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Document And Entity Information - shares	3 Months Ended
Document And Entity Information - shares	Mar. 31, 2018	Apr. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q1
Entity Registrant Name	CHIMERIX INC
Entity Central Index Key	0001117480
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding (in shares)		47,753,300

Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 22,112	$ 18,548
Short-term investments, available-for-sale	138,335	132,972
Accounts receivable	426	1,682
Prepaid expenses and other current assets	3,051	3,331
Total current assets	163,924	156,533
Long-term investments	49,225	76,731
Property and equipment, net of accumulated depreciation	1,688	1,894
Other long-term assets	61	72
Total assets	214,898	235,230
Current liabilities:
Accounts payable	1,892	3,812
Accrued liabilities	7,109	9,384
Total current liabilities	9,001	13,196
Lease-related obligations	206	224
Total liabilities	9,207	13,420
Stockholders’ equity:
Preferred stock, $0.001 par value, 10,000,000 shares authorized at March 31, 2018 and December 31, 2017; no shares issued and outstanding as of March 31, 2018 and December 31, 2017	0	0
Common stock, $0.001 par value, 200,000,000 shares authorized at March 31, 2018 and December 31, 2017; 47,753,300 and 47,505,532 shares issued and outstanding as of March 31, 2018 and December 31, 2017, respectively	48	47
Additional paid-in capital	713,323	709,514
Accumulated other comprehensive loss, net	(1,066)	(963)
Accumulated deficit	(506,614)	(486,788)
Total stockholders’ equity	205,691	221,810
Total liabilities and stockholders’ equity	$ 214,898	$ 235,230

Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value (in dollars 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 dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	200,000,000	200,000,000
Common stock, shares issued (in shares)	47,753,300	47,505,532
Common stock, shares outstanding (in shares)	47,753,300	47,505,532

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Contract revenue	$ 790	$ 1,078
Operating expenses:
Research and development	14,359	12,742
General and administrative	6,738	6,596
Total operating expenses	21,097	19,338
Loss from operations	(20,307)	(18,260)
Other (expense) income:
Unrealized loss on equity investment	(134)	0
Interest income	615	506
Net loss	(19,826)	(17,754)
Other comprehensive loss:
Unrealized (loss) gain on investments, net	(103)	331
Comprehensive loss	$ (19,929)	$ (17,423)
Per share information:
Net loss, basic and diluted (in dollars per share)	$ (0.42)	$ (0.38)
Weighted-average shares outstanding, basic and diluted (in shares)	47,637,907	46,573,394

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net loss	$ (19,826)	$ (17,754)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation of property and equipment	248	281
Amortization of premium/discount on investments	(73)	72
Share-based compensation	3,391	4,028
Unrealized loss on equity investment	134	0
Amortization of lease-related obligations	(11)	(47)
Changes in operating assets and liabilities:
Accounts receivable	1,256	787
Prepaid expenses and other assets	294	450
Accounts payable and accrued liabilities	(3,925)	(1,440)
Net cash used in operating activities	(18,512)	(13,623)
Cash flows from investing activities:
Purchases of property and equipment	(42)	(5)
Purchases of short-term investments	(1,989)	0
Purchases of long-term investments	(6,031)	(87,509)
Proceeds from sales of short-term investments	9,500	0
Proceeds from maturities of short-term investments	20,500	71,145
Net cash provided by (used in) investing activities	21,938	(16,369)
Cash flows from financing activities:
Proceeds from exercise of stock options	60	9
Proceeds from employee stock purchase plan	358	386
Payments of deferred offering costs	(280)	0
Net cash provided by financing activities	138	395
Net increase (decrease) in cash and cash equivalents	3,564	(29,597)
Cash and cash equivalents:
Beginning of period	18,548	51,463
End of period	$ 22,112	$ 21,866

The Business and Summary of Significant Accounting Policies

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

The Business and Summary of Significant Accounting Policies

Description of Business

Chimerix, Inc. (the Company) is a biopharmaceutical company committed to discovering, developing and commercializing medicines that improve outcomes for immunocompromised patients. The Company was founded in 2000 based on the promise of our proprietary lipid conjugate technology to unlock the potential of some of the most broad-spectrum antivirals by enhancing their antiviral activity and safety profiles in convenient dosing regimens. The Company's lead compound, brincidofovir, is in development as an oral and intravenous (IV) formulation for the prevention and treatment of DNA viruses, including smallpox, adenoviruses, and the human herpesviruses. The Company is in clinical development of CMX521, the first direct-acting antiviral specifically for the treatment and prevention of norovirus. In addition, the Company has an active discovery program focusing on viral targets for which limited or no therapies are currently available.

Basis of Presentation

The accompanying unaudited consolidated financial statements include the accounts of the Company and its wholly owned subsidiary. The accompanying unaudited consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (GAAP) for interim financial information, the instructions to Form 10-Q and Article 10 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements and should be read in conjunction with the Company’s audited financial statements and notes thereto included in its Annual Report on Form 10-K for the year ended December 31, 2017. In the opinion of the Company’s management, all adjustments, consisting of normal recurring adjustments, necessary for a fair presentation of its financial position, operating results and cash flows for the periods presented have been included. Operating results for the three months ended March 31, 2018 are not necessarily indicative of the results that may be expected for the full year, for any other interim period or for any future year.

Fair Value of Financial Instruments

The carrying amounts of certain financial instruments, including accounts receivable, accounts payable and accrued expenses approximate their fair values due to the short-term nature of such instruments.

