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Document And Entity Information - USD ($)	12 Months Ended
Document And Entity Information - USD ($)	Dec. 31, 2014	Feb. 24, 2015	Jun. 30, 2014	Dec. 31, 2013
Document Document And Entity Information [Abstract]
Share Price				$ 14.00
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2014
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Entity Registrant Name	CHIMERIX INC
Entity Central Index Key	0001117480
Entity Voluntary Filers	No
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Current Fiscal Year End Date	--12-31
Entity Filer Category	Accelerated Filer
Entity Public Float			$ 443,167,025
Entity Common Stock, Shares Outstanding		41,086,040
Common Stock, Par or Stated Value Per Share	$ 0.001			$ 0.001

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$ 128,462	$ 109,976
Short-term investments, available-for-sale	106,114	0
Accounts receivable	106	248
Prepaid expenses and other current assets	2,775	2,785
Total current assets	237,457	113,009
Long-term investments	52,973	0
Property and equipment, net of accumulated depreciation	1,310	338
Other long-term assets	138	40
Total assets	291,878	113,387
Current liabilities:
Accounts payable	5,938	2,214
Accrued liabilities	6,833	2,420
Loan payable, net, current portion	4,296	5,573
Total current liabilities	17,067	10,207
Loan payable, net, less current portion	0	4,294
Other long-term liabilities	175	347
Total liabilities	17,242	14,848
Commitments and contingencies	0	0
Stockholders’ equity:
Preferred stock, $0.001 par value, 10,000,000 shares authorized at December 31, 2014 and 2013; no shares issued and outstanding as of December 31, 2014 and 2013	0	0
Common stock, $0.001 par value; 200,000,000 shares authorized at December 31, 2014 and 2013; 41,031,798 and 26,664,972 shares issued and outstanding at December 31, 2014 and 2013, respectively	41	26
Additional paid-in capital	496,602	261,243
Accumulated other comprehensive gain	35	0
Accumulated deficit	(222,042)	(162,730)
Total stockholders’ equity	274,636	98,539
Total liabilities and stockholders’ equity	$ 291,878	$ 113,387

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares issued	41,031,770	26,664,972
Common stock, shares outstanding	41,031,770	26,664,972
Preferred stock, share authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues:
Contract revenue	$ 4,040	$ 4,370	$ 16,275
Collaboration and licensing revenue	0	0	17,445
Total revenues	4,040	4,370	33,720
Operating expenses:
Research and development	45,379	24,662	30,106
General and administrative	17,527	8,327	6,397
Total operating expenses	62,906	32,989	36,503
Loss from operations	(58,866)	(28,619)	(2,783)
Other expenses:
Interest expense, net	(445)	(1,232)	(776)
Fair value adjustments to preferred stock warrant liability	0	(6,590)	(847)
Loss on disposition of assets	(1)	(4)	0
Net loss	(59,312)	(36,445)	(4,406)
Other comprehensive loss:
Unrealized gain on investments, net	35	2	2
Comprehensive loss	(59,277)	(36,443)	(4,404)
Accretion of redeemable convertible preferred stock	0	(34,108)	(4,357)
Net loss attributable to common stockholders	$ (59,312)	$ (70,553)	$ (8,763)
Per share information:
Net loss, basic and diluted (in USD per share)	$ (1.80)	$ (3.65)	$ (5.75)
Weighted-average shares outstanding, basic and diluted	33,003,714	19,307,422	1,524,628

