Chimerix (CMRX) 10-K 25 Feb 20 2019 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2019	Feb. 21, 2020	Jun. 28, 2019
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY
Entity Registrant Name	CHIMERIX INC
Entity Central Index Key	0001117480
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 Small Business	true
Entity Emerging Growth	false
Entity Shell Company	false
Entity Public Float			$ 137,028,331
Entity Common Stock, Shares Outstanding		61,740,646

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 16,901	$ 81,106
Short-term investments, available-for-sale	96,574	105,424
Accounts receivable	1,233	330
Prepaid expenses and other current assets	3,385	2,598
Total current assets	118,093	189,458
Property and equipment, net of accumulated depreciation	540	1,210
Operating lease right-of-use assets	709
Other long-term assets	34	46
Total assets	119,376	190,714
Current liabilities:
Accounts payable	2,398	4,691
Accrued liabilities	6,830	8,275
Total current liabilities	9,228	12,966
Lease-related obligations	196	144
Total liabilities	9,424	13,110
Stockholders’ equity:
Preferred stock, $0.001 par value, 10,000,000 shares authorized at December 31, 2019 and 2018; no shares issued and outstanding as of December 31, 2019 and 2018	0	0
Common stock, $0.001 par value; 200,000,000 shares authorized at December 31, 2019 and 2018; 61,590,013 and 50,735,279 shares issued and outstanding at December 31, 2019 and 2018, respectively	62	51
Additional paid-in capital	778,693	733,907
Accumulated other comprehensive loss, net	35	(92)
Accumulated deficit	(668,838)	(556,262)
Total stockholders’ equity	109,952	177,604
Total liabilities and stockholders’ equity	$ 119,376	$ 190,714

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Preferred stock, par value (in USD per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	10,000,000	10,000,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	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)	61,590,013	50,735,279
Common stock, shares outstanding (in shares)	61,590,013	50,735,279

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Revenues	$ 12,519	$ 7,216	$ 4,494
Operating expenses:
Research and development	42,288	55,239	49,448
General and administrative	21,169	23,582	27,148
Acquired in-process research and development	65,045	0	0
Total operating expenses	128,502	78,821	76,596
Loss from operations	(115,983)	(71,605)	(72,102)
Other income:
Interest income and other, net	3,407	2,131	1,118
Net loss	(112,576)	(69,474)	(70,984)
Other comprehensive loss:
Unrealized gain (loss) on investments, net	127	871	(523)
Comprehensive loss	$ (112,449)	$ (68,603)	$ (71,507)
Per share information:
Net loss, basic and diluted (in USD per share)	$ (2.03)	$ (1.43)	$ (1.51)
Weighted-average shares outstanding, basic and diluted (in shares)	55,501,973	48,593,435	46,963,430
Contract revenue
Revenues:
Revenues	$ 7,604	$ 7,216	$ 4,494
Licensing revenue
Revenues:
Revenues	$ 4,915	$ 0	$ 0

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, 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, net of issuance costs	150		150
Comprehensive loss:
Unrealized gain (loss) 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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	13,074	1	13,073
Exercise of stock options	115		115
Employee stock purchase plan purchases	608		608
Issuance of common stock, net of issuance costs	10,600	3	10,597
Comprehensive loss:
Unrealized gain (loss) on investments, net	871			871
Net loss	(69,474)				(69,474)
Comprehensive loss	(68,603)
Ending Balance at Dec. 31, 2018	177,604	51	733,907	(92)	(556,262)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	9,529	1	9,528
Exercise of stock options	43		43
Employee stock purchase plan purchases	325		325
Issuance of common stock, net of issuance costs	34,900	10	34,890
Comprehensive loss:
Unrealized gain (loss) on investments, net	127			127
Net loss	(112,576)				(112,576)
Comprehensive loss	(112,449)
Ending Balance at Dec. 31, 2019	$ 109,952	$ 62	$ 778,693	$ 35	$ (668,838)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (112,576)	$ (69,474)	$ (70,984)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation of property and equipment	564	860	1,091
Amortization of discount/premium on investments	(1,842)	(852)	(5)
Share-based compensation	9,528	13,074	16,110
Fair value of common stock issued for license agreement	34,900	0	0
Loss on disposition of assets	264	5	10
Loss on sale of investments	31	378	0
Unrealized loss on equity investment	0	348
Unrealized loss on equity investment			1,160
Lease-related amortization	(76)	(59)	(319)
Changes in operating assets and liabilities:
Accounts receivable	(903)	1,352	(83)
Prepaid expenses and other assets	(777)	638	(178)
Accounts payable and accrued liabilities	(4,294)	5	3,073
Net cash used in operating activities	(75,181)	(53,725)	(50,125)
Cash flows from investing activities:
Purchases of property and equipment	(158)	(181)	(151)
Purchases of short-term investments	(167,528)	(125,611)	0
Purchases of long-term investments	0	(6,031)	(162,613)
Proceeds from sales of short-term investments	13,117	111,178	13,500
Proceeds from maturities of short-term investments	165,200	125,740	165,695
Net cash provided by investing activities	10,631	105,095	16,431
Cash flows from financing activities:
Proceeds from exercise of stock options	43	115	121
Proceeds from employee stock purchase plan	325	608	712
Proceeds from issuance of common stock, net of commissions	0	10,867	0
Payments of deferred offering costs	(23)	(402)	(54)
Net cash provided by financing activities	345	11,188	779
Net (decrease) increase in cash and cash equivalents	(64,205)	62,558	(32,915)
Beginning of period	81,106	18,548	51,463
End of period	16,901	81,106	18,548
Supplemental disclosure of cash flow information
Non-cash addition to deferred offering costs	$ 0	$ 22	$ 276