For assets and liabilities recorded at fair value, it is the Company’s policy to maximize the use of observable inputs and minimize the use of unobservable inputs when developing fair value measurements, in accordance with the fair value hierarchy. Fair value measurements for assets and liabilities where there exists limited or no observable market data are based primarily upon estimates and are often calculated based on the economic and competitive environment, the characteristics of the asset or liability and other factors. Therefore, fair value measurements cannot be determined with precision and may not be realized in an actual sale or immediate settlement of the asset or liability. Additionally, there may be inherent weaknesses in any calculation technique and changes in the underlying assumptions used, including discount rates and estimates of future cash flows, could significantly affect the calculated current or future fair values. The Company utilizes fair value measurements to record fair value adjustments to certain assets and liabilities and to determine fair value disclosures.

The Company groups assets and liabilities at fair value in three levels, based on the markets in which the assets and liabilities are traded and the reliability of the assumptions used to determine fair value. An adjustment to the pricing method used within either Level 1 or Level 2 inputs could generate a fair value measurement that effectively falls in a lower level in the hierarchy. These levels are:


•	Level 1 — Valuations based on unadjusted quoted prices in active markets for identical assets or liabilities that the Company has the ability to access.


•	Level 2 — Valuations based on quoted prices for similar assets or liabilities in active markets, quoted prices for identical or similar assets or liabilities in markets that are not active, and models for which all significant inputs are observable, either directly or indirectly.


•	Level 3 — Valuations based on inputs that are unobservable and significant to the overall fair value measurement.

The determination of where an asset or liability falls in the hierarchy requires significant judgment. The Company evaluates hierarchy disclosures and, based on various factors, it is possible that an asset or liability may be classified differently from period to period. However, the Company expects that changes in classification between levels will be rare. There were no transfers between Level 1 and Level 2 and no transfers to or from Level 3 during the three months ended March 31, 2018.

At March 31, 2018 and December 31, 2017, the Company had cash equivalents, consisting of money market accounts, and short-term and long-term investments consisting of U.S. Treasury securities, whose value is based on using quoted market prices. Accordingly, these securities are classified as Level 1.

At March 31, 2018 and December 31, 2017, the Company had cash equivalents consisting of commercial paper. 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.

In December 2014, the Company entered into a license agreement with ContraVir Pharmaceuticals (NASDAQ: CTRV) for the development and commercialization of CMX157 for certain antiviral indications. Under the terms of the agreement, ContraVir has sole responsibility with respect to the control of the development and commercialization of CMX157. In exchange for the license to CMX157 rights, the Company received an upfront payment consisting of ContraVir Series B Convertible Preferred Stock. On September 30, 2016, the Company converted its preferred stock investment in ContraVir into 1,071,429 shares of ContraVir common stock. The Company's investment in ContraVir common stock was categorized as a Level 1 asset and valued based on ContraVir's common stock value at March 31, 2018 and December 31, 2017. For the three months ended March 31, 2018, the Company recorded $0.1 million of unrealized loss to unrealized loss on equity investment in the Consolidated Statements of Operations and Comprehensive Loss related to the Company's investment in ContraVir common stock. For the three months ended March 31, 2017, the Company recorded $0.6 million of unrealized loss to unrealized (loss) gain on investments, net in the Consolidated Statements of Operations and Comprehensive Loss related to the Company's investment in ContraVir common stock.

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
	March 31, 2018
	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents
Money market funds	$	12,926	$	12,926	$	—	$	—
Commercial paper	6,486		—		6,486		—
Total cash equivalents	19,412		12,926		6,486		—
Short-term investments
U.S. treasury securities	138,083		138,083		—		—
Common stock of U.S. corporation	252		252		—		—
Total short-term investments	138,335		138,335		—		—
Long-term investments
U.S. treasury securities	49,225		49,225		—		—
Total long-term investments	49,225		49,225		—		—
Total assets	$	206,972	$	200,486	$	6,486	$	—

	Fair Value Measurements
	December 31, 2017
	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents
Money market funds	$	10,816	$	10,816	$	—	$	—
Commercial paper	3,995		—		3,995		—
Total cash equivalents	14,811		10,816		3,995		—
Short-term investments
U.S. treasury securities	132,586		132,586		—		—
Common stock of U.S. corporation	386		386		—		—
Total short-term investments	132,972		132,972		—		—
Long-term investments
U.S. treasury securities	76,731		76,731		—		—
Total long-term investments	76,731		76,731		—		—
Total assets	$	224,514	$	220,519	$	3,995	$	—

Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):



	March 31, 2018		December 31, 2017
Accrued research and development expenses	$	4,084	$	3,384
Accrued compensation	1,911		3,678
Other accrued liabilities	1,114		1,322
Accrued indemnification claim	—		1,000
Total accrued liabilities	$	7,109	$	9,384

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 4 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. 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. As such, the Company does not disclose the value of unsatisfied performance obligations for contracts for which the Company recognizes revenue at the amount to which the Company has a right to invoice. 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 $21.6 million and $11.6 million in expense reimbursement and fees, respectively. The second and third option segments are expected to end on September 30, 2018.