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Additional Paid-in Capital	Accumulated Other Comprehensive Gain (Loss)	Accumulated Deficit	Redeemable Convertible Preferred Stock	Common Stock	The 2013 Employee Stock Purchase Plan	The 2013 Employee Stock Purchase Plan Additional Paid-in Capital	The 2013 Employee Stock Purchase Plan Common Stock	Common stock offering, May 27, 2014	Common stock offering, May 27, 2014 Additional Paid-in Capital	Common stock offering, May 27, 2014 Common Stock	Common stock offering, November 5, 2014	Common stock offering, November 5, 2014 Additional Paid-in Capital	Common stock offering, November 5, 2014 Common Stock
Beginning balance at Dec. 31, 2011	$ (93,680)	$ 0	$ (4)	$ (93,678)	$ 103,366	$ 2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	1,396	1,396
Exercise of stock options	14	13				1
Dividends on redeemable preferred stock	(3,600)	1,409		2,191	(3,600)
Adjustment of redeemable preferred stock to redemption value	(757)			757	(757)
Comprehensive loss:
Unrealized gain on investments, net	2		2
Net loss	(4,406)			(4,406)
Total comprehensive loss	(4,404)
Ending balance at Dec. 31, 2012	(101,031)	0	(2)	(101,032)	107,723	3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	3,071	3,071
Exercise of stock options	1,946	1,946
Exercise of warrants	1,537	1,537
Dividends on redeemable preferred stock	(976)	349		627	(976)
Issuance of common stock, value	107,634	107,611				23
Adjustment of redeemable preferred stock to redemption value	(33,132)	8,506		24,626	(33,132)
Conversion of redeemable preferred stock into common stock	141,831	141,831			$ (141,831)
Reclassification of redeemable preferred stock warrant liability to additional paid in capital	14,102	14,102
Comprehensive loss:
Unrealized gain on investments, net	2		2
Net loss	(36,445)			(36,445)
Total comprehensive loss	(36,443)
Ending balance at Dec. 31, 2013	98,539	261,243		(162,730)		26
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	4,411	4,411
Exercise of stock options	4,593	4,591				2
Exercise of warrants	6	6
Stock Issued During Period, Value, Employee Stock Purchase Plan							$ 426	$ 425	$ 1
Issuance of common stock, value										$ 111,845	$ 111,837	$ 8	$ 114,093	$ 114,089	$ 4
Comprehensive loss:
Unrealized gain on investments, net	35		35
Net loss	(59,312)			(59,312)
Total comprehensive loss	(59,277)
Ending balance at Dec. 31, 2014	$ 274,636	$ 496,602	$ 35	$ (222,042)		$ 41

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) - USD ($)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Issuance of common stock, shares		8,418,000
Issuance of common stock, per share price		$ 14.00
Issuance of common stock, issuance costs		$ 10,217,696
Common stock offering, November 5, 2014
Issuance of common stock, shares	4,197,500
Issuance of common stock, per share price	$ 29
Issuance of common stock, issuance costs	$ 7,633,815
Common stock offering, May 27, 2014
Issuance of common stock, shares	8,395,000,000
Issuance of common stock, per share price	$ 14.22
Issuance of common stock, issuance costs	$ 7,531,172,000

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net loss	$ (59,312)	$ (36,445)	$ (4,406)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation of property and equipment	283	258	280
Amortization of debt costs	148	248	238
Amortization of premium/discount on investments	1,175	196	84
Loss on disposition of assets	1	4	0
Share-based compensation	4,411	3,071	1,396
Amortization of deferred lease obligation	(19)	10	0
Fair value adjustments to preferred stock warrant liability	0	6,590	847
Changes in operating assets and liabilities:
Accounts receivable	142	535	3,404
Prepaid expenses and other assets	(108)	(1,790)	65
Accounts payable and accrued liabilities	6,202	1,764	(3,784)
Net cash used in operating activities	(47,077)	(25,559)	(1,876)
Cash flows from investing activities:
Purchases of property and equipment	(1,018)	(193)	(126)
Purchases of short-term investments	(155,433)	(1,852)	(9,907)
Purchases of long-term investments	(55,337)	0	0
Purchases of long-term investments	3,499	750	0
Maturities of short-term investments	48,589	10,757	5,894
Net cash (used in) provided by investing activities	(159,700)	9,462	(4,139)
Cash flows from financing activities:
Proceeds from exercise of stock options	4,592	1,946	14
Proceeds from employee stock purchase plan stock purchases	426	0	0
Proceeds from exercise of warrants	6	1,537	0
Proceeds from issuance of debt	0	0	15,000
Proceeds from public offerings, net of offering costs	225,939	107,634	0
Debt discount	0	0	(75)
Repayments of debt	(5,700)	(4,950)	(2,601)
Payments for stock offering and deferred financing costs	0	0	24
Net cash provided by financing activities	225,263	106,167	12,314
Net increase in cash and cash equivalents	18,486	90,070	6,299
Beginning of period	109,976	19,906	13,607
End of period	128,462	109,976	19,906
Supplemental disclosure of cash flow information
Cash paid for interest	614	1,092	448
Noncash or Part Noncash Acquisition, Investments Acquired	$ 1,545	$ 0	$ 0