The Business and Summary of Sig

The Business and Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
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 development-stage biopharmaceutical company dedicated to accelerating the advancement of innovative medicines that make a meaningful impact in the lives of patients living with cancer and other serious diseases. The Company has two clinical-stage product candidates, dociparstat sodium (DSTAT) and brincidofovir (BCV). Dociparstat sodium is a potential first-in-class glycosaminoglycan compound derived from porcine heparin that has low anticoagulant activity but retains the ability to inhibit activities of several key proteins implicated in the retention and viability of AML blasts and leukemic stem cells in the bone marrow during chemotherapy (e.g., CXCL12, selectins, HMGB1. elastase). Mobilization of AML blasts and leukemic stem cells from the bone marrow has been associated with enhanced chemosensitivity and may be a primary mechanism accounting for the observed increases in event-free survival (EFS) and overall survival (OS) in a Phase 2 study with DSTAT versus placebo. Randomized Phase 2 data suggests that DSTAT may also accelerate platelet recovery post chemotherapy via inhibition of platelet factor 4, a negative regulator of platelet production that impairs platelet recovery following chemotherapy. BCV is a lipid conjugate DNA polymerase inhibitor in development as a medical countermeasure for smallpox. The Company expects to continue its evaluation of external innovation in order to license, acquire or otherwise gain access to molecules that further broaden its pipeline of investigational agents in cancer or other serious diseases. 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, U.S. Treasury securities, commercial paper, and corporate bonds. Investments Investments consist primarily of commercial paper, corporate bonds, 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 debt 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 and other, net. For the year ended December 31, 2019 , approximately $2,000 of realized gains were reclassified from accumulated other comprehensive loss, net in the Consolidated Balance Sheets to interest income and other, net in the Consolidated Statements of Operations and Comprehensive Loss. 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 debt 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 debt securities in an unrealized loss position before recovery of the amortized cost bases of the debt securities, which may be maturity. Any such declines in value judged to be other-than-temporary on available-for-sale debt securities are reported in other-than-temporary impairment of investment. For the year ended December 31, 2018 , the Company recorded $348,000 of unrealized loss related to the Company's investment in ContraVir common stock to interest income and other, net in the Consolidated Statements of Operations and Comprehensive Loss. The Company sold its investment in ContraVir in September 2019. For the year ended December 31, 2019 , the Company recorded $33,000 of realized loss related to the sale of the Company’s investment in ContraVir common stock to interest income and other, net in the Consolidated Statements of Operations and Comprehensive Loss. The Company recognizes interest income on an accrual basis in interest income and other, 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, 2019 and December 31, 2018 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. The determination of where an asset or liability falls in the hierarchy requires significant judgment. 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. At December 31, 2019 , the Company had cash equivalents, including money market funds, and short-term investments, including U.S. Treasury securities. At December 31, 2018 , the Company had cash equivalents, including money market funds and U.S. Treasury securities, and short-term investments, including U.S. Treasury securities, whose value is based on using quoted market prices. Accordingly, these securities are classified as Level 1. At December 31, 2018 , the Company had short-term investments, including stock of a U.S. corporation. The Company’s investment in ContraVir Pharmaceuticals (ContraVir) common stock was categorized as a Level 1 asset and had a value based on ContraVir's common stock value at December 31, 2018 . The Company sold its investment in ContraVir in September 2019. For the twelve months ended December 31, 2019 the Company recorded a realized loss related to the Company’s investment in ContraVir common stock of approximately $33,000 . For the twelve months ended December 31, 2018 and 2017 , the Company recorded $348,000 and $1,160,000 , respectively, of unrealized loss related to the Company’s investment in ContraVir common stock to interest income and other, net in the Consolidated Statements of Operations and Comprehensive Loss. At December 31, 2019 , the Company had short-term investments, including commercial paper and corporate bonds. At December 31, 2018 , the Company had cash equivalents and short-term investments including commercial paper and corporate bonds. As quoted prices are not available for these securities, they 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. 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, 2019 Total Quoted Prices in Significant Other Significant Cash equivalents Money market funds $ 11,854 $ 11,854 $ — $ — Total cash equivalents 11,854 11,854 — — Short-term investments U.S. Treasury securities 22,493 22,493 — — Commercial paper 43,119 — 43,119 — Corporate bonds 30,962 — 30,962 — Total short-term investments 96,574 22,493 74,081 — Total assets $ 108,428 $ 34,347 $ 74,081 $ — Fair Value Measurements December 31, 2018 Total Quoted Prices in Significant Other Significant Cash equivalents Money market funds $ 30,726 $ 30,726 $ — $ — U.S. Treasury securities 11,482 11,482 — — Commercial paper 29,677 — 29,677 — Corporate bonds 4,008 — 4,008 — Total cash equivalents 75,893 42,208 33,685 — Short-term investments U.S. Treasury securities 12,589 12,589 — — Common stock of U.S. corporation 38 38 — — Commercial paper 60,114 — 60,114 — Corporate bonds 32,683 — 32,683 — Total short-term investments 105,424 12,627 92,797 — Total assets $ 181,317 $ 54,835 $ 126,482 $ — Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2019 2018 Prepaid research and development expenses $ 936 $ 1,071 Interest receivable 323 259 Prepaid insurance 344 382 Other prepaid expenses and current assets 1,782 886 Total prepaid expenses and other current assets $ 3,385 $ 2,598 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 Property and Equipment The Company evaluates property and equipment 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. Leases At the inception of an arrangement, we determine if an arrangement is, or contains, a lease based on the unique facts and circumstances present in that arrangement. Lease classification, recognition, and measurement are then determined at the lease commencement date. For arrangements that contain a lease we (i) identify lease and non-lease components, (ii) determine the consideration in the contract, (iii) determine whether the lease is an operating or financing lease; and (iv) recognize lease right-of-use (ROU) assets and liabilities. Lease liabilities and their corresponding ROU assets are recorded based on the present value of lease payments over the expected lease term. The interest rate implicit in lease contracts is typically not readily determinable and as such, we use our incremental borrowing rate based on the information available at the lease commencement date, which represents an internally developed rate that would be incurred to borrow, on a collateralized basis, over a similar term, an amount equal to the lease payments in a similar economic environment. Most leases include options to renew and, or, terminate the lease, which can impact the lease term. The exercise of these options is at our discretion and we do not include any of these options within the expected lease term as we are not reasonably certain we will exercise these options. Operating lease assets and liabilities are included on our Consolidated Balance Sheet beginning January 1, 2019. The current portion of our operating lease liabilities is included in accrued liabilities and the long-term portion is included in lease-related obligations. Accrued Liabilities Accrued liabilities consisted of the following (in thousands): December 31, 2019 2018 Accrued compensation $ 3,626 $ 2,469 Accrued research and development expenses 1,868 4,525 Other accrued liabilities 1,336 1,281 Total accrued liabilities $ 6,830 $ 8,275 Revenue Recognition Policy The Company’s revenues generally consist of (i) contract revenue - revenue generated under federal contracts, 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 Accounting Standards Codification (ASC) 606 issued by the Financial Accounting Standards Board (FASB). Following this accounting pronouncement, a five-step approach is applied 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. 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, the Company may receive up to $75.8 million in expense reimbursement and $5.3 million in fees over the performance of 1 base segment and 4 option segments. Exercise of each option segment is solely at the discretion of BARDA. Currently, option segments 1 through 3 have been exercised. The Company assessed the services in accordance with the authoritative guidance and concluded that there is a potential of 5 separate contracts ( 1 base segment and four option segments) within this agreement, each of which has a single performance obligation. The transaction price for each segment, based on the transaction price as defined in each segment contract, is allocated to the single performance obligation for each contract. The transaction price is recognized over time by measuring the progress toward complete satisfaction of the performance obligation. For reimbursable expenses, this occurs as qualifying research activities are conducted based on invoices from company vendors. For the fixed fee, the progress toward complete satisfaction is estimated based on the costs incurred to date relative to the total estimated costs per the terms of each contract. The Company typically invoices BARDA monthly as costs are incurred. Any amounts received in advance of performance are recorded as deferred revenue until earned. The base segment and first option segment were completed prior to adoption of ASC 606. 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 $23.9 million and $14.1 million in expense reimbursement and fees, respectively. The second and third option segments are scheduled to end on May 31, 2020. SymBio Pharmaceuticals On September 30, 2019, the Company entered into a license agreement with SymBio Pharmaceuticals Limited (SymBio) for the exclusive worldwide rights to develop, manufacture and commercialize BCV for all human indications, excluding the prevention and treatment of orthopoxviruses, including smallpox. The Company assessed the agreement in accordance with the authoritative guidance and concluded that the SymBio contract includes multiple performance obligations. The SymBio contract has one fixed transaction amount of a $5.0 million upfront payment received in October 2019 and several variable transaction amounts, up to $180.0 million , due to the Company at certain regulatory and commercial milestones, along with low double-digit royalties, due to the Company on annual net sales. All variable transaction amounts are fully constrained, therefore the allocated transaction price is $5.0 million . The majority of the transaction price of the contract has been allocated to the combined performance obligation of the granting of the license to BCV and associated technology transfer which was recognized when the technology transfer was completed in the fourth quarter of 2019. The remainder of the upfront payment will be recognized when the inventory transfer and technical assistance performance obligations are complete, which is expected to occur in the first half of 2020. The revenue from regulatory and commercial milestones and royalties from net sales will be recognized upon occurrence of the triggering events or when those transaction amounts are no longer fully constrained. As of December 31, 2019 , the Company had recognized $4.9 million of revenue related to the upfront payment and $0.1 million remained in deferred revenue as of December 31, 2019 . 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, 2019 , 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 are 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 and Other, Net Interest income and other, net consists primarily of interest earned on our cash, cash equivalents, short-term investments and long-term investments and decreases in fair value as well as realized losses of our investment in ContraVir Pharmaceuticals common stock. 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, 2019 , 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. The FASB Staff Q&A, Topic 740, No. 5, Accounting for Global Intangible Low-Taxed Income (GILTI), states that an entity can make an accounting policy election to either recognized deferred taxes for temporary basis differences expected to reverse as GILTI in future years or to provide for the tax expense related to GILTI in the year the tax is incurred as a period expense only. The Company has elected to account for GILTI as a period expense in the year the tax is incurred. 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 records 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, 2019 , 2018 and 2017 , 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”). For the years ended December 31, 2019 , 2018 and 2017 , 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, 2019 , 2018 and 2017 . The calculation of weighted-average diluted shares outstanding excludes the dilutive effect of non-vested restricted stock, stock options to purchase common stock, and employee stock purchase plan purchase rights as the impact of such items are anti-dilutive during periods of net loss. Potential common shares excluded from the calculations were 1,571,356 , 749,110 , and 1,687,887 for the years ended December 31, 2019 , 2018 and 2017 , respectively. Segments The Company operates in only one segment, pharmaceuticals. Impact of Recently Issued Accounting Standards In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments , which amends the impairment model by requiring entities to use a forward-looking approach on expected losses to estimate credit losses on certain financial instruments, including trade receivables and available-for-sale debt securities. The new guidance was originally due to become effective for the Company beginning in the first quarter of 2020, however the FASB in November 2019 issued ASU 2019-10 which moved the effective date for smaller reporting companies to the first quarter of 2023. The Company is currently evaluating the potential impact that this standard may have on its consolidated financial statements. Impact of Recently Adopted Accounting Standards In February 2016, the FASB issued Accounting Standards Update (ASU) No. 2016-02, “Leases (Topic 842)”, which has been amended through subsequent ASUs, and which increases transparency and comparability among companies accounting for lease transactions. The most significant change of this update requires 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 ASU is effective for financial statements issued for annual periods and interim periods within those annual periods, beginning after December 15, 2018. The Company adopted this standard effective January 1, 2019 using the alternative modified retrospective adoption method allowed by ASU 2018-11. The Company elected to use the package of three practical expedients which allows the Company not to reassess whether contracts are or contain leases, lease classification, and whether initial direct costs qualify for capitalization. The Company has completed its assessment of the impact of the standard and determined that the only material leases that the Company holds are real estate operating leases. Upon adoption of the standard, the Company recorded a right of use asset of $1.4 million and lease liability of $1.6 million on its consolidated balance sheets with no adjustment to beginning retained earnings in the period of adoption.

Investments

Investments	12 Months Ended
	Dec. 31, 2019
Investments [Abstract]
Investments	Investments The following tables summarize the Company’s short-term and long-term debt investments (in thousands): December 31, 2019 Amortized Cost Gross Unrealized Gross Unrealized Estimated Corporate bonds $ 30,952 $ 19 $ (9 ) $ 30,962 Commercial paper 43,109 14 (4 ) 43,119 U.S. Treasury securities 22,478 17 (2 ) 22,493 Total investments $ 96,539 $ 50 $ (15 ) $ 96,574 December 31, 2018 Amortized Cost Gross Unrealized Gross Unrealized Estimated Corporate bonds $ 32,724 $ — $ (41 ) $ 32,683 Commercial paper 60,159 — (45 ) 60,114 U.S. Treasury securities 12,592 — (3 ) 12,589 Total investments $ 105,475 $ — $ (89 ) $ 105,386 The following tables summarize the Company’s debt 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, 2019 Less than 12 Months Greater than 12 Months Total Fair Value Unrealized Loss Fair Value Unrealized Loss Fair Value Unrealized Loss Corporate bonds $ 9,657 $ (9 ) $ — $ — $ 9,657 $ (9 ) Commercial paper 10,147 (4 ) — — 10,147 (4 ) U.S. Treasury securities 2,994 (2 ) — — 2,994 (2 ) Total $ 22,798 $ (15 ) $ — $ — $ 22,798 $ (15 ) Number of securities with unrealized losses 9 — 9 December 31, 2018 Less than 12 Months Greater than 12 Months Total Fair Value Unrealized Loss Fair Value Unrealized Loss Fair Value Unrealized Loss Corporate bonds $ 32,683 $ (41 ) $ — $ — $ 32,683 $ (41 ) Commercial paper 60,114 (45 ) — — 60,114 (45 ) U.S. Treasury securities 12,589 (3 ) — — 12,589 (3 ) Total $ 105,386 $ (89 ) $ — $ — $ 105,386 $ (89 ) Number of securities with unrealized losses 36 — 36 The following table summarizes the scheduled maturity for the Company’s debt investments at December 31, 2019 (in thousands): December 31, 2019 Maturing in one year or less $ 96,574 Total debt investments $ 96,574

Property and Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Lab equipment $ 2,329 $ 2,599 Leasehold improvements 1,550 1,550 Computer equipment 1,182 1,181 Office furniture and equipment 520 520 Property and equipment 5,581 5,850 Less accumulated depreciation (5,041 ) (4,640 ) Property and equipment, net of accumulated depreciation $ 540 $ 1,210

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Leases The Company leases its facilities under long-term operating leases that expire at various dates through 2021. The Company generally has options to renew lease terms on its facilities, which may be exercised at the Company’s sole discretion. In addition, certain lease arrangements may be terminated prior to their original expiration date at the Company’s discretion. The Company evaluates renewal and termination options at the lease commencement date to determine if it is reasonably certain to exercise the option and has concluded on all operating leases that it is not reasonably certain that any options will be exercised. The weighted-average remaining lease term for the Company’s operating leases as of December 31, 2019 was 1.27 years . Expense related to leases is recorded on a straight-line basis over the lease term. Lease expense under operating leases, including common area maintenance fees, totaled approximately $724,000 and $692,000 , respectively, for the twelve months ended December 31, 2019 and 2018 , respectively. The discount rate implicit within the Company’s leases is generally not determinable and therefore the Company determines the discount rate based on its incremental borrowing rate based on the information available at commencement date. As of December 31, 2019 , the operating lease liabilities reflect a weighted-average discount rate of 13.06% . The following table sets forth the operating lease right-of-use assets and liabilities as of December 31, 2019 (in thousands): Assets Operating Lease Right-of-Use Assets $ 709 Liabilities Operating Lease Short-term Liabilities (recorded within Accrued liabilities) $ 656 Operating Lease Long-term Liabilities (recorded within Lease-related obligations) 179 Total Operating Lease Liabilities $ 835 Operating lease payments over the remainder of the lease terms are as follows (in thousands): Years Ending December 31, As of December 31, 2019 2020 719 2021 182 Total future minimum rental payments $ 901 Less amount of lease payments representing interest 66 Total present value of lease payments $ 835 As of December 31, 2018, future minimum payments under operating leases under ASC 840 were as follows (in thousands): Years Ending December 31, As of December 31, 2018 2019 $ 786 2020 797 2021 235 Total future minimum rental payments $ 1,818 For the twelve months ended December 31, 2019 and 2018, the Company made lease payments of approximately $751,000 and $750,000 , respectively, which are included in operating cash flows. Sublease The Company subleases 3,537 square feet of its office space under a non-cancelable operating lease that expires February 2021. For the twelve months ended December 31, 2019 , the Company recognized approximately $71,000 of income in Interest income and other, net on the Consolidated Statement of Operations and Comprehensive Loss. For the twelve months ended December 31, 2018 , the Company recognized approximately $71,000 of a reduction of rent expense in operating expenses on the Consolidated Statement of Operations and Comprehensive Loss. Total future minimum rentals under the non-cancelable operating sublease are presented below (in thousands): Years Ending December 31, Minimum Sublease Rentals 2020 $ 81 2021 14 Total future minimum sublease rentals $ 95 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, 2019 and 2018 .