ContraVir Pharmaceuticals

The Company entered into a license agreement with ContraVir Pharmaceuticals on December 17, 2014 for the development and commercialization of CMX157 for certain antiviral indications. The Company is eligible to receive up to approximately $20 million in clinical, regulatory and initial commercial milestones as well as royalties and additional milestones based on commercial sales. The Company assessed the agreement in accordance with the authoritative guidance and concluded that the ContraVir contract includes multiple performance obligations, which had all been satisfied in 2015 prior to the adoption of ASC 606. The ContraVir contract has one fixed and several variable transaction amounts. The fixed fee portion of the contract was for the license to CMX157 rights. The Company recognized revenue for the fixed fee portion of the contract in 2015 when the performance obligations were satisfied. All variable transaction amounts, which relate to clinical, regulatory and commercial milestones as well as royalties and milestones based on commercial sales, are fully constrained. The Company will begin recognizing revenue on the variable transaction amounts when those transaction amounts are no longer fully constrained.

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 March 31, 2018, 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.

Basic and Diluted 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 for the three months ended March 31, 2018 and 2017.

Impact of Recently Issued Accounting Standards

In February 2016, the FASB issued ASU No. 2016-02, “Leases (Topic 842)”, which increases transparency and comparability among companies accounting for lease transactions. The most significant change of this update will require the recognition of lease assets and liabilities on the balance sheet for lessees for operating lease arrangements with lease terms greater than 12 months. This update will require a modified retrospective application which includes a number of optional practical expedients related to the identification and classification of leases commenced before the effective date. This ASU is effective for financial statements issued for annual periods and interim periods within those annual periods, beginning after December 15, 2018. The Company is currently analyzing the impact of the adoption of ASU No. 2016-02 on its consolidated financial statements.

Impact of Recently Adopted Accounting Standards

In May 2014, the FASB issued Accounting Standards Update (ASU) No. 2014-09, “Revenue from Contracts with Customers (Topic 606).” The ASU establishes a principles-based approach for accounting for revenue arising from contracts with customers and supersedes existing revenue recognition guidance. The ASU provides that an entity should apply a five-step approach for recognizing revenue, including (1) identify the contract with a customer; (2) identify the performance obligations in the contract; (3) determine the transaction price; (4) allocate the transaction price to the performance obligations in the contract; and (5) recognize revenue when, or as, the entity satisfies a performance obligation. Also, the entity must provide various disclosures concerning the nature, amount and timing of revenue and cash flows arising from contracts with customers. The FASB has issued several updates to the standard which (1) defer the original effective date to annual periods and interim periods within those annual periods beginning after December 15, 2017, while allowing for early adoption as of January 1, 2017 (ASU 2015-14); (2) clarify the application of the principal versus agent guidance (ASU 2016-08); and (3) clarify the guidance on inconsequential and perfunctory promises and licensing (ASU 2016-10). The Company adopted ASC 606 as of January 1, 2018 using the full retrospective approach and determined that there was no impact on its financial statements. In preparation for adoption of the standard, the Company has implemented internal controls to enable the preparation of financial information, including the assessment of the impact of the standard.

In January 2016, the FASB issued ASU No. 2016-01, “Financial Instruments-Overall (Subtopic 825-10)-Recognition and Measurement of Financial Assets and Financial Liabilities.” The new standard enhances reporting for financial instruments. The ASU is effective for financial statements issued for annual periods and interim periods within those annual periods beginning after December 15, 2017. Earlier adoption is permitted for interim and annual reporting periods as of the beginning of the fiscal year of adoption. The Company adopted ASU No. 2016-01 on January 1, 2018 on a prospective basis. As a result of this standard, changes in fair value of available-for-sale equity securities that were previously recognized in other comprehensive income are now recognized in earnings. As of January 1, 2018, the Company had no unrealized gains or losses in other comprehensive income that had to be reclassified to retained earnings.

Investments

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Investments

The following tables summarize the Company's short-term and long-term debt investments (in thousands):



	March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. treasury securities	$	188,372	$	—	$	(1,064	)	$	187,308
Total investments	$	188,372	$	—	$	(1,064	)	$	187,308

	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. treasury securities	$	210,280	$	—	$	(963	)	$	209,317
Total investments	$	210,280	$	—	$	(963	)	$	209,317

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):



	March 31, 2018
	Less than 12 Months					Greater than 12 Months					Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
U.S. treasury securities	$	103,949	$	(798	)	$	83,359	$	(266	)	$	187,308	$	(1,064	)
Total	$	103,949	$	(798	)	$	83,359	$	(266	)	$	187,308	$	(1,064	)
Number of securities with unrealized losses			25					18					43

	December 31, 2017
	Less than 12 Months					Greater than 12 Months					Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
U.S. treasury securities	$	170,390	$	(871	)	$	38,927	$	(92	)	$	209,317	$	(963	)
Total	$	170,390	$	(871	)	$	38,927	$	(92	)	$	209,317	$	(963	)
Number of securities with unrealized losses			39					7					46

The Company periodically reviews available-for-sale securities for other-than-temporary declines in fair value below the cost basis and whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. The Company evaluates, among other things, the duration and extent to which the fair value of a security is less than its cost; the financial condition of the issuer and any changes thereto; and the Company’s intent to sell, or whether it will more likely than not be required to sell, the security before recovery of its cost basis. At March 31, 2018, the Company did not intend to sell, and was not more likely than not to be required to sell, the available-for-sale debt securities in an unrealized loss position before recovery of the cost basis of the securities, which may be at maturity. There were no such declines in value for the three months ended March 31, 2018 and 2017. Unrealized gains and losses on debt investments are recorded to unrealized (loss) gain on investments, net in the Consolidated Statements of Operations and Comprehensive Loss. The Company recognizes interest income on an accrual basis in interest income in the Consolidated Statements of Operations and Comprehensive Loss.