The Business and Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2014

Accounting Policies [Abstract]

The Business and Summary of Significant Accounting Policies

Description of Business

Chimerix, Inc. (the Company) is a biopharmaceutical company dedicated to discovering, developing and commercializing novel, oral antivirals to address unmet medical needs. The Company was founded in 2000 based on the promise of its proprietary lipid technology to unlock the potential of some of the most broad-spectrum antivirals by enhancing their antiviral activity and safety profiles in convenient, orally administered dosing regimens. Based on the Company’s lipid conjugate technology, its lead compound, brincidofovir (BCV, CMX001), is in Phase 3 clinical development. In addition, the Company has an active discovery program focusing on viral targets for which limited or no therapies are currently available.

Basis of Presentation

The consolidated financial statements include the accounts of the Company, and its wholly owned subsidiaries. The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (GAAP). The preparation of the Company’s consolidated financial statements requires estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and the disclosure of contingent assets and liabilities in the consolidated financial statements and accompanying notes. Although these estimates are based on knowledge of current events and actions the Company may undertake in the future, actual results may ultimately differ from these estimates and assumptions.

Reclassifications

Certain prior period amounts in the accompanying consolidated financial statements have been reclassified to conform to the current year presentation. These reclassifications had no effect on previously reported net income or stockholders' equity (deficit).

Cash and Cash Equivalents

The Company considers any highly liquid instrument with an original maturity of three months or less at acquisition to be a cash equivalent. Cash equivalents consist of money market accounts.

Investments

Investments consist primarily of corporate bonds, brokered certificates of deposit, U.S. Treasury securities, commercial paper, and preferred stock of a U.S. corporation. The Company invests in high-credit quality investments in accordance with its investment policy which minimizes the probability of loss.

Available-for-sale securities are carried at fair value as determined by quoted market prices, with the unrealized gains and losses, net of tax, reported as a separate component of stockholders deficit. Realized gains and losses are determined using the specific identification method and transactions are recorded on a settlement date basis in interest income or expense, net. Investments with original maturities beyond three months at the date of purchase and which mature on, or less than twelve months from, the balance sheet date are classified as short-term. Investments with a maturity beyond twelve months from the balance sheet date are classified as long-term. The Company periodically reviews available-for-sale securities for other-than-temporary declines in fair value below the cost basis and whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. The Company evaluates, among other things, the duration and extent to which the fair value of a security is less than its cost; the financial condition of the issuer and any changes thereto; and the Company’s intent to sell, or whether it will more likely than not be required to sell, the security before recovery of its amortized cost basis. Any such declines in value judged to be other-than-temporary on available-for-sale securities are reported in interest income or expense, net. There were no such declines in value for the years ended December 31, 2014, 2013 and 2012.

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, 2014 and December 31, 2013 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, notes receivable, accounts payable and accrued expenses approximate their fair values due to the short-term nature of such instruments. The carrying amount of borrowings under loans payable approximates its fair value based on the determination that the stated rate on such loans payable is consistent with current interest rates for similar borrowing arrangements available to the Company.

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.