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit)	12 Months Ended
	Dec. 31, 2019
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, 2019 and 2018 , and 61.6 million and 50.7 million shares issued and outstanding at December 31, 2019 and December 31, 2018 , respectively. Shares Reserved for Future Issuance The Company has reserved shares of common stock for future issuances as follows: December 31, 2019 2018 For exercise of common stock warrants — — For exercise of outstanding common stock options 8,390,304 6,429,638 For delivery upon vesting of outstanding restricted stock units 1,561,237 656,169 For future equity awards under the 2013 Equity Incentive Plan 1,855,688 1,587,670 For future purchases under the 2013 Employee Stock Purchase Plan 2,333,750 2,120,290 Total shares of common stock reserved for future issuances 14,140,979 10,793,767 Stock Options The Company maintains a 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. The number of shares of common stock reserved for issuance under the 2013 Plan will automatically increase on January 1 of each year, continuing through and including January 1, 2023, by 4.0% 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 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 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. 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, 2019 2018 2017 Expected volatility 88.77 % 85.83 % 85.51 % Expected term (in years) 6.0 5.9 5.9 Weighted-average risk-free interest rate 2.42 % 2.52 % 2.02 % Expected dividend yield — % — % — % Weighted-average fair value per option $ 1.71 $ 3.43 $ 3.71 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, 2017 4,996,661 $ 15.51 7.59 Granted 2,050,995 4.73 — Exercised (29,262 ) 3.93 — Forfeited (588,756 ) 12.31 — Balance, December 31, 2018 6,429,638 $ 12.41 7.37 Granted 4,029,200 2.31 — Exercised (19,284 ) 2.21 — Forfeited (2,049,250 ) 9.26 — Balance, December 31, 2019 8,390,304 $ 8.36 7.47 $ 15,600 Exercisable at December 31, 2019 5,004,867 $ 12.18 6.44 $ 650 Vested or expected to vest at December 31, 2019 8,112,168 $ 8.56 7.41 $ 12,610 As of December 31, 2019 , there was approximately $5.2 million of total unrecognized compensation cost related to non-vested stock options granted under the 2013 Plan. That compensation cost is expected to be recognized over a weighted-average period of approximately 2.72 years . Other information regarding the Company’s stock options is as follows (in thousands, except per share data): Years Ended December 31, 2019 2018 2017 Weighted-average grant-date fair value per share of options granted $ 1.71 $ 3.43 $ 3.71 Total intrinsic value of options exercised $ 10 $ 31 $ 48 Total fair value of shares vested $ 6,798 $ 11,021 $ 11,786 The following table summarizes, at December 31, 2019 , 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 Exercise Price Range ($) Number Weighted-Average Remaining Contractual Life (in years) Weighted-Average Exercise Price Number Weighted-Average Exercise Price 1.51 to 7.57 5,764,618 8.41 $ 3.31 2,393,762 $ 4.21 7.58 to 8.06 1,361,550 6.02 8.06 1,346,969 8.06 8.07 to 18.75 210,117 4.08 18.75 210,117 18.75 18.76 to 39.17 462,350 4.50 24.55 462,350 24.55 39.18 to 53.74 591,669 5.20 41.81 591,669 41.81 1.51 to 53.74 8,390,304 7.47 $ 8.36 5,004,867 $ 12.18 In April 2019, the Company granted stock options covering a total of 1,750,000 shares in connection with the hiring of its Chief Executive Officer and Chief Business Officer. These grants were non-qualified stock options, have a 10 -year term and will vest over four years , with one-fourth vesting on the one-year anniversary of the grant date and remaining three-fourths vesting over the following three years in equal monthly installments. These stock options are subject to the terms of the 2013 Plan, but were granted outside of the 2013 Plan, as they constituted inducement grants in accordance with Nasdaq Stock Market rules. 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, 2018 and 2019, bringing the total number of shares of common stock that may be purchased under the ESPP to 2,648,796 and 3,071,331 , respectively. The Company has reserved a total of 3,071,331 shares of common stock to be purchased under the ESPP, of which 2,333,750 and 2,120,290 shares remained available for purchase at December 31, 2019 and 2018 , 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 209,075 and 163,717 shares of common stock pursuant to the ESPP for the year ended December 31, 2019 and 2018 , 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, 2019 2018 2017 Expected volatility 57.22 % 44.01 % 77.18 % Expected term (in years) 1.23 1.23 0.97 Weighted-average risk-free interest rate 2.36 % 2.56 % 0.99 % Expected dividend yield — % — % — % Weighted-average option value per share $ 1.00 $ 1.36 $ 2.65 As of December 31, 2019 , the Company had a liability of $0.1 million representing employees' contributions to the ESPP. Restricted Stock Units For the years ended December 31, 2019 and 2018 , 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, 2019 and 2018 , the Company issued 626,375 and 233,050 shares of common stock pursuant to the vesting of RSUs, respectively. 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, 2018 656,169 $ 4.92 Granted 1,754,575 2.43 Share issuance (626,375 ) 3.51 Forfeited (223,132 ) 4.13 Balance, December 31, 2019 1,561,237 $ 2.80 The total unrecognized compensation cost related to the non-vested RSUs as of December 31, 2019 was $3 million and will be recognized over a weighted average period of approximately 3.00 years . 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, 2019 2018 2017 Income Statement Classification: Research and development expense $ 4,089 $ 5,343 $ 7,047 General and administrative expense 5,439 7,731 9,063 Total stock-based compensation expense $ 9,528 $ 13,074 $ 16,110 Cash received from exercises under all share-based payment arrangements for 2019 , 2018 and 2017 was $0.4 million , $0.7 million and $0.8 million , respectively. There was no actual tax benefit realized for the tax deductions from exercises of the share-based payment arrangements during 2019 , 2018 or 2017 . On February 5, 2019, Dr. M. Michelle Berrey, the Company’s then President and Chief Executive Officer, resigned. The Company entered into a severance agreement with Dr. Berrey that provides for severance benefits to her in connection with her resignation. Among other benefits, Dr. Berrey received accelerated vesting of her outstanding stock options and RSUs as if she had continued service for an additional 15 -month period. In addition, Dr. Berrey's vested options were modified to extend her exercise period to May 5, 2020. The Company recorded a charge of $1.8 million to compensation expense on the date of her resignation related to the acceleration of vesting and the modifications of her outstanding stock options and RSUs. In May 2019, related to the Company’s reduction in workforce further discussed in Note 8, certain outstanding stock option and RSU grants received accelerated vesting as if the service period of the terminated employee continued for an additional 12 -month period. In addition, certain vested options were modified to extend their exercise period for 12 months. The Company recorded a charge of $0.7 million to compensation expense on the date of the reduction in workforce related to the acceleration of vesting and the modifications of the outstanding stock options and RSUs. At-The-Market Equity Offering On November 8, 2017, the Company entered into an at-the-market (ATM) sales agreement with Cowen and Company, LLC to sell up to $75 million of the Company’s common stock under a shelf registration statement filed in November 2017. During the year ended December 31, 2018 , the Company sold 2.8 million shares of common stock at a weighted average price per share of $4.00 for $10.9 million of proceeds net of commissions. The Company did not sell any shares of common stock pursuant to the ATM in 2019.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes No income tax expense or benefit has been recorded for the years ended December 31, 2019 , 2018 or 2017 . This is due to the establishment of a valuation allowance against the deferred tax assets generated during those periods. At December 31, 2019 , 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, 2019 , 2018 , and 2017 (in thousands, except percentages): 2019 2018 2017 Amount % of Pretax Amount % of Pretax Amount % of Pretax Income tax benefit at statutory rate $ (23,641 ) 21.0 % $ (14,590 ) 21.0 % $ (24,134 ) 34.0 % State income taxes (1,596 ) 1.4 % (792 ) 1.1 % (1,090 ) 1.5 % Research and development credits (1,190 ) 1.1 % (1,798 ) 2.6 % (2,039 ) 2.9 % Foreign rate differential — — % 2 — % 60 (0.1 )% Permanent items 696 (0.6 )% 1,164 (1.7 )% 1,646 (2.3 )% Provision to return adjustments 937 (0.8 )% 621 (0.9 )% 1,212 (1.7 )% Effect of change in federal tax rate — — % — — % 57,950 (81.6 )% Effect of change in state tax rate (117 ) 0.1 % 151 (0.2 )% 193 (0.3 )% Removal of excess tax benefit — — % — — % (12,930 ) 18.2 % Increase in unrecognized tax benefits 298 (0.3 )% 450 (0.7 )% 403 (0.6 )% Change in valuation allowance 24,613 (21.9 )% 14,792 (21.2 )% (21,271 ) 30.0 % Net benefit $ — — % $ — — % $ — — % The components of deferred tax assets and liabilities at December 31, 2019 and 2018 were as follows (in thousands): December 31, 2019 2018 Deferred tax assets: Domestic net operating loss carryforwards $ 114,292 $ 104,708 Foreign net operating loss carryforwards — 71 Research and development expenses 419 1,002 Capitalized Section 174 expenses 24 25 License fees 14,176 — Research and development credits 14,682 13,789 Capital Loss Carryforwards 431 — Accrued bonuses 751 500 Share-based compensation 7,557 7,144 Other 456 778 Total gross deferred tax assets 152,788 128,017 Valuation allowance (152,629 ) (128,017 ) Total deferred tax assets 159 — Deferred tax liabilities: Other (159 ) — Total deferred tax liabilities (159 ) — Total deferred tax assets and liabilities, net $ — $ — At December 31, 2019 , the Company had net operating loss carryforwards for federal and state tax purposes of approximately $508.1 million and $384.3 million , respectively. At December 31, 2018 , the Company had net operating loss carryforwards for federal, state, and foreign tax purposes of approximately $465.3 million , $353.8 million and $0.4 million , respectively. Federal losses of $408.1 million begin to expire in 2020 and $100.0 million of the federal losses carryforward indefinitely. The state losses begin to expire in 2020 . In addition, the Company has tax credit carryforwards for federal tax purposes of approximately $19.6 million as of December 31, 2019 , which begin to expire in 2022 . The Company also has capital loss carryforwards for federal tax purposes of $1.9 million , 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 was liquidated during 2019, and all related tax attributes have been removed. The Company incorporated a subsidiary in Ireland during 2018. However, the subsidiary had no activity during both 2018 and 2019 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 of 1986, as amended (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, 2019 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 operating 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, 2019 and 2018 , as follows (in thousands): Balance at December 31, 2017 $ 3,276 Increases related to 2018 450 Increases related to prior periods — Balance at December 31, 2018 3,726 Increases related to 2019 297 Increases related to prior periods — Balance at December 31, 2019 $ 4,023 The Company has determined that it had no other material uncertain tax benefits for the year ended December 31, 2019 . As of January 1, 2019, 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 2019 . 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, 2019 . On December 22, 2017, the Tax 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 revised tax rate, the Company adjusted its deferred tax assets as of December 31, 2017 by applying the revised 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 Act 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 the transition to the territorial tax system the Tax Act included a mandatory deemed repatriation of all undistributed foreign earnings that are subject to a U.S. income tax. The Company has determined that the deemed repatriation applicable to the year ended December 31, 2017 does not result in an additional U.S. income tax liability as it has no undistributed foreign earnings. The Tax Act subjects a US shareholder to tax on global intangible low-taxed income (GILTI) earned by certain foreign subsidiaries. The FASB Staff Q&A, Topic 740, No. 5, Accounting for Global Intangible Low-Taxed Income, states that an entity can make an accounting policy election to either recognized deferred taxes for temporary basis differences expected to reverse as GILTI in future years or to provide for the tax expense related to GILTI in the year the tax is incurred as a period expense only. Because the Company was evaluating the provision of GILTI as of December 31, 2017, no GILTI-related deferred amounts were recorded in 2017. The Company has elected to account for GILTI in the year the tax is incurred. The Company does not have a GILTI inclusion in 2018 or 2019; therefore, no GILTI tax has been recorded for the years ended December 31, 2018 and 2019. The SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (SAB 118) which allows the Company to record provisional amounts during a measurement period which is similar to the measurement period used when accounting for business combinations. At December 31, 2017 provisional amounts were recorded related to deferred taxes for stock compensation and the deferred rate change. At December 31, 2018 the measurement period has ended and the Company's accounting related to the Tax Act is complete. The Company did not make any measurement-period adjustments related to the provisional items recorded as of December 31, 2017.

