liabilities, net	$ 0	$ 0

Income Taxes (Summary of Uncert

Income Taxes (Summary of Uncertain Tax Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 2,400	$ 1,956
Increase related to current year tax positions	403	444
Increase related to prior periods	473	0
Ending balance	$ 3,276	$ 2,400

Significant Agreements (Details

Significant Agreements (Details) $ in Thousands	Dec. 17, 2014USD ($)shares	Jan. 31, 2017shares	Dec. 31, 2016USD ($)	Apr. 30, 2015USD ($)	May 31, 2002USD ($)shares	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2017USD ($)period	Sep. 30, 2018USD ($)extension	Sep. 30, 2016shares
Long-term Purchase Commitment [Line Items]
Contract revenue						$ 4,494	$ 5,702	$ 9,214
Collaboration and licensing revenue						0	0	$ 1,548
Fair value of investments			$ 227,965			209,703	227,965		$ 209,703
Regents of University of California
Long-term Purchase Commitment [Line Items]
Stock issued during period, issued for services (in shares) | shares					64,788
BARDA
Long-term Purchase Commitment [Line Items]
Contract revenue									$ 56,100
Number of extension periods | period									3
ContraVir Pharmaceuticals
Long-term Purchase Commitment [Line Items]
License agreement, termination notice period	60 days
Collaboration and licensing revenue				$ 1,500
The Regents of the University of Michigan
Long-term Purchase Commitment [Line Items]
Stock issued during period, issued for services (in shares) | shares		33,058
License fees			50
Scenario, Forecast | BARDA
Long-term Purchase Commitment [Line Items]
Number of extension periods | extension										4
Preferred stock of U.S. corporation
Long-term Purchase Commitment [Line Items]
Convertible preferred stock, shares issued upon conversion (in shares) | shares											1,071,429
Preferred stock of U.S. corporation | ContraVir Pharmaceuticals
Long-term Purchase Commitment [Line Items]
Shares received as consideration for license agreement (in shares) | shares	120,000
Value of shares received as consideration for license agreement	$ 1,500
Fair value of investments			$ 1,300			$ 400	$ 1,300		$ 400
Maximum | Regents of University of California
Long-term Purchase Commitment [Line Items]
Milestone payments					$ 3,400
Sublicense fee percentage					50.00%
Royalties percentage					50.00%
Maximum | ContraVir Pharmaceuticals
Long-term Purchase Commitment [Line Items]
Future licenses revenue	20,000
Maximum | The Regents of the University of Michigan
Long-term Purchase Commitment [Line Items]
Royalties percentage						2.00%
Maximum | Brincidofovir | Regents of University of California
Long-term Purchase Commitment [Line Items]
Sublicense fee amount payable fee percentage					5.00%
Maximum | Scenario, Forecast | BARDA
Long-term Purchase Commitment [Line Items]
Reimbursement revenue										$ 75,800
Fees and commissions										5,300
Maximum | Second Option Segment | Scenario, Forecast | BARDA
Long-term Purchase Commitment [Line Items]
Contract revenue										21,600
Maximum | Third Option Segment | Scenario, Forecast | BARDA
Long-term Purchase Commitment [Line Items]
Contract revenue										$ 11,600
Minimum | The Regents of the University of Michigan
Long-term Purchase Commitment [Line Items]
Royalties percentage						0.25%
Stated Value | Preferred stock of U.S. corporation | ContraVir Pharmaceuticals
Long-term Purchase Commitment [Line Items]
Value of shares received as consideration for license agreement	$ 1,200

Selected Quarterly Financial 53

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 1,844	$ 897	$ 675	$ 1,078	$ 1,980	$ 653	$ 1,841	$ 1,228	$ 4,494	$ 5,702	$ 10,762
Operating loss	(18,687)	(17,910)	(17,245)	(18,260)	(15,373)	(17,422)	(18,525)	(26,632)	(72,102)	(77,952)	(118,251)
Net loss	$ (19,238)	$ (17,312)	$ (16,680)	$ (17,754)	$ (14,957)	$ (17,025)	$ (18,148)	$ (26,260)	$ (70,984)	$ (76,390)	$ (117,372)
Net loss per share, basic and diluted (in USD per share)	$ (0.41)	$ (0.37)	$ (0.36)	$ (0.38)	$ (0.32)	$ (0.37)	$ (0.39)	$ (0.57)	$ (1.51)	$ (1.65)	$ (2.67)
Weighted-average shares outstanding, basic and diluted (in shares)	47,341,271	47,065,756	46,863,753	46,573,394	46,431,809	46,236,749	46,214,086	46,184,134	46,963,430	46,267,064	43,878,326