At December 31, 2014 and 2013, 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 December 31, 2014, the Company had cash equivalents, short-term investments and long-term investments comprised of corporate bonds, commercial paper and brokered certificates of deposit for which quoted prices are not available that are valued using independent pricing models or other model-based valuation techniques such as the present value of future cash flows, adjusted for the security’s credit rating, prepayment assumptions and other factors such as credit loss assumptions. Accordingly, these securities are classified as Level 2.

The warrants issued for Series F redeemable convertible preferred stock included in the Company’s 2012 consolidated financial statements are categorized as Level 3 as there are significant unobservable inputs. The valuation of the warrants at December 31, 2012 reflected a two stage process. Using a contingent claims model in combination with the Company’s Series F financing, which occurred in February 2011, the fair value of total equity and all components of the Company’s capital structure, including the warrants, was determined as of the time of the financing event. Using this value as a starting point, a series of equity values and associated probabilities were calculated using simulation methodologies that incorporate both Monte Carlo and risk neutral frameworks. Based on assessments of expected returns and volatilities consistent with market practice, a distribution of equity values was produced which covered the range of values that an informed market participant might expect. These outcomes were organized into ranges and a probability calculated based on the percent of the total falling into each range. This process created a range of equity values. Using a contingent claims framework, each equity value was allocated to the various components of the capital structure including the warrants. Each warrant value was weighted by its respective probability to determine the final fair value of the warrants as of December 31, 2012. The key unobservable inputs used in the determination of the fair value were (i) volatility – 79%, (ii) range of implied fair value of the Series F redeemable convertible preferred stock – $2.19 to $2.85 , (iii) time to liquidity – 8 months to 5 years, and (iv) range of probabilities of liquidity event outcomes – 2% to 31%. The warrants were valued again at April 10, 2013, just prior to the Company’s IPO, using a Black-Scholes valuation model. The key unobservable inputs used in determination of the fair value at that time were (i) volatility – 79%, (ii) fair value of the Series F redeemable convertible preferred stock – $3.94 , (iii) expected life – 2.5 years, (iv) risk-free interest rate – 0.24%, and (v) dividend yield – 0%. As the warrants for Series F redeemable convertible preferred stock converted to warrants for common stock upon the IPO, no future valuations are necessary.

The Company’s preferred stock investment in ContraVir Pharmaceuticals (NASDAQ: CTRV) (ContraVir), included in the Company’s 2014 consolidated financial statements is categorized as Level 3 as there are significant unobservable inputs. The Company performed a valuation of the investment at December 17, 2014, which was the effective date of the licensing agreement upon which the Company acquired its investment in ContraVir. The value of the Preferred Series B Convertible shares was calculated on an as if converted to common share basis with a discount for lack of marketability applied due to the 18 month restriction from the date of the agreement on selling the converted common shares. An option pricing model was used to determine the discount for lack of marketability of 25%. The key unobservable inputs used in the option pricing model were (i) exercise price - $1.79, (ii) dividend yield - 0%, (iii) expected holding period - 1.5 years, (iv) risk-free rate - 0.43%, and (v) volatility - 66%. The Preferred Series B Convertible shares were valued again at December 31, 2014, using a similar valuation model. The key unobservable inputs used in determination of the discount for lack of marketability at that time were (i) exercise price - $2.22, (ii) dividend yield - 0%, (iii) expected holding period - 1.5 years, (iv) risk-free rate - 0.44%, and (v) volatility - 65%. The increase in valuation of the Preferred Series B Convertible shares from the effective date of the agreement to year-end was recorded as an unrealized gain in investment in the Consolidated Statements of Operations and Comprehensive Loss.

There was no material re-measurement to fair value of financial assets and liabilities that are not measured at fair value on a recurring basis.