Significant Agreements

Significant Agreements	12 Months Ended
	Dec. 31, 2019
Contractors [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 BCV. The license agreement was terminated effective September 29, 2019. The termination of the license to the UC Patent Rights does not affect the Chimerix solely-owned patents covering BCV composition of matter that currently are set to expire in 2034. 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 $23.9 million and $14.1 million in expense reimbursement and fees, respectively. The second and third option segments are scheduled to end on May 31, 2020. Of the $75.8 million in expense reimbursement and $5.3 million in fees that the Company may receive, approximately $74.3 million in expense reimbursement and fees has been funded. As of December 31, 2019 , of the total funding the Company had invoiced an aggregate of $70.1 million with respect to the base performance segment and the first three extension periods. For the years ended December 31, 2019 , 2018 , and 2017 , the Company recognized revenue under this contract of $7.6 million , $7.2 million and $4.5 million , respectively. License and Development Agreement with Cantex Pharmaceuticals, Inc. On July 26, 2019, the Company entered into a License and Development Agreement with Cantex Pharmaceuticals, Inc. (Cantex) pursuant to which the Company acquired exclusive worldwide rights to develop and commercialize, for any and all uses, a glycosaminoglycan compound known as DSTAT, which is currently being studied for the treatment of acute myeloid leukemia. Under the terms of the license agreement, the Company is responsible for, and bears the future costs of, worldwide development and commercialization of DSTAT. In connection with the transaction, Cantex assigned to the Company all of its rights under its DSTAT supply agreements, including its bulk API agreement with Scientific Protein Laboratories LLC (SPL), pursuant to which SPL will exclusively produce DSTAT for the Company through October 2030. In consideration for the license rights, the Company made an upfront cash payment of $30.0 million to Cantex and issued to Cantex 10.0 million shares of its common stock. For the twelve months ended, the Company recognized $65.0 million of acquired in-process research and development expenses for the $30.0 million upfront cash payment, the fair value of the 10.0 million shares of common stock issued to Cantex and $0.1 million of transaction costs. The license agreement obligates the Company to pay Cantex regulatory milestone payments of up to $202.5 million upon receipt of product approvals in the United States, the European Union and Japan, and sales milestone payments of up to $385.0 million upon achievement of specified net sales levels. The Company also agreed to pay Cantex tiered royalties based on percentages of net sales beginning at 10% and not to exceed the high-teens. SymBio Pharmaceuticals On September 30, 2019, the Company entered into a license agreement with Symbio for the exclusive worldwide rights to develop, manufacture and commercialize BCV for all human indications, excluding the prevention and treatment of orthopoxviruses, including smallpox. Under the terms of the license agreement, SymBio will be responsible for, and bear the future costs of, worldwide development and commercialization of BCV in the licensed indications. 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. SymBio may also terminate the license agreement without cause on a country-by-country basis upon ninety days prior notice. In exchange for the license to BCV rights, the Company received an upfront payment of $5.0 million in the fourth quarter of 2019. In addition, the Company is eligible to receive up to $180.0 million in clinical, regulatory and commercial milestones worldwide, as well as low double-digit royalties and additional milestones based on commercial sales. At December 31, 2019 , the Company had recognized $4.9 million of revenue related to the upfront payment and had $0.1 million remaining as deferred revenue at December 31, 2019 . 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.

Restructuring Costs

Restructuring Costs	12 Months Ended
	Dec. 31, 2019
Restructuring and Related Activities [Abstract]
Restructuring Costs	Restructuring Costs In May 2019, the Company made the decision to discontinue the development of oral and IV BCV development programs for the treatment of Adenovirus (AdV) in stem-cell transplant (HCT) patients. The Company’s development efforts with respect to BCV are now focused on the treatment of smallpox. As a result, the Company restructured its operations, which included a reduction in workforce of 43 full-time employees and the accrual of expenses to close-out the clinical trials for the oral and IV development programs of BCV in AdV (study 210, study 211, AdAPT) and other supportive BCV development programs. The Company recorded charges for one-time employee termination benefits of $3.3 million , contract close-out costs of $2.0 million , other BCV development costs of $0.3 million , and losses on disposals of fixed assets of $0.3 million during the twelve months ended December 31, 2019 . The $2.0 million of contract close-out costs were recorded through an increase in liabilities of $1.5 million with the remainder recognized through the expensing of prepaid balances. As of December 31, 2019 , the Company had a clinical trial accrual balance related to the AdAPT, 210 and 211 trial terminations of $27,000 and other development costs accrual balance of $0.1 million , which are expected to be paid during the first quarter of 2020. As of December 31, 2019 , the Company had a severance accrual balance of $0.2 million , which is expected to be fully paid by June 2020. The following table summarizes the restructuring charges (in thousands) recorded for the twelve months ended December 31, 2019 : Employee Termination Benefits Clinical Trial Close-out Costs Other Development Costs Fixed Asset Disposals Total Research and development 1,437 2,021 339 — 3,797 General and administrative 1,909 — — — 1,909 Interest income and other, net — — — 250 250 Total restructuring expenses 3,346 2,021 339 250 5,956 The following table sets forth the accrual activity for employee termination benefits and contract close-out costs (in thousands) for the twelve months ended December 31, 2019 . No additional charges are expected to be incurred. Employee Termination Benefits Clinical Trial Close-out Costs Other Development Costs Fixed Asset Disposals Total Balance at January 1, 2019 — — — — — Accruals 3,335 2,131 315 — 5,781 Revised estimates 11 (621 ) 24 250 (336 ) Payments (3,163 ) (1,483 ) (229 ) (250 ) (5,125 ) Balance at December 31, 2019 183 27 110 — 320 In addition to the approximately $621,000 of revised estimates to accrued liabilities, the Company revised estimates of prepaid clinical trial balances included in prepaid expenses and other current assets, which resulted in the reduction of originally estimated clinical trial close-out costs by approximately $196,000 for the twelve months ended December 31, 2019 . As of December 31, 2019 , prepaid balances of $1.3 million for unused deposits have been reclassed to other receivables, which is recorded in prepaid expenses and other current assets on the Consolidated Balance Sheet and are expected to be refunded during the first quarter of 2020.

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2019
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 2019 and 2018 are as follows (in thousands, except share and per share data): 2019 Quarters Fourth Third Second First Revenue $ 6,767 $ 1,958 $ 1,438 $ 2,356 Operating loss (3,873 ) (74,564 ) (18,701 ) (18,845 ) Net loss (3,503 ) (73,730 ) (17,650 ) (17,693 ) Net loss per share, basic and diluted $ (0.06 ) $ (1.26 ) $ (0.35 ) $ (0.35 ) Weighted-average shares outstanding, basic and diluted 61,385,616 58,457,110 51,130,104 50,887,221 2018 Quarters Fourth Third Second First Revenue $ 4,864 $ 369 $ 1,193 $ 790 Operating loss (15,419 ) (16,710 ) (19,169 ) (20,307 ) Net loss (14,956 ) (16,079 ) (18,613 ) (19,826 ) Net loss per share, basic and diluted $ (0.29 ) $ (0.33 ) $ (0.39 ) $ (0.42 ) Weighted-average shares outstanding, basic and diluted 50,722,655 48,172,354 47,811,552 47,637,907 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 2019 and 2018 .

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2019
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, 2019 , and events which occurred subsequently but were not recognized in the financial statements.

The Business and Summary of S_2

The Business and Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2019
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, U.S. Treasury securities, commercial paper, and corporate bonds.
Investments and Unrealized Loss on Equity Investment	Investments Investments consist primarily of commercial paper, corporate bonds, 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 debt 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 and other, net. For the year ended December 31, 2019 , approximately $2,000 of realized gains were reclassified from accumulated other comprehensive loss, net in the Consolidated Balance Sheets to interest income and other, net in the Consolidated Statements of Operations and Comprehensive Loss. 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 debt 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 debt securities in an unrealized loss position before recovery of the amortized cost bases of the debt securities, which may be maturity. Any such declines in value judged to be other-than-temporary on available-for-sale debt securities are reported in other-than-temporary impairment of investment.
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, 2019 and December 31, 2018 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. The determination of where an asset or liability falls in the hierarchy requires significant judgment. 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.
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 Property and Equipment	Impairment of Property and Equipment The Company evaluates property and equipment 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.
Leases	Leases At the inception of an arrangement, we determine if an arrangement is, or contains, a lease based on the unique facts and circumstances present in that arrangement. Lease classification, recognition, and measurement are then determined at the lease commencement date. For arrangements that contain a lease we (i) identify lease and non-lease components, (ii) determine the consideration in the contract, (iii) determine whether the lease is an operating or financing lease; and (iv) recognize lease right-of-use (ROU) assets and liabilities. Lease liabilities and their corresponding ROU assets are recorded based on the present value of lease payments over the expected lease term. The interest rate implicit in lease contracts is typically not readily determinable and as such, we use our incremental borrowing rate based on the information available at the lease commencement date, which represents an internally developed rate that would be incurred to borrow, on a collateralized basis, over a similar term, an amount equal to the lease payments in a similar economic environment. Most leases include options to renew and, or, terminate the lease, which can impact the lease term. The exercise of these options is at our discretion and we do not include any of these options within the expected lease term as we are not reasonably certain we will exercise these options. Operating lease assets and liabilities are included on our Consolidated Balance Sheet beginning January 1, 2019. The current portion of our operating lease liabilities is included in accrued liabilities and the long-term portion is included in lease-related obligations.
Revenue Recognition	Revenue Recognition Policy The Company’s revenues generally consist of (i) contract revenue - revenue generated under federal contracts, 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 Accounting Standards Codification (ASC) 606 issued by the Financial Accounting Standards Board (FASB). Following this accounting pronouncement, a five-step approach is applied 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.
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, 2019 , 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 are 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 and Other, Net	Interest Income and Other, Net Interest income and other, net consists primarily of interest earned on our cash, cash equivalents, short-term investments and long-term investments and decreases in fair value as well as realized losses of our investment in ContraVir Pharmaceuticals common stock.
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, 2019 , 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. The FASB Staff Q&A, Topic 740, No. 5, Accounting for Global Intangible Low-Taxed Income (GILTI), states that an entity can make an accounting policy election to either recognized deferred taxes for temporary basis differences expected to reverse as GILTI in future years or to provide for the tax expense related to GILTI in the year the tax is incurred as a period expense only. The Company has elected to account for GILTI as a period expense in the year the tax is incurred.
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 records 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, 2019 , 2018 and 2017 , 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”).
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, 2019 , 2018 and 2017 .
Segments	Segments The Company operates in only one segment, pharmaceuticals.
Impact of Recently Issued Accounting Standards and Impact of Recently Adopted Accounting Standards	Impact of Recently Issued Accounting Standards In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments , which amends the impairment model by requiring entities to use a forward-looking approach on expected losses to estimate credit losses on certain financial instruments, including trade receivables and available-for-sale debt securities. The new guidance was originally due to become effective for the Company beginning in the first quarter of 2020, however the FASB in November 2019 issued ASU 2019-10 which moved the effective date for smaller reporting companies to the first quarter of 2023. The Company is currently evaluating the potential impact that this standard may have on its consolidated financial statements. Impact of Recently Adopted Accounting Standards In February 2016, the FASB issued Accounting Standards Update (ASU) No. 2016-02, “Leases (Topic 842)”, which has been amended through subsequent ASUs, and which increases transparency and comparability among companies accounting for lease transactions. The most significant change of this update requires 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 ASU is effective for financial statements issued for annual periods and interim periods within those annual periods, beginning after December 15, 2018. The Company adopted this standard effective January 1, 2019 using the alternative modified retrospective adoption method allowed by ASU 2018-11. The Company elected to use the package of three practical expedients which allows the Company not to reassess whether contracts are or contain leases, lease classification, and whether initial direct costs qualify for capitalization. The Company has completed its assessment of the impact of the standard and determined that the only material leases that the Company holds are real estate operating leases. Upon adoption of the standard, the Company recorded a right of use asset of $1.4 million and lease liability of $1.6 million on its consolidated balance sheets with no adjustment to beginning retained earnings in the period of adoption.