The following table summarizes the scheduled maturity for the Company's debt investments at March 31, 2018 (in thousands):



Maturing in one year or less	$	138,083
Maturing after one year through two years	49,225
Total debt investments	187,308
Common stock of U.S. corporation	252
Total investments	$	187,560

Commitments and Contingencies	3 Months Ended
Commitments and Contingencies	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Leases The Company leases its facilities and certain office equipment under long-term non-cancelable operating leases that expire at various dates through 2021. Rent expense under non-cancelable operating leases and other month-to-month equipment rental agreements, including common area maintenance fees, totaled approximately $0.2 million for both the three months ended March 31, 2018 and 2017. Significance of Revenue Source The Company is the recipient of federal research contract funds from BARDA, the sole source of the Company's contract revenue. Periodic audits are required under the Company’s BARDA agreement and certain costs may be questioned as appropriate under the BARDA agreement. Management believes that such amounts in the current year, if any, are not significant. Accordingly, no provision for refundable amounts under the BARDA agreement had been made as of March 31, 2018 and December 31, 2017.

Equity Transactions and Share-based Compensation

3 Months Ended

Mar. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Equity Transactions and Share-based Compensation

Warrants

During the three months ended March 31, 2018, there were no exercises of warrants for the purchase of shares of the Company's common stock. All remaining outstanding warrants expired in February 2018. As of March 31, 2018 no warrants were outstanding.

Stock Options

In connection with the Company’s IPO, the Company adopted the 2013 Equity Incentive Plan (the 2013 Plan). The 2013 Plan provides for the grant of incentive stock options (ISOs), non-statutory 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. The number of shares of common stock reserved for future issuance automatically increases on January 1 of each calendar year by 4% 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. On January 1, 2018, the common stock reserved for issuance under the 2013 Plan was automatically increased by 1.9 million shares. As of March 31, 2018, there was a total of 1.1 million shares reserved for future issuance under the 2013 Plan. The Company issued approximately 14,000 shares of common stock pursuant to the exercise of stock options during the three months ended March 31, 2018.

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. Initially, the ESPP 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 automatically increases on January 1 of each calendar year, from January 1, 2014 through January 1, 2023 by the lesser 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). On January 1, 2018, the common stock reserved for issuance under the ESPP was automatically increased by an additional 0.4 million shares.

The Company has reserved a total of 2.6 million shares of common stock to be purchased under the ESPP, of which 2.2 million shares remained available for purchase as of March 31, 2018. The ESPP 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. 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 Company issued approximately 87,000 shares of common stock pursuant to the ESPP during the three months ended March 31, 2018. Compensation expense for shares purchased under the ESPP related to the purchase discount and the “look-back” option and were determined using a Black-Scholes option pricing model.

Restricted Stock Units

The Company has 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. The Company issued 146,800 shares of common stock pursuant to the vesting of RSUs during the three months ended March 31, 2018.

In January 2017, the Company also granted performance-based RSUs which, when vested, represent the right to be issued the number of shares of the Company’s common stock that is equal to the number of RSUs granted. The grant date fair value for performance-based RSUs is based upon the market price of the Company’s common stock on the date of the grant. For the portion of the performance-based RSUs of which the achievement of the performance condition is considered probable, the Company recognizes stock-based compensation expense on the related estimated fair value of such RSUs ratably for each vesting tranche from the service inception date to the end of the requisite service period. For the performance conditions that are not considered probable of achievement at the grant date or upon quarterly re-evaluation, prior to the event actually occurring, the Company begins recognizing the related stock-based compensation expense ratably when the event occurs or when the Company can determine that achievement of the performance condition is probable. In those cases, the Company recognizes the change in estimate at the time it determines the performance condition is probable of achievement (by recognizing stock-based compensation expense as cumulative catch-up adjustment as if the Company had estimated at the grant date that the performance condition would have been achieved) and recognize the remaining compensation cost through the end of the requisite service period. The Company issued no shares of common stock pursuant to the vesting of performance-based RSUs during the three months ended March 31, 2018.

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 share-based compensation expense recognized related to stock options, the ESPP and RSUs was as follows (in thousands):



	Three Months Ended March 31,
	2018		2017
Research and development expense	$	1,320	$	1,763
General and administrative expense	2,071		2,265
Total share-based compensation expense	$	3,391	$	4,028

Income Taxes	3 Months Ended
Income Taxes	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company estimates an annual effective tax rate of 0% for the year ending December 31, 2018 as the Company incurred losses for the three month period ended March 31, 2018 and is forecasting additional losses through the fourth quarter, resulting in an estimated net loss for both financial statement and tax purposes for the year ending December 31, 2018. Therefore, no federal or state income taxes are expected and none have been recorded at this time. Income taxes have been accounted for using the liability method in accordance with FASB ASC 740. Due to the Company's history of losses since inception, there is not enough evidence at this time to support that the Company will generate future income of a sufficient amount and nature to utilize the benefits of its net deferred tax assets. Accordingly, the deferred tax assets have been reduced by a full valuation allowance, since the Company does not currently believe that realization of its deferred tax assets is more likely than not. At March 31, 2018, the Company had no unrecognized tax benefits that would reduce the Company’s effective tax rate if recognized. At March 31, 2018, the Company’s accounting for the 2017 Tax Cuts and Jobs Act is incomplete; however, it expects to complete the accounting by December 31, 2018. As discussed in our 2017 Annual Report on Form 10-K, the Company recorded provisional adjustments related to deferred taxes for stock compensation and the deferred rate change. The Company is continuing to evaluate the impact of the recently enacted tax law on its business and consolidated financial statements. For the first quarter 2018, the Company has not made any measurement-period adjustments related to these provisional items. Updates to the Company’s calculations may result in changes to the provisional adjustments recorded at December 31, 2017.