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, 2014
	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	$	125,606	$	125,606	$	—	$	—
Certificates of deposit	480		—		480		—
Total cash equivalents	126,086		125,606		480		—
Short-term investments
Certificates of deposit	16,981		—		16,981		—
Corporate bonds	69,892		—		69,892		—
Commercial paper	11,240		—		11,240		—
U.S. Treasury securities	8,000		8,000		—		—
Total short-term investments	106,114		8,000		98,114		—
Long-term investments
Certificates of deposit	10,996		—		10,996		—
U.S. Treasury securities	40,197		40,197		—		—
Preferred stock of U.S. corporation	1,781		—		—		1,781
Total long-term investments	52,973		40,197		10,996		1,781
Total assets	$	285,173	$	173,803	$	109,590	$	1,781



			Fair Value Measurements
			December 31, 2013
	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	$	107,349	$	107,349	$	—	$	—
Total cash equivalents	$	107,349	$	107,349	$	—	$	—

Below is a table that presents a reconciliation of the beginning and ending balances of assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs (Level 3) (in thousands):



	Fair Value Measurements (Level 3)
Redeemable convertible preferred stock warrant liability:
Fair value at January 1, 2012	$	6,491
Issuance	174
Fair value increase recorded in other expense	847
Fair value at December 31, 2012	7,512
Fair value increase recorded in other expense	6,590
Reclassification of warrant liability to additional paid-in capital	(14,102		)
Fair value at December 31, 2013	$	—

Preferred stock of U.S. corporation:
Fair value at January 1, 2014	$	—
Investment acquired	1,545
Fair value increase recorded in other comprehensive loss	236
Fair value at December 31, 2014	$	1,781

Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consist of the following (in thousands):



	December 31,
	2014		2013
Prepaid research and development expenses	$	1,685	$	2,433
Interest receivable	553		—
Prepaid insurance	342		176
Other prepaid expenses and current assets	195		176
Total prepaid expenses and other current assets	$	2,775	$	2,785

Property and Equipment

Property and equipment are stated at cost, less accumulated depreciation. Depreciation is determined on a straight-line basis over the estimated useful lives of the assets, which generally range from three to five years. Leasehold improvements are amortized over the shorter of the useful life of the asset or the term of the related lease. Maintenance and repairs are charged against expense as incurred.

Impairment of Long-Lived Assets

The Company evaluates long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying value of an asset may not be recoverable. If the estimated future cash flows (undiscounted and without interest charges) from the use of an asset are less than the carrying value, a write-down would be recorded to reduce the related asset to its estimated fair value. To date, no such write-downs have occurred.

Deferred Lease Obligation

The Company recognizes rent expense on a straight-line basis over the non-cancelable term of its operating lease and records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. The Company also records landlord-funded lease incentives, such as reimbursable leasehold improvements, as a deferred rent liability, which is amortized as a reduction of rent expense over the non-cancelable term of its operating lease.

Accrued Liabilities

Accrued liabilities consist of the following (in thousands):



	December 31,
	2014		2013
Accrued compensation	$	3,057	$	1,779
Deferred revenue	1,545		—
Accrued research and development expenses	1,116		350
Other accrued liabilities	1,115		291
Total accrued liabilities	$	6,833	$	2,420

Redeemable Convertible Preferred Stock Warrant Liability

Freestanding warrants for shares that are either putable or redeemable are classified as liabilities on the balance sheet at fair value. As further discussed in Note 6, the preferred stock underlying certain warrants was redeemable in certain circumstances, and as such the freestanding warrants that are related to the purchase of the Company’s Series F preferred stock were liabilities that should be recorded at the estimated fair value. At the end of each reporting period, changes in the estimated fair value during the period are recorded in other income.

Redeemable Convertible Preferred Stock

The Company classified its redeemable convertible preferred stock, for which the Company did not control the redemption, outside of permanent equity. The Company recorded redeemable convertible preferred stock at fair value upon issuance, net of any offering costs, and the carrying value was adjusted to the redemption value at the end of each reporting period. These adjustments were effected through charges against additional paid-in capital and accumulated deficit.