The Business and Summary of S_3

The Business and Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 Total Quoted Prices in Significant Other Significant Cash equivalents Money market funds $ 11,854 $ 11,854 $ — $ — Total cash equivalents 11,854 11,854 — — Short-term investments U.S. Treasury securities 22,493 22,493 — — Commercial paper 43,119 — 43,119 — Corporate bonds 30,962 — 30,962 — Total short-term investments 96,574 22,493 74,081 — Total assets $ 108,428 $ 34,347 $ 74,081 $ — Fair Value Measurements December 31, 2018 Total Quoted Prices in Significant Other Significant Cash equivalents Money market funds $ 30,726 $ 30,726 $ — $ — U.S. Treasury securities 11,482 11,482 — — Commercial paper 29,677 — 29,677 — Corporate bonds 4,008 — 4,008 — Total cash equivalents 75,893 42,208 33,685 — Short-term investments U.S. Treasury securities 12,589 12,589 — — Common stock of U.S. corporation 38 38 — — Commercial paper 60,114 — 60,114 — Corporate bonds 32,683 — 32,683 — Total short-term investments 105,424 12,627 92,797 — Total assets $ 181,317 $ 54,835 $ 126,482 $ —
Schedule of Prepaid and Other Current Assets	Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2019 2018 Prepaid research and development expenses $ 936 $ 1,071 Interest receivable 323 259 Prepaid insurance 344 382 Other prepaid expenses and current assets 1,782 886 Total prepaid expenses and other current assets $ 3,385 $ 2,598
Schedule of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): December 31, 2019 2018 Accrued compensation $ 3,626 $ 2,469 Accrued research and development expenses 1,868 4,525 Other accrued liabilities 1,336 1,281 Total accrued liabilities $ 6,830 $ 8,275

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2019
Investments [Abstract]
Summary of Short-term and Long-term Investments	The following tables summarize the Company’s short-term and long-term debt investments (in thousands): December 31, 2019 Amortized Cost Gross Unrealized Gross Unrealized Estimated Corporate bonds $ 30,952 $ 19 $ (9 ) $ 30,962 Commercial paper 43,109 14 (4 ) 43,119 U.S. Treasury securities 22,478 17 (2 ) 22,493 Total investments $ 96,539 $ 50 $ (15 ) $ 96,574 December 31, 2018 Amortized Cost Gross Unrealized Gross Unrealized Estimated Corporate bonds $ 32,724 $ — $ (41 ) $ 32,683 Commercial paper 60,159 — (45 ) 60,114 U.S. Treasury securities 12,592 — (3 ) 12,589 Total investments $ 105,475 $ — $ (89 ) $ 105,386
Summary of Investments with Unrealized Losses, Aggregated by Investment Type and the Length of Time	The following tables summarize the Company’s debt 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, 2019 Less than 12 Months Greater than 12 Months Total Fair Value Unrealized Loss Fair Value Unrealized Loss Fair Value Unrealized Loss Corporate bonds $ 9,657 $ (9 ) $ — $ — $ 9,657 $ (9 ) Commercial paper 10,147 (4 ) — — 10,147 (4 ) U.S. Treasury securities 2,994 (2 ) — — 2,994 (2 ) Total $ 22,798 $ (15 ) $ — $ — $ 22,798 $ (15 ) Number of securities with unrealized losses 9 — 9 December 31, 2018 Less than 12 Months Greater than 12 Months Total Fair Value Unrealized Loss Fair Value Unrealized Loss Fair Value Unrealized Loss Corporate bonds $ 32,683 $ (41 ) $ — $ — $ 32,683 $ (41 ) Commercial paper 60,114 (45 ) — — 60,114 (45 ) U.S. Treasury securities 12,589 (3 ) — — 12,589 (3 ) Total $ 105,386 $ (89 ) $ — $ — $ 105,386 $ (89 ) Number of securities with unrealized losses 36 — 36
Summary of the Scheduled Maturity of Company Investments	The following table summarizes the scheduled maturity for the Company’s debt investments at December 31, 2019 (in thousands): December 31, 2019 Maturing in one year or less $ 96,574 Total debt investments $ 96,574

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Lab equipment $ 2,329 $ 2,599 Leasehold improvements 1,550 1,550 Computer equipment 1,182 1,181 Office furniture and equipment 520 520 Property and equipment 5,581 5,850 Less accumulated depreciation (5,041 ) (4,640 ) Property and equipment, net of accumulated depreciation $ 540 $ 1,210

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Operating Lease Right-of-Use Assets and Liabilities	The following table sets forth the operating lease right-of-use assets and liabilities as of December 31, 2019 (in thousands): Assets Operating Lease Right-of-Use Assets $ 709 Liabilities Operating Lease Short-term Liabilities (recorded within Accrued liabilities) $ 656 Operating Lease Long-term Liabilities (recorded within Lease-related obligations) 179 Total Operating Lease Liabilities $ 835
Operating Lease Maturity	Operating lease payments over the remainder of the lease terms are as follows (in thousands): Years Ending December 31, As of December 31, 2019 2020 719 2021 182 Total future minimum rental payments $ 901 Less amount of lease payments representing interest 66 Total present value of lease payments $ 835
Operating Lease Maturity	As of December 31, 2018, future minimum payments under operating leases under ASC 840 were as follows (in thousands): Years Ending December 31, As of December 31, 2018 2019 $ 786 2020 797 2021 235 Total future minimum rental payments $ 1,818
Sublease Income	Total future minimum rentals under the non-cancelable operating sublease are presented below (in thousands): Years Ending December 31, Minimum Sublease Rentals 2020 $ 81 2021 14 Total future minimum sublease rentals $ 95

Stockholders' Equity (Deficit)

Stockholders' Equity (Deficit) (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 For exercise of common stock warrants — — For exercise of outstanding common stock options 8,390,304 6,429,638 For delivery upon vesting of outstanding restricted stock units 1,561,237 656,169 For future equity awards under the 2013 Equity Incentive Plan 1,855,688 1,587,670 For future purchases under the 2013 Employee Stock Purchase Plan 2,333,750 2,120,290 Total shares of common stock reserved for future issuances 14,140,979 10,793,767
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, 2019 2018 2017 Expected volatility 88.77 % 85.83 % 85.51 % Expected term (in years) 6.0 5.9 5.9 Weighted-average risk-free interest rate 2.42 % 2.52 % 2.02 % Expected dividend yield — % — % — % Weighted-average fair value per option $ 1.71 $ 3.43 $ 3.71
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, 2017 4,996,661 $ 15.51 7.59 Granted 2,050,995 4.73 — Exercised (29,262 ) 3.93 — Forfeited (588,756 ) 12.31 — Balance, December 31, 2018 6,429,638 $ 12.41 7.37 Granted 4,029,200 2.31 — Exercised (19,284 ) 2.21 — Forfeited (2,049,250 ) 9.26 — Balance, December 31, 2019 8,390,304 $ 8.36 7.47 $ 15,600 Exercisable at December 31, 2019 5,004,867 $ 12.18 6.44 $ 650 Vested or expected to vest at December 31, 2019 8,112,168 $ 8.56 7.41 $ 12,610
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, 2019 2018 2017 Weighted-average grant-date fair value per share of options granted $ 1.71 $ 3.43 $ 3.71 Total intrinsic value of options exercised $ 10 $ 31 $ 48 Total fair value of shares vested $ 6,798 $ 11,021 $ 11,786
Schedule of Share-based Compensation, Shares Authorized	The following table summarizes, at December 31, 2019 , 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 Exercise Price Range ($) Number Weighted-Average Remaining Contractual Life (in years) Weighted-Average Exercise Price Number Weighted-Average Exercise Price 1.51 to 7.57 5,764,618 8.41 $ 3.31 2,393,762 $ 4.21 7.58 to 8.06 1,361,550 6.02 8.06 1,346,969 8.06 8.07 to 18.75 210,117 4.08 18.75 210,117 18.75 18.76 to 39.17 462,350 4.50 24.55 462,350 24.55 39.18 to 53.74 591,669 5.20 41.81 591,669 41.81 1.51 to 53.74 8,390,304 7.47 $ 8.36 5,004,867 $ 12.18
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, 2019 2018 2017 Expected volatility 57.22 % 44.01 % 77.18 % Expected term (in years) 1.23 1.23 0.97 Weighted-average risk-free interest rate 2.36 % 2.56 % 0.99 % Expected dividend yield — % — % — % Weighted-average option value per share $ 1.00 $ 1.36 $ 2.65
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, 2018 656,169 $ 4.92 Granted 1,754,575 2.43 Share issuance (626,375 ) 3.51 Forfeited (223,132 ) 4.13 Balance, December 31, 2019 1,561,237 $ 2.80
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, 2019 2018 2017 Income Statement Classification: Research and development expense $ 4,089 $ 5,343 $ 7,047 General and administrative expense 5,439 7,731 9,063 Total stock-based compensation expense $ 9,528 $ 13,074 $ 16,110

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 , 2018 , and 2017 (in thousands, except percentages): 2019 2018 2017 Amount % of Pretax Amount % of Pretax Amount % of Pretax Income tax benefit at statutory rate $ (23,641 ) 21.0 % $ (14,590 ) 21.0 % $ (24,134 ) 34.0 % State income taxes (1,596 ) 1.4 % (792 ) 1.1 % (1,090 ) 1.5 % Research and development credits (1,190 ) 1.1 % (1,798 ) 2.6 % (2,039 ) 2.9 % Foreign rate differential — — % 2 — % 60 (0.1 )% Permanent items 696 (0.6 )% 1,164 (1.7 )% 1,646 (2.3 )% Provision to return adjustments 937 (0.8 )% 621 (0.9 )% 1,212 (1.7 )% Effect of change in federal tax rate — — % — — % 57,950 (81.6 )% Effect of change in state tax rate (117 ) 0.1 % 151 (0.2 )% 193 (0.3 )% Removal of excess tax benefit — — % — — % (12,930 ) 18.2 % Increase in unrecognized tax benefits 298 (0.3 )% 450 (0.7 )% 403 (0.6 )% Change in valuation allowance 24,613 (21.9 )% 14,792 (21.2 )% (21,271 ) 30.0 % Net benefit $ — — % $ — — % $ — — %
Components of Deferred Tax Assets and Liabilities	The components of deferred tax assets and liabilities at December 31, 2019 and 2018 were as follows (in thousands): December 31, 2019 2018 Deferred tax assets: Domestic net operating loss carryforwards $ 114,292 $ 104,708 Foreign net operating loss carryforwards — 71 Research and development expenses 419 1,002 Capitalized Section 174 expenses 24 25 License fees 14,176 — Research and development credits 14,682 13,789 Capital Loss Carryforwards 431 — Accrued bonuses 751 500 Share-based compensation 7,557 7,144 Other 456 778 Total gross deferred tax assets 152,788 128,017 Valuation allowance (152,629 ) (128,017 ) Total deferred tax assets 159 — Deferred tax liabilities: Other (159 ) — Total deferred tax liabilities (159 ) — 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, 2019 and 2018 , as follows (in thousands): Balance at December 31, 2017 $ 3,276 Increases related to 2018 450 Increases related to prior periods — Balance at December 31, 2018 3,726 Increases related to 2019 297 Increases related to prior periods — Balance at December 31, 2019 $ 4,023