Significant Agreements	3 Months Ended
Significant Agreements	Mar. 31, 2018
Revenue Recognition [Abstract]
Significant Agreements	Significant Agreements The Regents of the University of California In May 2002, the Company entered into a license agreement with The Regents of the University of California (UC) under which the Company obtained an exclusive, worldwide license to UC’s patent rights in certain inventions (the UC Patent Rights) related to lipid-conjugated antiviral compounds and their use, including certain patents relating to brincidofovir. The license agreement was amended in September 2002 in order to expand the scope of the license and again in December 2010 in order to modify certain financial terms. The agreement was amended a third time in September 2011 to add additional patents related to certain metabolically stable lipid-conjugate compounds. In April 2018, a fifth amendment was executed to alter the rights and obligations of the parties in light of the Company's current business plans and to extend the term of the agreement to the later of the longest-lived Patent Rights (as defined in the agreement) or May 2028. Under the license agreement, the Company is permitted to research, develop, manufacture and commercialize products utilizing the UC Patent Rights for all human and veterinary uses, and to sublicense such rights. UC retained the right, on behalf of itself and other non-profit institutions, to use the UC Patent Rights for educational and research purposes and to publish information about the UC Patent Rights. In consideration for the rights granted under the license agreement, the Company has issued UC an aggregate of 64,788 shares of common stock. As additional consideration, the Company is required to pay certain cash milestone payments in connection with the development and commercialization of compounds that are covered by the UC Patent Rights, plus certain annual fees to maintain such patents until the Company commercializes a product utilizing UC Patent Rights. In connection with the development and commercialization of brincidofovir and CMX157, the Company could be required to pay UC up to an aggregate of $3.4 million in milestone payments, assuming the achievement of all applicable milestone events under the license agreement. In addition, upon commercialization of any product utilizing the UC Patent Rights (which would include the commercialization of brincidofovir), the Company will be required to pay low single digit royalties on net sales of such product. The license agreement requires that we diligently develop, manufacture and commercialize compounds that are covered by the UC Patent Rights, and we have agreed to meet certain development and commercialization milestones. UC may, at its option, either terminate the license agreement or change the license granted from an exclusive license to a non-exclusive license if we fail to meet such development and commercialization milestones. We are currently in compliance with these milestone requirements. In the event the Company sublicenses a UC Patent Right (including UC Patent Rights relating to brincidofovir or CMX157) the Company is obligated to pay to UC a fee, which amount will vary depending upon the amount of any payments the Company receives and the clinical development stage of the compound being sublicensed, but which could be up to approximately 50% of the sublicense fee in certain circumstances. With respect to brincidofovir, the fee payable to UC will not exceed 5% of the sublicense fee. In addition, the Company will also be required to pay to UC a low single digit sublicense royalty on net sales of products that use the sublicensed UC Patent Rights, but in no event will the Company be required to pay more than 50% of the royalties it receives in connection with the relevant sublicense. Any such royalty payment will be reduced by other payments the Company is required to make to third parties until a minimum royalty has been reached. Biomedical Advanced Research and Development Authority (BARDA) In February 2011, the Company entered into a contract with BARDA for the advanced development of brincidofovir as a medical countermeasure in the event of a smallpox release. Under the contract, BARDA will reimburse the Company, plus pay a fixed fee, for the research and development of brincidofovir as a broad-spectrum therapeutic antiviral for the treatment of smallpox infections. The contract consists of an initial performance period, referred to as the base performance segment, plus up to four extension periods, referred to as option segments, each of which may be exercised at BARDA’s sole discretion. The Company must complete the agreed upon milestones and deliverables in each discrete work segment before the next option segment is eligible to be exercised. Under the contract as currently in effect, the Company may receive up to $75.8 million in expense reimbursement and $5.3 million in fees. The Company is currently performing under the second and third option segments of the contract during which the Company may receive up to a total of $21.6 million and $11.6 million in expense reimbursement and fees, respectively. The second and third option segments are expected to end on September 30, 2018. For the three months ended March 31, 2018 and 2017, the Company recognized revenue under this contract of $0.8 million and $1.1 million, respectively. ContraVir Pharmaceuticals On December 17, 2014, the Company entered into a license agreement with ContraVir Pharmaceuticals (NASDAQ:CTRV) for the development and commercialization of CMX157 for certain antiviral indications. Under the terms of the agreement, ContraVir has sole responsibility with respect to the control of the development and commercialization of CMX157. In exchange for the license to CMX157 rights, the Company received an upfront payment consisting of 120,000 shares of ContraVir Series B Convertible Preferred Stock with a stated value of $1.2 million. In addition, the Company is eligible to receive up to approximately $20 million in clinical, regulatory and initial commercial milestones in the United States and Europe, as well as royalties and additional milestones based on commercial sales in those territories. Either party may terminate the license agreement upon the occurrence of a material breach by the other party (subject to standard cure periods), or upon certain events involving the bankruptcy or insolvency of the other party. ContraVir may also terminate the license agreement without cause on a country-by-country basis upon sixty days’ prior written notice. The upfront payment of 120,000 shares of ContraVir Series B Convertible Preferred Stock was valued at $1.5 million at the time of the agreement. The Company recorded this amount as a long-term investment and deferred revenue, which was included in accrued liabilities in the Consolidated Balance Sheets. Upon completion of the transfer of the IND and technical know-how related to CMX157 in April 2015, the Company recognized the $1.5 million upfront payment as revenue. In September 2016, the Company converted its shares of ContraVir Series B convertible preferred stock into 1,071,429 shares of ContraVir common stock. As of March 31, 2018 and December 31, 2017, the fair value of the investment was recorded as a short-term investment of $0.3 million and $0.4 million, respectively. University of Michigan In 2006, the Company entered into a license agreement with The Regents of the University of Michigan (UM) under which the Company obtained an exclusive, worldwide license to UM’s patent rights in certain inventions (UM Patent Rights) related to certain compounds originally synthesized at UM. Under the license agreement, the Company is permitted to research, develop, manufacture and commercialize products utilizing the UM Patent Rights, and to sublicense such rights subject to certain sublicensing fees and royalty payments. The agreement was amended in 2016. 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.