Revenue Recognition

The Company’s revenues generally consist of (i) contract and grant revenue – revenue generated under federal contracts and other awarded grants, and (ii) collaboration and licensing revenue – revenue related to non-refundable upfront fees, royalties and milestone payments earned under license agreements. Revenue is recognized in accordance with the criteria outlined in the Securities and Exchange Commission (SEC)’s Topic 13 and Accounting Standards Codification (ASC) 605-25 and by the Financial Accounting Standards Board. Following these accounting pronouncements, revenue is recognized when all four of the following criteria are met: (i) persuasive evidence of an arrangement exists; (ii) delivery of the products and/or services has occurred and risk of loss has passed; (iii) the selling price is fixed or determinable; and (iv) collectability is reasonably assured.

For arrangements that involve the delivery of more than one element, each product, service and/or right to use assets is evaluated to determine whether it qualifies as a separate unit of accounting. This determination is based on whether the deliverable has “stand-alone value” to the customer. The consideration that is fixed or determinable is then allocated to each separate unit of accounting based on the relative selling prices of each deliverable. The consideration allocated to each unit of accounting is recognized as the related goods and services are delivered, limited to the consideration that is not contingent upon future deliverables. If the arrangement constitutes a single unit of accounting, the revenue recognition policy must be determined for the entire arrangement and the consideration received is recognized over the period of inception through the date the last deliverable within the single unit of accounting is expected to be delivered. Revisions to the estimated period of recognition are reflected in revenue prospectively.

Non-refundable upfront fees are recorded as deferred revenue and recognized into revenue as license fees from collaborations on a straight-line basis over the estimated period of the Company’s substantive performance obligations. If the Company does not have substantive performance obligations, the Company recognizes non-refundable upfront fees into revenue through the date the deliverable is satisfied. Analyzing the arrangement to identify deliverables requires the use of judgment and each deliverable may be an obligation to deliver services, a right or license to use an asset, or another performance obligation.

Milestone payments are recognized when earned, provided that (i) the milestone event is substantive; (ii) there is no ongoing performance obligation related to the achievement of the milestone earned; and (iii) it would result in additional payments. Milestone payments are considered substantive if all of the following conditions are met: the milestone payment is non-refundable; achievement of the milestone was not reasonably assured at the inception of the arrangement; substantive effort is involved to achieve the milestone; and the amount of the milestone appears reasonable in relation to the effort expended, the other milestones in the arrangement; and the related risk associated with the achievement of the milestone. Contingent based event payments the Company may receive under a license or collaboration agreement will be recognized when received.

For the years ended December 31, 2014, 2013 and 2012, contract and grant revenue consisted only of revenue from the BARDA contract as there was no grant revenue. The Company recognizes contract and grant revenue as qualifying research activities are conducted based on invoices received from the Company’s vendors. Changes in fringe and indirect rates are recognized as a change in estimate in the period such rate changes are approved by BARDA.

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, 2014, there had been no material adjustments to the Company’s prior period estimates of prepaid and accruals for research and development expenses. The Company’s research and development prepaids and accruals are dependent upon the timely and accurate reporting of contract research organizations and other third-party vendors.

Research and Development Expenses

Major components of research and development costs include cash compensation, stock based compensation, pre-clinical studies, clinical trial and related clinical manufacturing, drug development, materials and supplies, legal, regulatory compliance, and fees paid to consultants and other entities that conduct certain research and development activities on the Company’s behalf. Research and development costs, including upfront fees and milestones paid to contract research organizations, are expensed as goods as received or services rendered. Costs incurred in connection with clinical trial activities for which the underlying nature of the activities themselves do not directly relate to active research and development, such as costs incurred for market research and focus groups linked to clinical strategy as well as costs to build the Company’s brand, are not included in research and development costs but are reflected as general and administrative costs.

Interest Income (Expense), Net

Interest income (expense), net includes interest earned on short-term and long-term investments, interest incurred on loans payable, the amortization of deferred financing costs related to fees paid to attorneys and other non-lender entities in order to acquire debt, and the amortization of debt discount related to fees paid to the lender in order to acquire debt.