Restructuring Costs (Tables)

Restructuring Costs (Tables)	12 Months Ended
	Dec. 31, 2019
Restructuring and Related Activities [Abstract]
Restructuring and Related Costs	The following table summarizes the restructuring charges (in thousands) recorded for the twelve months ended December 31, 2019 : Employee Termination Benefits Clinical Trial Close-out Costs Other Development Costs Fixed Asset Disposals Total Research and development 1,437 2,021 339 — 3,797 General and administrative 1,909 — — — 1,909 Interest income and other, net — — — 250 250 Total restructuring expenses 3,346 2,021 339 250 5,956
Schedule of Restructuring Reserve by Type of Cost	The following table sets forth the accrual activity for employee termination benefits and contract close-out costs (in thousands) for the twelve months ended December 31, 2019 . No additional charges are expected to be incurred. Employee Termination Benefits Clinical Trial Close-out Costs Other Development Costs Fixed Asset Disposals Total Balance at January 1, 2019 — — — — — Accruals 3,335 2,131 315 — 5,781 Revised estimates 11 (621 ) 24 250 (336 ) Payments (3,163 ) (1,483 ) (229 ) (250 ) (5,125 ) Balance at December 31, 2019 183 27 110 — 320

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2019
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 2019 and 2018 are as follows (in thousands, except share and per share data): 2019 Quarters Fourth Third Second First Revenue $ 6,767 $ 1,958 $ 1,438 $ 2,356 Operating loss (3,873 ) (74,564 ) (18,701 ) (18,845 ) Net loss (3,503 ) (73,730 ) (17,650 ) (17,693 ) Net loss per share, basic and diluted $ (0.06 ) $ (1.26 ) $ (0.35 ) $ (0.35 ) Weighted-average shares outstanding, basic and diluted 61,385,616 58,457,110 51,130,104 50,887,221 2018 Quarters Fourth Third Second First Revenue $ 4,864 $ 369 $ 1,193 $ 790 Operating loss (15,419 ) (16,710 ) (19,169 ) (20,307 ) Net loss (14,956 ) (16,079 ) (18,613 ) (19,826 ) Net loss per share, basic and diluted $ (0.29 ) $ (0.33 ) $ (0.39 ) $ (0.42 ) Weighted-average shares outstanding, basic and diluted 50,722,655 48,172,354 47,811,552 47,637,907

The Business and Summary of S_4

The Business and Summary of Significant Accounting Policies (Narrative) (Details)	12 Months Ended			107 Months Ended	112 Months Ended
	Dec. 31, 2019USD ($)product_candidatesegmentshares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)shares	Dec. 31, 2019USD ($)	May 31, 2020USD ($)	May 31, 2020base_segment	May 31, 2020option_segment	May 31, 2020extension	May 31, 2020revenue_contract	Sep. 30, 2019USD ($)	Jan. 01, 2019USD ($)
Class of Stock [Line Items]
Number of product candidates | product_candidate	2
Investments
Realized gains on available for sale debt securities	$ 2,000
Unrealized loss on equity investment	0	$ 348,000
Unrealized loss on equity investment			$ 1,160,000
Property and Equipment
Impairment of long-lived assets	0
Revenue Recognition
Contract revenue	12,519,000	7,216,000	4,494,000
Defined Contribution Plan [Abstract]
Defined contribution plan, cost recognized	$ 300,000	$ 400,000	$ 300,000
Basic and Diluted Net Loss Per Share of Common Stock
Antidilutive shares excluded from earnings per share calculation (in shares) | shares	1,571,356	749,110	1,687,887
Segments
Number of operating segments | segment	1
Leases
Operating lease right-of-use assets	$ 709,000			$ 709,000
Total present value of lease payments	$ 835,000			835,000
Minimum
Property and Equipment
Property and equipment, useful life	3 years
Maximum
Property and Equipment
Property and equipment, useful life	5 years
License
Revenue Recognition
Contract revenue	$ 4,915,000	$ 0	$ 0
Accounting Standards Update 2016-02
Leases
Operating lease right-of-use assets											$ 1,400,000
Total present value of lease payments											$ 1,600,000
BARDA
Revenue Recognition
Contract revenue	7,600,000	7,200,000	$ 4,500,000	70,100,000
Second Option Segment
Revenue Recognition
Contract revenue	23,900,000
Third Option Segment
Revenue Recognition
Contract revenue	14,100,000
SymBio Pharmaceuticals
Revenue Recognition
Initial upfront payment to be received										$ 5,000,000
Payment to be received upon achievement of milestones										$ 180,000,000
SymBio Pharmaceuticals | License
Revenue Recognition
Contract revenue	4,900,000
Deferred revenue	100,000			$ 100,000
ContraVir Pharmaceuticals
Investments
Unrealized loss on equity investment	$ 33,000	$ 348,000
Forecast | BARDA
Revenue Recognition
Contract revenue					$ 74,300,000
Number of base segments | base_segment						1
Number of option segments							4	4
Number of contracts | revenue_contract									5
Forecast | BARDA | Expense Reimbursement
Revenue Recognition
Contract revenue					75,800,000
Forecast | BARDA | Fees
Revenue Recognition
Contract revenue					$ 5,300,000

The Business and Summary of S_5

The Business and Summary of Significant Accounting Policies (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - Fair value, measurements, recurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Separate Account Investment [Line Items]
Cash equivalents	$ 11,854	$ 75,893
Short-term investments	96,574	105,424
Total assets	108,428	181,317
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents	11,854	42,208
Short-term investments	22,493	12,627
Total assets	34,347	54,835
Significant Other Observable Inputs (Level 2)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents	0	33,685
Short-term investments	74,081	92,797
Total assets	74,081	126,482
Significant Unobservable Inputs (Level 3)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents	0	0
Short-term investments	0	0
Total assets	0	0
Money market funds
Fair Value, Separate Account Investment [Line Items]
Cash equivalents	11,854	30,726
Money market funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents	11,854	30,726
Money market funds | Significant Other Observable Inputs (Level 2)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents	0	0
Money market funds | Significant Unobservable Inputs (Level 3)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents	0	0
U.S. Treasury securities
Fair Value, Separate Account Investment [Line Items]
Cash equivalents		11,482
Short-term investments	22,493	12,589
U.S. Treasury securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents		11,482
Short-term investments	22,493	12,589
U.S. Treasury securities | Significant Other Observable Inputs (Level 2)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents		0
Short-term investments	0	0
U.S. Treasury securities | Significant Unobservable Inputs (Level 3)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents		0
Short-term investments	0	0
Common stock of U.S. corporation
Fair Value, Separate Account Investment [Line Items]
Short-term investments		38
Common stock of U.S. corporation | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Separate Account Investment [Line Items]
Short-term investments		38
Common stock of U.S. corporation | Significant Other Observable Inputs (Level 2)
Fair Value, Separate Account Investment [Line Items]
Short-term investments		0
Common stock of U.S. corporation | Significant Unobservable Inputs (Level 3)
Fair Value, Separate Account Investment [Line Items]
Short-term investments		0
Commercial paper
Fair Value, Separate Account Investment [Line Items]
Cash equivalents		29,677
Short-term investments	43,119	60,114
Commercial paper | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Separate Account Investment [Line Items]
Short-term investments	0	0
Commercial paper | Significant Other Observable Inputs (Level 2)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents		29,677
Short-term investments	43,119	60,114
Commercial paper | Significant Unobservable Inputs (Level 3)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents		0
Short-term investments	0	0
Corporate bonds
Fair Value, Separate Account Investment [Line Items]
Cash equivalents		4,008
Short-term investments	30,962	32,683
Corporate bonds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents		0
Short-term investments	0	0
Corporate bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents		4,008
Short-term investments	30,962	32,683
Corporate bonds | Significant Unobservable Inputs (Level 3)
Fair Value, Separate Account Investment [Line Items]
Cash equivalents		0
Short-term investments	$ 0	$ 0

The Business and Summary of S_6

The Business and Summary of Significant Accounting Policies (Schedule of Prepaid and Other Current Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Prepaid research and development expenses	$ 936	$ 1,071
Interest receivable	323	259
Prepaid insurance	344	382
Other prepaid expenses and current assets	1,782	886
Total prepaid expenses and other current assets	$ 3,385	$ 2,598

The Business and Summary of S_7

The Business and Summary of Significant Accounting Policies (Schedule of Accrued Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Accrued compensation	$ 3,626	$ 2,469
Accrued research and development expenses	1,868	4,525
Other accrued liabilities	1,336	1,281
Total accrued liabilities	$ 6,830	$ 8,275

Investments - Summary of Availa

Investments - Summary of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Amortized Cost	$ 96,539	$ 105,475
Gross Unrealized Gains	50	0
Gross Unrealized Losses	(15)	(89)
Estimated Fair Value	96,574	105,386
Corporate bonds
Schedule of Investments [Line Items]
Amortized Cost	30,952	32,724
Gross Unrealized Gains	19	0
Gross Unrealized Losses	(9)	(41)
Estimated Fair Value	30,962	32,683
Commercial paper
Schedule of Investments [Line Items]
Amortized Cost	43,109	60,159
Gross Unrealized Gains	14	0
Gross Unrealized Losses	(4)	(45)
Estimated Fair Value	43,119	60,114
U.S. Treasury securities
Schedule of Investments [Line Items]
Amortized Cost	22,478	12,592
Gross Unrealized Gains	17	0
Gross Unrealized Losses	(2)	(3)
Estimated Fair Value	$ 22,493	$ 12,589

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, 2019USD ($)security	Dec. 31, 2018USD ($)security
Schedule of Investments [Line Items]
Fair Value, Less than 12 Months	$ 22,798	$ 105,386
Unrealized Loss, Less than 12 Months	(15)	(89)
Fair Value, Greater than 12 Months	0	0
Unrealized Loss, Greater than 12 Months	0	0
Fair Value, Total	22,798	105,386
Unrealized Loss, Total	$ (15)	$ (89)
Number of securities with unrealized losses, Less than 12 Months | security	9	36
Number of securities with unrealized losses, Greater than 12 Months | security	0	0
Number of securities with unrealized losses, Total | security	9	36
Corporate bonds
Schedule of Investments [Line Items]
Fair Value, Less than 12 Months	$ 9,657	$ 32,683
Unrealized Loss, Less than 12 Months	(9)	(41)
Fair Value, Greater than 12 Months	0	0
Unrealized Loss, Greater than 12 Months	0	0
Fair Value, Total	9,657	32,683
Unrealized Loss, Total	(9)	(41)
Commercial paper
Schedule of Investments [Line Items]
Fair Value, Less than 12 Months	10,147	60,114
Unrealized Loss, Less than 12 Months	(4)	(45)
Fair Value, Greater than 12 Months	0	0
Unrealized Loss, Greater than 12 Months	0	0
Fair Value, Total	10,147	60,114
Unrealized Loss, Total	(4)	(45)
U.S. Treasury securities
Schedule of Investments [Line Items]
Fair Value, Less than 12 Months	2,994	12,589
Unrealized Loss, Less than 12 Months	(2)	(3)
Fair Value, Greater than 12 Months	0	0
Unrealized Loss, Greater than 12 Months	0	0
Fair Value, Total	2,994	12,589
Unrealized Loss, Total	$ (2)	$ (3)