Subsequent Events	3 Months Ended
Subsequent Events	Mar. 31, 2018
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 March 31, 2018, and events which occurred subsequently but were not recognized in the financial statements.

The Business and Summary of Significant Accounting Policies (Policy)

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

Fair Value of Financial Instruments


•	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.

Revenue Recognition

Policy

Biomedical Advanced Research and Development Authority (BARDA)

ContraVir Pharmaceuticals

Research and Development Prepaids and Accruals

Basic and Diluted Net Loss Per Share of Common Stock

Impact of Recently Issued Accounting Standards and Impact of Recently Adopted Accounting Standards

Impact of Recently Issued Accounting Standards

Impact of Recently Adopted Accounting Standards

The Business and Summary of Significant Accounting Policies (Tables)

3 Months Ended

Mar. 31, 2018

Accounting Policies [Abstract]

Schedule of certain assets 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
	March 31, 2018
	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents
Money market funds	$	12,926	$	12,926	$	—	$	—
Commercial paper	6,486		—		6,486		—
Total cash equivalents	19,412		12,926		6,486		—
Short-term investments
U.S. treasury securities	138,083		138,083		—		—
Common stock of U.S. corporation	252		252		—		—
Total short-term investments	138,335		138,335		—		—
Long-term investments
U.S. treasury securities	49,225		49,225		—		—
Total long-term investments	49,225		49,225		—		—
Total assets	$	206,972	$	200,486	$	6,486	$	—

	Fair Value Measurements
	December 31, 2017
	Total		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents
Money market funds	$	10,816	$	10,816	$	—	$	—
Commercial paper	3,995		—		3,995		—
Total cash equivalents	14,811		10,816		3,995		—
Short-term investments
U.S. treasury securities	132,586		132,586		—		—
Common stock of U.S. corporation	386		386		—		—
Total short-term investments	132,972		132,972		—		—
Long-term investments
U.S. treasury securities	76,731		76,731		—		—
Total long-term investments	76,731		76,731		—		—
Total assets	$	224,514	$	220,519	$	3,995	$	—

Schedule of accrued liabilities

Accrued liabilities consisted of the following (in thousands):



	March 31, 2018		December 31, 2017
Accrued research and development expenses	$	4,084	$	3,384
Accrued compensation	1,911		3,678
Other accrued liabilities	1,114		1,322
Accrued indemnification claim	—		1,000
Total accrued liabilities	$	7,109	$	9,384

Investments (Tables)

3 Months Ended

Mar. 31, 2018

Investments, Debt and Equity Securities [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):



	March 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. treasury securities	$	188,372	$	—	$	(1,064	)	$	187,308
Total investments	$	188,372	$	—	$	(1,064	)	$	187,308

	December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. treasury securities	$	210,280	$	—	$	(963	)	$	209,317
Total investments	$	210,280	$	—	$	(963	)	$	209,317

Summary of investments with unrealized losses, aggregated by investment type and the length of time



	March 31, 2018
	Less than 12 Months					Greater than 12 Months					Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
U.S. treasury securities	$	103,949	$	(798	)	$	83,359	$	(266	)	$	187,308	$	(1,064	)
Total	$	103,949	$	(798	)	$	83,359	$	(266	)	$	187,308	$	(1,064	)
Number of securities with unrealized losses			25					18					43

	December 31, 2017
	Less than 12 Months					Greater than 12 Months					Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
U.S. treasury securities	$	170,390	$	(871	)	$	38,927	$	(92	)	$	209,317	$	(963	)
Total	$	170,390	$	(871	)	$	38,927	$	(92	)	$	209,317	$	(963	)
Number of securities with unrealized losses			39					7					46

Summary of the scheduled maturity of Company investments

The following table summarizes the scheduled maturity for the Company's debt investments at March 31, 2018 (in thousands):



Maturing in one year or less	$	138,083
Maturing after one year through two years	49,225
Total debt investments	187,308
Common stock of U.S. corporation	252
Total investments	$	187,560

- Definition

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

+ References + Details

Equity Transactions and Share-based Compensation (Tables)

3 Months Ended

Mar. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of employee and non-employee share-based compensation expense recognized related to stock options, the ESPP and RSUs

Total share-based compensation expense recognized related to stock options, the ESPP and RSUs was as follows (in thousands):



	Three Months Ended March 31,
	2018		2017
Research and development expense	$	1,320	$	1,763
General and administrative expense	2,071		2,265
Total share-based compensation expense	$	3,391	$	4,028