Income Taxes

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, 2014, and therefore has not recorded any current provision for income taxes.

Additionally, the Company recognizes the tax benefit from an uncertain tax position only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities based on the technical merits of the position. The tax benefit recognized in the financial statements for a particular tax position is based on the largest benefit that is more likely than not to be realized upon settlement. Accordingly, the Company establishes reserves for uncertain tax positions.

Share-Based Compensation

The Company measures and recognizes compensation expense for all share-based payment awards made to employees and directors, including employee stock options, restricted stock units and the employee stock purchase plan purchase rights, based on estimated fair values. The fair value of share-based awards is estimated on the grant date using the Black-Scholes valuation model. The value of the portion of the award that is ultimately expected to vest is recorded as expense over the requisite service periods.

The Company also accounts for equity instruments issued to non-employees using a fair value approach. The Company values equity instruments, stock options and warrants granted to lenders and consultants using the Black-Scholes valuation model. The measurement of non-employee share-based compensation is subject to periodic adjustments as the underlying equity instruments vest and is recognized as an expense over the term of the related financing or the period over which services are received.

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 converting redeemable convertible preferred stock, warrants to purchase redeemable convertible preferred stock and 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 redeemable convertible preferred stock and warrants to purchase redeemable convertible preferred stock and 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, 2014, 2013 and 2012.

The calculation of weighted-average diluted shares outstanding excludes the dilutive effect of converting redeemable convertible preferred stock, warrants to purchase convertible preferred stock and common stock, 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 2,170,660, 7,550,950 and 15,694,862 for the years ended December 31, 2014, 2013 and 2012, respectively.

Segments

The Company operates in only one segment. The chief operating decision-maker, who is the Company’s Chief Executive Officer, and management use cash flows as the primary measure to manage the business and do not segment the business for internal reporting or decision making.

Impact of Recently Issued Accounting Standards

In May 2014, the FASB issued 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. This ASU is effective for fiscal years, and interim periods within those years, beginning after December 15, 2016 and early application is not permitted. The Company is currently analyzing the impact of this new accounting guidance.

Investments

12 Months Ended

Dec. 31, 2014

Investments [Abstract]

Investments

The following table summarizes the Company's short-term and long-term investments as of December 31, 2014 (in thousands):



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Corporate bonds	$	69,947	$	—	$	(56	)	$	69,891
Certificates of deposit	28,039		1		(62		)	27,978
U.S. Treasury securities	48,279		—		(82		)	48,197
Commercial paper	11,242		—		(2		)	11,240
Preferred stock of U.S. corporation	$	1,545	$	236	$	—		$	1,781
Total investments	$	159,052	$	237	$	(202	)	$	159,087

The Company had no short-term or long-term investments at December 31, 2013.

The following table summarizes the scheduled maturity for the Company's investments at December 31, 2014 (in thousands):



	December 31, 2014
Maturing in one year or less	$	106,114
Maturing after one year through two years	52,973
Total investments	$	159,087

- Definition

The entire disclosure for investments in certain debt and equity securities.

+ References + Details

Property and Equipment

12 Months Ended

Dec. 31, 2014

Property, Plant and Equipment [Abstract]

Property and Equipment

Property and equipment, net of accumulated depreciation consists of the following (in thousands):



	December 31,
	2014			2013
Lab equipment	$	1,049		$	873
Leasehold improvements	777			78
Computer equipment	512			324
Office furniture and equipment	343			222
Property and equipment	2,681			1,497
Less accumulated depreciation	(1,371		)	(1,159		)
Property and equipment, net of accumulated depreciation	$	1,310		$	338