Investments - Available for Sal

Investments - Available for Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments [Abstract]
Maturing in one year or less	$ 96,574
Total debt investments	$ 96,574	$ 105,386

Property and Equipment (Details

Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Property and equipment	$ 5,581	$ 5,850
Less accumulated depreciation	(5,041)	(4,640)
Property and equipment, net of accumulated depreciation	540	1,210
Lab equipment
Property, Plant and Equipment [Line Items]
Property and equipment	2,329	2,599
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment	1,550	1,550
Computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment	1,182	1,181
Office furniture and equipment
Property, Plant and Equipment [Line Items]
Property and equipment	$ 520	$ 520

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details)	12 Months Ended
	Dec. 31, 2019USD ($)ft²	Dec. 31, 2018USD ($)
Commitments and Contingencies Disclosure [Abstract]
Weighted average remaining lease term	1 year 99 days
Lease expense under operating leases	$ 724,000	$ 692,000
Weighted average discount rate	13.06%
Lease payments	$ 751,000	750,000
Area of sublease space (in sq ft) | ft²	3,537
Sublease income	$ 71,000
Reduction in rent expense for sublease		71,000
BARDA | Refundable Agreements
Disaggregation of Revenue [Line Items]
Provision for refundable amounts	$ 0	$ 0

Commitments and Contingencies_2

Commitments and Contingencies - Operating Lease Right-of-Use Assets and Liabilities (Details) $ in Thousands	Dec. 31, 2019USD ($)
Assets
Operating Lease Right-of-Use Assets	$ 709
Liabilities
Operating Lease Short-term Liabilities (recorded within Accrued liabilities)	656
Operating Lease Long-term Liabilities (recorded within Lease-related obligations)	179
Total Operating Lease Liabilities	$ 835

Commitments and Contingencies_3

Commitments and Contingencies - Maturity Analysis of Operating Lease Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020	$ 719
2021	182
Total future minimum rental payments	901
Less amount of lease payments representing interest	66
Total present value of lease payments	$ 835
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019		$ 786
2020		797
2021		235
Total future minimum rental payments		$ 1,818

Commitments and Contingencies_4

Commitments and Contingencies - Minimum Sublease Rentals (Details) $ in Thousands	Dec. 31, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
2019	$ 81
2020	14
Total future minimum sublease rentals	$ 95

Stockholders' Equity (Deficit_2

Stockholders' Equity (Deficit) (Narrative) (Details) - USD ($)	Feb. 05, 2019	Jan. 01, 2019	Jan. 01, 2018	May 31, 2019	Apr. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 08, 2017	Feb. 28, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized (in shares)						200,000,000	200,000,000
Common stock, shares issued (in shares)						61,590,013	50,735,279
Common stock, shares outstanding (in shares)						61,590,013	50,735,279
Shares reserved for future issuance (in shares)						14,140,979	10,793,767
Liability						$ 6,830,000	$ 8,275,000
Shares issued pursuant to vesting of RSUs (in shares)						626,375	233,050
Proceeds from exercise of all share-based payment arrangements						$ 400,000	$ 700,000	$ 800,000
Tax benefit from share-based payment arrangements						$ 0	$ 0	$ 0
Accelerated compensation period	15 months			12 months
Accelerated compensation	$ 1,800,000			$ 700,000
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued pursuant to vesting of RSUs (in shares)							0
Restricted stock units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation costs, period of recognition						3 years
Shares reserved for future issuance (in shares)						1,561,237	656,169
Unrecognized compensation costs						$ 3,000,000
Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)						8,390,304	6,429,638
The 2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, equity increase percentage						4.00%
Unrecognized compensation costs						$ 5,200,000
Unrecognized compensation costs, period of recognition						2 years 263 days
Shares reserved for future issuance (in shares)						1,855,688	1,587,670
The 2013 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, equity increase percentage						1.00%
Number of shares authorized to be granted (in shares)										704,225
Number of shares authorized, increase amount (in shares)						422,535
Number of additional shares authorized (in shares)		422,535	422,535
Number of shares available for grant (in shares)		3,071,331	2,648,796			3,071,331
Shares reserved for future issuance (in shares)						2,333,750	2,120,290
Percentage of pay that employee can contribute, maximum						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)						209,075	163,717
Liability						$ 100,000
Private Placement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock sold (in shares)							2,800,000
Sale of common stock, amount authorized									$ 75,000,000
Sale of common stock, weighted average price of shares sold (in usd per share)							$ 4
Sale of common stock, net offering proceeds							$ 10,900,000
Chief Executive Officer
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)					1,750,000
Chief Executive Officer | Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period					10 years
Vesting period					4 years
Year One | Chief Executive Officer | Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage					25.00%
Year Two | Chief Executive Officer | Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage					25.00%
Year Three | Chief Executive Officer | Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage					25.00%
Year Four | Chief Executive Officer | Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting percentage					25.00%

Stockholders' Equity (Deficit_3

Stockholders' Equity (Deficit) (Schedule of Shares Reserved for Future Issuance) (Details) - shares	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	14,140,979	10,793,767
The 2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	1,855,688	1,587,670
The 2013 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	2,333,750	2,120,290
Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	8,390,304	6,429,638
Restricted stock units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	1,561,237	656,169
Warrant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares reserved for future issuance (in shares)	0	0

Stockholders' Equity (Deficit_4

Stockholders' Equity (Deficit) (Assumption Used to Determine the Fair Value of Options Granted) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average fair value per option (in usd per share)	$ 1.71	$ 3.43	$ 3.71
Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	88.77%	85.83%	85.51%
Expected term (in years)	6 years 4 days	5 years 329 days	5 years 329 days
Weighted-average risk-free interest rate	2.42%	2.52%	2.02%
Expected dividend yield	0.00%	0.00%	0.00%
Weighted-average fair value per option (in usd per share)	$ 1.71	$ 3.43	$ 3.71

Stockholders' Equity (Deficit_5

Stockholders' Equity (Deficit) (Summary of Activity Related to Stock Options) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Options Outstanding
Beginning balance (in shares)	6,429,638	4,996,661
Granted (in shares)	4,029,200	2,050,995
Exercised (in shares)	(19,284)	(29,262)
Forfeited (in shares)	(2,049,250)	(588,756)
Ending balance (in shares)	8,390,304	6,429,638	4,996,661
Exercisable (in shares)	5,004,867
Vested and expected to vest (in shares)	8,112,168
Weighted-Average Exercise Price
Beginning balance (in usd per share)	$ 12.41	$ 15.51
Granted (in usd per share)	2.31	4.73
Exercised (in usd per share)	2.21	3.93
Forfeited (in usd per share)	9.26	12.31
Ending balance (in usd per share)	8.36	$ 12.41	$ 15.51
Exercisable (in usd per share)	12.18
Vested or expected to vest (in usd per share)	$ 8.56
Additional Disclosure
Weighted-average remaining contractual life (in years)	7 years 172 days	7 years 135 days	7 years 215 days
Weighted-average remaining contractual life (in years), Exercisable	6 years 161 days
Weighted-average remaining contractual life (in years), Vested or expected to vest	7 years 150 days
Total intrinsic value of options outstanding	$ 15,600
Total intrinsic value of options exercisable	650
Total intrinsic value of options vested or expected to vest	$ 12,610

Stockholders' Equity (Deficit_6

Stockholders' Equity (Deficit) (Schedule of other information regarding stock options) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stockholders' Equity Note [Abstract]
Weighted-average fair value per option (in usd per share)	$ 1.71	$ 3.43	$ 3.71
Total intrinsic value of options exercised	$ 10	$ 31	$ 48
Total fair value of shares vested	$ 6,798	$ 11,021	$ 11,786

Stockholders' Equity (Deficit_7

Stockholders' Equity (Deficit) (Schedule of Share-based Compensation, Shares Authorized) (Details)	12 Months Ended
	Dec. 31, 2019$ / sharesshares
1.51 to 7.57
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit (in usd per share)	$ 1.51
Share-based compensation, upper range limit (in usd per share)	$ 7.57
Share-based compensation, number of outstanding options (in shares) | shares	5,764,618
Share-based compensation, outstanding options, weighted average remaining contractual term	8 years 150 days
Share-based compensation, outstanding options, weighted average exercise price (in usd per share)	$ 3.31
Share-based compensation, number of exercisable options (in shares) | shares	2,393,762
Share-based compensation, exercisable options, weighted average exercise price (in usd per share)	$ 4.21
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
Share-based compensation, upper range limit (in usd per share)	$ 8.06
Share-based compensation, number of outstanding options (in shares) | shares	1,361,550
Share-based compensation, outstanding options, weighted average remaining contractual term	6 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	1,346,969
Share-based compensation, exercisable options, weighted average exercise price (in usd per share)	$ 8.06
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
Share-based compensation, upper range limit (in usd per share)	$ 18.75
Share-based compensation, number of outstanding options (in shares) | shares	210,117
Share-based compensation, outstanding options, weighted average remaining contractual term	4 years 29 days
Share-based compensation, outstanding options, weighted average exercise price (in usd per share)	$ 18.75
Share-based compensation, number of exercisable options (in shares) | shares	210,117
Share-based compensation, exercisable options, weighted average exercise price (in usd per share)	$ 18.75
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
Share-based compensation, upper range limit (in usd per share)	$ 39.17
Share-based compensation, number of outstanding options (in shares) | shares	462,350
Share-based compensation, outstanding options, weighted average remaining contractual term	4 years 183 days
Share-based compensation, outstanding options, weighted average exercise price (in usd per share)	$ 24.55
Share-based compensation, number of exercisable options (in shares) | shares	462,350
Share-based compensation, exercisable options, weighted average exercise price (in usd per share)	$ 24.55
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
Share-based compensation, upper range limit (in usd per share)	$ 53.74
Share-based compensation, number of outstanding options (in shares) | shares	591,669
Share-based compensation, outstanding options, weighted average remaining contractual term	5 years 73 days
Share-based compensation, outstanding options, weighted average exercise price (in usd per share)	$ 41.81
Share-based compensation, number of exercisable options (in shares) | shares	591,669
Share-based compensation, exercisable options, weighted average exercise price (in usd per share)	$ 41.81
1.51 to 53.74
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit (in usd per share)	1.57
Share-based compensation, upper range limit (in usd per share)	$ 53.74
Share-based compensation, number of outstanding options (in shares) | shares	8,390,304
Share-based compensation, outstanding options, weighted average remaining contractual term	7 years 172 days
Share-based compensation, outstanding options, weighted average exercise price (in usd per share)	$ 8.36
Share-based compensation, number of exercisable options (in shares) | shares	5,004,867
Share-based compensation, exercisable options, weighted average exercise price (in usd per share)	$ 12.18