The Business and Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2016
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Unrealized loss on equity investment	$ 134	$ 0
Unrealized loss on investments, net	103	(331)
Common stock of U.S. corporation \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Convertible preferred stock, shares issued upon conversion (in shares)			1,071,429
Unrealized loss on equity investment	$ 100
Unrealized loss on investments, net		$ 600

The Business and Summary of Significant Accounting Policies - Schedule of Accrued Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Accrued research and development expenses	$ 4,084	$ 3,384
Accrued compensation	1,911	3,678
Other accrued liabilities	1,114	1,322
Accrued indemnification claim	0	1,000
Total accrued liabilities	$ 7,109	$ 9,384

The Business and Summary of Significant Accounting Policies - Revenue Recognition (Details) $ in Thousands		3 Months Ended		92 Months Ended
	Dec. 17, 2014 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2018 USD ($) base_segment revenue_contract option_segment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract revenue		$ 790	$ 1,078
BARDA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract revenue		$ 800	$ 1,100
Maximum \| ContraVir Pharmaceuticals
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Collaboration and licensing revenue	$ 20,000
Scenario, Forecast \| BARDA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Total number of base segments \| base_segment				1
Total number of option segments \| option_segment				4
Total number of separate contracts \| revenue_contract				5
Scenario, Forecast \| Maximum \| BARDA
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Reimbursement revenue				$ 75,800
Fees and commissions, other				5,300
Scenario, Forecast \| Maximum \| BARDA Second Option Segment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract revenue				21,600
Scenario, Forecast \| Maximum \| BARDA Third Option Segment
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Contract revenue				$ 11,600

The Business and Summary of Significant Accounting Policies - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments	$ 138,335	$ 132,972
Long-term investments	49,225	76,731
Fair value, measurements, recurring
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	19,412	14,811
Short-term investments	138,335	132,972
Long-term investments	49,225	76,731
Total investments	206,972	224,514
Fair value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	12,926	10,816
Short-term investments	138,335	132,972
Long-term investments	49,225	76,731
Total investments	200,486	220,519
Fair value, measurements, recurring \| Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	6,486	3,995
Short-term investments	0	0
Long-term investments	0	0
Total investments	6,486	3,995
Fair value, measurements, recurring \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	0	0
Short-term investments	0	0
Long-term investments	0	0
Total investments	0	0
Money market funds \| Fair value, measurements, recurring
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	12,926	10,816
Money market funds \| Fair value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	12,926	10,816
Money market funds \| Fair value, measurements, recurring \| Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	0	0
Money market funds \| Fair value, measurements, recurring \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	0	0
Commercial paper \| Fair value, measurements, recurring
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	6,486	3,995
Commercial paper \| Fair value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	0	0
Commercial paper \| Fair value, measurements, recurring \| Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	6,486	3,995
Commercial paper \| Fair value, measurements, recurring \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash equivalents	0	0
U.S. treasury securities \| Fair value, measurements, recurring
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments	138,083	132,586
Long-term investments	49,225	76,731
U.S. treasury securities \| Fair value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments	138,083	132,586
Long-term investments	49,225	76,731
U.S. treasury securities \| Fair value, measurements, recurring \| Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments	0	0
Long-term investments	0	0
U.S. treasury securities \| Fair value, measurements, recurring \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments	0	0
Long-term investments	0	0
Common stock of U.S. corporation \| Fair value, measurements, recurring
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments	252	386
Common stock of U.S. corporation \| Fair value, measurements, recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments	252	386
Common stock of U.S. corporation \| Fair value, measurements, recurring \| Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments	0	0
Common stock of U.S. corporation \| Fair value, measurements, recurring \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Short-term investments	$ 0	$ 0

Investments - Summary of Available-for-Sale securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Amortized Cost	$ 188,372
Gross Unrealized Gains	0
Gross Unrealized Losses	(1,064)
Estimated Fair Value	187,308
Amortized Cost		$ 210,280
Gross Unrealized Gains		0
Gross Unrealized Losses		(963)
Estimated Fair Value		209,317
U.S. treasury securities
Schedule of Investments [Line Items]
Amortized Cost	188,372
Gross Unrealized Gains	0
Gross Unrealized Losses	(1,064)
Estimated Fair Value	$ 187,308
Amortized Cost		210,280
Gross Unrealized Gains		0
Gross Unrealized Losses		(963)
Estimated Fair Value		$ 209,317

Investments - Summary of Investments with Unrealized Losses, Aggregated by Investment Type and the Length of Time (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	$ 103,949,000		$ 170,390,000
Unrealized Loss, Less than 12 Months	(798,000)		(871,000)
Fair Value, Greater than 12 Months	83,359,000		38,927,000
Unrealized Loss, Greater than 12 Months	(266,000)		(92,000)
Fair Value, Total	187,308,000		209,317,000
Unrealized Loss, Total	$ (1,064,000)		$ (963,000)
Number of securities with unrealized losses, Less than 12 Months \| security	25		39
Number of securities with unrealized losses, Greater than 12 Months \| security	18		7
Number of securities with unrealized losses, Total \| security	43		46
Other than temporary impairment losses	$ 0	$ 0
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Fair Value, Less than 12 Months	103,949,000		$ 170,390,000
Unrealized Loss, Less than 12 Months	(798,000)		(871,000)
Fair Value, Greater than 12 Months	83,359,000		38,927,000
Unrealized Loss, Greater than 12 Months	(266,000)		(92,000)
Fair Value, Total	187,308,000		209,317,000
Unrealized Loss, Total	$ (1,064,000)		$ (963,000)

Investments - Schedule of Investment Maturity (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Investments, Debt and Equity Securities [Abstract]
Maturing in one year or less	$ 138,083
Maturing after one year through two years	49,225
Total debt investments	187,308
Common stock of U.S. corporation	252
Total investments	$ 187,560

Commitments and Contingencies - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Operating leases, rent expense	$ 200,000	$ 200,000
BARDA
Other Commitments [Line Items]
Deferred revenue	$ 0		$ 0

Equity Transactions and Share-based Compensation (Details) - shares		3 Months Ended
	Jan. 01, 2018	Mar. 31, 2018	Feb. 28, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants outstanding (in shares)		0
Stock issued during period, shares, restricted stock award, gross (in shares)		146,800
The 2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, evergreen option provision, equity increase		4.00%
Additional shares authorized (in shares)	1,900,000.0
Number of shares reserved for future issuance (in shares)		1,100,000.0
Shares issued pursuant to the exercise of stock options (in shares)		14,000
The 2013 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, evergreen option provision, equity increase		1.00%
Additional shares authorized (in shares)	400,000.0
Number of shares reserved for future issuance (in shares)		2,200,000.0
Number of shares authorized to be granted (in shares)		2,600,000.0	704,225
Number of shares reserved for issuance, annual increase (in shares)		422,535
Employee stock purchase plan, automatic reset purchase period		24 months
Percentage of pay that employee can contribute, maximum		15.00%
Employee stock purchase plan, purchase period		6 months
Discounted purchase price from market price, offering date		15.00%
Discounted purchase price from market price, purchase date		15.00%
Shares issued pursuant to employee stock purchase plan (in shares)		87,000
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award, gross (in shares)		0
Common stock of U.S. corporation
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued for warrants exercised (in shares)		0

Equity Transactions and Share-based Compensation - Schedule of Share-based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total share-based compensation expense	$ 3,391	$ 4,028
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total share-based compensation expense	1,320	1,763
General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total share-based compensation expense	$ 2,071	$ 2,265

Income Taxes - Narrative (Details) - USD ($)	3 Months Ended		12 Months Ended
Income Taxes - Narrative (Details) - USD ($)	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2018
Income Tax Contingency [Line Items]
Federal or state income taxes	$ 0	$ 0
Unrecognized tax benefits	$ 0
Scenario, Forecast
Income Tax Contingency [Line Items]
Estimated annual effective tax rate			0.00%

Significant Agreements (Details) - USD ($)		1 Months Ended
Significant Agreements (Details) - USD ($)	Dec. 17, 2014	Dec. 31, 2016	Apr. 30, 2015	May 31, 2002	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2016
Long-term Purchase Commitment [Line Items]
Estimated fair value at the time of agreement						$ 209,317,000
Fair value of investment					$ 252,000
The Regents of University of California
Long-term Purchase Commitment [Line Items]
Number of options outstanding, granted (in shares)				64,788
The Regents of University of California \| Maximum
Long-term Purchase Commitment [Line Items]
Milestone payments				$ 3,400,000
Sublicense fee percentage				50.00%
Royalties percentage				50.00%
The Regents of University of California \| Maximum \| Brincidofovir
Long-term Purchase Commitment [Line Items]
Sublicense fee amount, payable fee percentage				5.00%
ContraVir Pharmaceuticals
Long-term Purchase Commitment [Line Items]
License agreement, termination notice	60 days
Licenses revenues			$ 1,500,000
ContraVir Pharmaceuticals \| Maximum
Long-term Purchase Commitment [Line Items]
Collaboration and licensing revenue	$ 20,000,000
The Regents of the University of Michigan \| Maximum
Long-term Purchase Commitment [Line Items]
Royalties percentage		2.00%
The Regents of the University of Michigan \| Minimum
Long-term Purchase Commitment [Line Items]
Royalties percentage		0.25%
Preferred Stock \| ContraVir Pharmaceuticals
Long-term Purchase Commitment [Line Items]
Other significant noncash transaction, shares received as consideration (in shares)	120,000
Estimated fair value at the time of agreement	$ 1,500,000
Preferred Stock \| ContraVir Pharmaceuticals \| Stated Value
Long-term Purchase Commitment [Line Items]
Other significant noncash transaction, value of consideration received	$ 1,200,000
Common stock of U.S. corporation \| ContraVir Pharmaceuticals
Long-term Purchase Commitment [Line Items]
Estimated fair value at the time of agreement						$ 400,000
Convertible preferred stock, shares issued upon conversion (in shares)							1,071,429
Fair value of investment					$ 300,000

Significant Agreements - BARDA (Details) $ in Thousands	1 Months Ended	3 Months Ended		92 Months Ended
Significant Agreements - BARDA (Details) $ in Thousands	Feb. 28, 2011 extension	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Contract revenue		$ 790	$ 1,078
BARDA
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Number of extension periods \| extension	4
Contract revenue		$ 800	$ 1,100
Maximum \| Scenario, Forecast \| BARDA
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Reimbursement revenue				$ 75,800
Fees and commissions, other				5,300
Maximum \| Scenario, Forecast \| BARDA Second Option Segment
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Contract revenue				21,600
Maximum \| Scenario, Forecast \| BARDA Third Option Segment
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Contract revenue				$ 11,600