Stockholders' Equity (Deficit_8

Stockholders' Equity (Deficit) (Schedule of Share-Based Compensation) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average fair value per option (in usd per share)	$ 1.71	$ 3.43	$ 3.71
The 2013 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected volatility	57.22%	44.01%	77.18%
Expected term (in years)	1 year 84 days	1 year 84 days	355 days
Weighted-average risk-free interest rate	2.36%	2.56%	0.99%
Expected dividend yield	0.00%	0.00%	0.00%
Weighted-average fair value per option (in usd per share)	$ 1	$ 1.36	$ 2.65

Stockholders' Equity (Deficit_9

Stockholders' Equity (Deficit) (Summary of Activity Related to Restricted Stock Units) (Details) - Restricted stock units (RSUs)	12 Months Ended
	Dec. 31, 2019$ / sharesshares
Number of Restricted Stock Units Outstanding
Beginning Balance (in shares) | shares	656,169
Granted (in shares) | shares	1,754,575
Shares issuance (in shares) | shares	(626,375)
Forfeited (in shares) | shares	(223,132)
Ending Balance (in shares) | shares	1,561,237
Weighted-Average Grant-Date Fair Value
Beginning Balance (in usd per share) | $ / shares	$ 4.92
Granted (in usd per share) | $ / shares	2.43
Shares issuance (in usd per share) | $ / shares	3.51
Forfeited (in usd per share) | $ / shares	4.13
Ending Balance (in usd per share) | $ / shares	$ 2.80

Stockholders' Equity (Defici_10

Stockholders' Equity (Deficit) (Schedule of Allocation of Recognized Period Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 9,528	$ 13,074	$ 16,110
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	4,089	5,343	7,047
General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 5,439	$ 7,731	$ 9,063

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 ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Amount
Income tax benefit at statutory rate	$ (23,641)	$ (14,590)	$ (24,134)
State income taxes	(1,596)	(792)	(1,090)
Research and development credits	(1,190)	(1,798)	(2,039)
Foreign rate differential	0	2	60
Permanent items	696	1,164	1,646
Provision to return adjustments	937	621	1,212
Effect of change in federal tax rate	0	0	57,950
Effect of change in state tax rate	(117)	151	193
Removal of excess tax benefit	0	0	(12,930)
Increase in unrecognized tax benefits	298	450	403
Change in valuation allowance	24,613	14,792	(21,271)
Net benefit	$ 0	$ 0	$ 0
% of Pretax Earnings
Income tax benefit at statutory rate	21.00%	21.00%	34.00%
State income taxes	1.40%	1.10%	1.50%
Research and development credits	1.10%	2.60%	2.90%
Foreign rate differential	0.00%	0.00%	(0.10%)
Permanent items	(0.60%)	(1.70%)	(2.30%)
Provision to return adjustments	(0.80%)	(0.90%)	(1.70%)
Effect of change in federal tax rate	0.00%	0.00%	(81.60%)
Effect of change in state tax rate	0.10%	(0.20%)	(0.30%)
Removal of excess tax benefit	0.00%	0.00%	18.20%
Increase in unrecognized tax benefits	(0.30%)	(0.70%)	(0.60%)
Change in valuation allowance	(21.90%)	(21.20%)	30.00%
Net benefit	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, 2019	Dec. 31, 2018
Deferred tax assets:
Domestic net operating loss carryforwards	$ 114,292	$ 104,708
Foreign net operating loss carryforwards	0	71
Research and development expenses	419	1,002
Capitalized Section 174 expenses	24	25
License fees	14,176	0
Research and development credits	14,682	13,789
Capital Loss Carryforwards	431	0
Accrued bonuses	751	500
Share-based compensation	7,557	7,144
Other	456	778
Total gross deferred tax assets	152,788	128,017
Valuation allowance	(152,629)	(128,017)
Total deferred tax assets	159	0
Deferred tax liabilities:
Other	(159)	0
Total deferred tax liabilities	(159)	0
Total deferred tax assets and liabilities, net	$ 0	$ 0

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2013	Dec. 31, 2007	Dec. 31, 2002
Operating Loss Carryforwards [Line Items]
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	0	$ 0	$ 57,950,000
Undisturbed foreign earnings			$ 0
Federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	508,100,000	465,300,000
Operating loss carryforwards, subject to expiration		408,100,000
Operating loss carryforwards, not subject to expiration	100,000,000
State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	384,300,000	353,800,000
Foreign
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards		$ 400,000
Tax Credit Carryforward | Federal
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	19,600,000
Capital Loss Carryforward | Federal
Operating Loss Carryforwards [Line Items]
Tax credit carryforward	$ 1,900,000

Income Taxes (Summary of Uncert

Income Taxes (Summary of Uncertain Tax Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 3,726	$ 3,276
Increase related to current year tax positions	297	450
Increases related to prior periods	0	0
Ending balance	$ 4,023	$ 3,726

Significant Agreements (Details

Significant Agreements (Details)	Sep. 30, 2019USD ($)	Jul. 26, 2019USD ($)shares	Jan. 31, 2017shares	Dec. 31, 2016USD ($)	Dec. 31, 2019USD ($)period	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2019USD ($)	May 31, 2020USD ($)	May 31, 2020option_segment	May 31, 2020extension
Long-term Purchase Commitment [Line Items]
Contract revenue					$ 12,519,000	$ 7,216,000	$ 4,494,000
License
Long-term Purchase Commitment [Line Items]
Contract revenue					4,915,000	0	0
BARDA
Long-term Purchase Commitment [Line Items]
Contract revenue					$ 7,600,000	$ 7,200,000	$ 4,500,000	$ 70,100,000
Number of extension periods | period					3
Second Option Segment
Long-term Purchase Commitment [Line Items]
Contract revenue					$ 23,900,000
Third Option Segment
Long-term Purchase Commitment [Line Items]
Contract revenue					14,100,000
Cantex Pharmaceuticals
Long-term Purchase Commitment [Line Items]
Upfront cash payment for license and development agreement		$ 30,000,000
Shares issued for license and development agreement (in shares) | shares		10,000,000
Acquired in-process research and development					65,000,000
Transaction costs		$ 100,000
Payments to be made for regulatory milestones (up to)		202,500,000
Payments to be made upon achievement of specified net sales levels (up to)		$ 385,000,000
Royalty percentage to be paid (at least)		10.00%
SymBio Pharmaceuticals
Long-term Purchase Commitment [Line Items]
Termination notice period	90 days
Initial upfront payment to be received	$ 5,000,000
Payment to be received upon achievement of milestones	$ 180,000,000
SymBio Pharmaceuticals | License
Long-term Purchase Commitment [Line Items]
Contract revenue					4,900,000
Deferred revenue					$ 100,000			$ 100,000
The Regents of the University of Michigan
Long-term Purchase Commitment [Line Items]
Upfront cash payment for license and development agreement				$ 50,000
Shares issued for license and development agreement (in shares) | shares			33,058
The Regents of the University of Michigan | Maximum
Long-term Purchase Commitment [Line Items]
Royalty percentage to be paid (at least)				2.00%
The Regents of the University of Michigan | Minimum
Long-term Purchase Commitment [Line Items]
Royalty percentage to be paid (at least)				0.25%
Forecast | BARDA
Long-term Purchase Commitment [Line Items]
Number of option segments										4	4
Contract revenue									$ 74,300,000
Forecast | BARDA | Expense Reimbursement
Long-term Purchase Commitment [Line Items]
Contract revenue									75,800,000
Forecast | BARDA | Fees
Long-term Purchase Commitment [Line Items]
Contract revenue									$ 5,300,000

Restructuring Costs (Details)

Restructuring Costs (Details) $ in Thousands	1 Months Ended	12 Months Ended
	May 31, 2019employee	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Restructuring Cost and Reserve [Line Items]
Reduction in workforce | employee	43
One-time employee termination benefits		$ 3,346
Accrued contract close-out costs		2,021
Other BCV development costs		339
Fixed Asset Disposals		250
Accrual balance		320	$ 0
Revised estimates		(336)
Reduction in clinical trial close-out costs from change in estimates		196
Trial Terminations
Restructuring Cost and Reserve [Line Items]
Accrual balance		27	0
Revised estimates		(621)
Other Development Costs
Restructuring Cost and Reserve [Line Items]
Accrual balance		110	0
Revised estimates		24
Severance Accrual Balance
Restructuring Cost and Reserve [Line Items]
Accrual balance		183	$ 0
Revised estimates		11
Restatement Adjustment
Restructuring Cost and Reserve [Line Items]
Deposits		(1,300)
Other receivables		$ 1,300

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Revenue	$ 6,767	$ 1,958	$ 1,438	$ 2,356	$ 4,864	$ 369	$ 1,193	$ 790
Operating loss	(3,873)	(74,564)	(18,701)	(18,845)	(15,419)	(16,710)	(19,169)	(20,307)	$ (115,983)	$ (71,605)	$ (72,102)
Net loss	$ (3,503)	$ (73,730)	$ (17,650)	$ (17,693)	$ (14,956)	$ (16,079)	$ (18,613)	$ (19,826)	$ (112,576)	$ (69,474)	$ (70,984)
Net loss per share, basic and diluted (in USD per share)	$ (0.06)	$ (1.26)	$ (0.35)	$ (0.35)	$ (0.29)	$ (0.33)	$ (0.39)	$ (0.42)	$ (2.03)	$ (1.43)	$ (1.51)
Weighted-average shares outstanding, basic and diluted (in shares)	61,385,616	58,457,110	51,130,104	50,887,221	50,722,655	48,172,354	47,811,552	47,637,907	55,501,973	48,593,435	46,963,430

Restructuring Costs - Summary o

Restructuring Costs - Summary of Restructuring Charges (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Restructuring Cost and Reserve [Line Items]
Employee Termination Benefits	$ 3,346
Clinical Trial Close-out Costs	2,021
Other Development Costs	339
Fixed Asset Disposals	250
Total	5,956
Research and development expense
Restructuring Cost and Reserve [Line Items]
Employee Termination Benefits	1,437
Clinical Trial Close-out Costs	2,021
Other Development Costs	339
Fixed Asset Disposals	0
Total	3,797
General and administrative expense
Restructuring Cost and Reserve [Line Items]
Employee Termination Benefits	1,909
Clinical Trial Close-out Costs	0
Other Development Costs	0
Fixed Asset Disposals	0
Total	1,909
Interest income and other, net
Restructuring Cost and Reserve [Line Items]
Employee Termination Benefits	0
Clinical Trial Close-out Costs	0
Other Development Costs	0
Fixed Asset Disposals	250
Total	$ 250

Restructuring Costs - Accrual A

Restructuring Costs - Accrual Activity for Restructuring Accrual (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019USD ($)
Restructuring Reserve [Roll Forward]
Beginning balance	$ 0
Accruals	5,781
Revised estimates	(336)
Payments	(5,125)
Ending balance	320
Employee Termination Benefits
Restructuring Reserve [Roll Forward]
Beginning balance	0
Accruals	3,335
Revised estimates	11
Payments	(3,163)
Ending balance	183
Clinical Trial Close-out Costs
Restructuring Reserve [Roll Forward]
Beginning balance	0
Accruals	2,131
Revised estimates	(621)
Payments	(1,483)
Ending balance	27
Other Development Costs
Restructuring Reserve [Roll Forward]
Beginning balance	0
Accruals	315
Revised estimates	24
Payments	(229)
Ending balance	110
Fixed Asset Disposals
Restructuring Reserve [Roll Forward]
Beginning balance	0
Accruals	0
Revised estimates	250
Payments	(250)
Ending balance	$ 0