Loan Payable	12 Months Ended
Loan Payable	Dec. 31, 2014
Debt Disclosure [Abstract]
Loan Payable	Loan Payable On January 27, 2012, the Company entered into a Loan and Security Agreement (LSA) with Silicon Valley Bank (SVB) and MidCap Financial SBIC, LP (MidCap) allowing for borrowings up to $15.0 million, split between a first tranche of $3.0 million borrowed at the time of the agreement, and a second tranche of up to $12.0 million that was available to be drawn by December 31, 2012 upon meeting one of three stated financial and/or operational goals. The borrowings under the LSA are collateralized by a security interest in all of the Company's assets, excluding its intellectual property. The first tranche was used to repay the remaining principal balance outstanding of $2.6 million under a previous loan. This repayment was deemed a modification of debt and therefore the remaining related deferred financing costs totaling $0.1 million remained in deferred financing costs and are being amortized over the term of the LSA through interest expense. The first tranche has an interest-only period of twelve months followed by a 30 months principal and interest amortization period with interest being charged at 8.25% per year for the full period of the LSA. The Company met one of the financial and/or operational goals mentioned above and, in September 2012, the remaining $12.0 million was borrowed in the second tranche. The second tranche has a 6 months interest-only period followed by a 32 months principal and interest amortization period with interest being charged at the same rate as the first tranche. There are certain fees in accordance with the LSA which are being recorded as discounts or other long and short-term liabilities depending on the nature of the fees. The fees are being accreted through interest expense. Approximately $0.1 million , $0.1 million , and $0.1 million was included in interest expense in the Consolidated Statements of Operations and Comprehensive Loss for the years ended December 31, 2014, 2013, and 2012, respectively. Concurrently with entering into the LSA, the Company also granted SVB a warrant to purchase shares of Series F preferred stock at a price of $2.045 per share equal to 2% of the aggregate amount of the advances made to the Company pursuant to the LSA, divided by the exercise price. In relation to the first tranche, the warrant became exercisable to purchase an aggregate of 29,340 shares of Series F preferred stock, and in relation to the second tranche, the warrant became exercisable to purchase an additional 117,360 shares of Series F preferred stock. As discussed in Note 1, the warrant is classified as a liability and is required to be measured at fair value. Therefore, the warrant was recorded as a debt discount at its fair value at the time of grant and accreted over the life of the LSA using the effective interest method. Upon the completion of the Company’s IPO, this warrant was converted into a warrant to purchase 41,323 shares of common stock at an exercise price equal to $7.26. In May 2013, SVB exercised the warrant in full and it is no longer outstanding. At December 31, 2014, the remaining principal amount of $4.4 million under the LSA is due and payable throughout 2015, and as such is included in current liabilities in the Consolidated Balance Sheets.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2014

Commitments and Contingencies Disclosure [Abstract]

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 2018. The Company has the following minimum rental payments under noncancelable operating lease obligations that existed at December 31, 2014 (in thousands):



Years Ending December 31,	Minimal Rental Payment
2015	$	541
2016	511
2017	516
2018	128
Total future minimum rental payments	$	1,696

Rent expense under non-cancelable operating leases and other month-to-month equipment rental agreements, including common area maintenance fees, totaled approximately $0.4 million, $0.5 million, and $0.4 million for the years ended December 31, 2014, 2013 and 2012, respectively.

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, 2014 and 2013.

Redeemable Convertible Preferred Stock	12 Months Ended
Redeemable Convertible Preferred Stock	Dec. 31, 2014
Temporary Equity Disclosure [Abstract]
Redeemable Convertible Preferred Stock	Redeemable Convertible Preferred Stock In February 2011, the Company issued 22,004,895 shares of $0.001 par value Series F redeemable convertible preferred stock at $2.045 per share and warrants to purchase an aggregate of 5,501,215 shares of Series F redeemable convertible preferred stock at an exercise price of $2.045 per share for proceeds of $45.0 million, less issuance costs of $0.2 million. Upon the completion of the Company’s IPO in April 2013, these warrants were converted into warrants to purchase an aggregate of 1,549,628 shares of common stock at an exercise price of $7.26 per share. The warrants are exercisable at any time and expire on February 4, 2018. In January 2012, the Company issued a warrant to SVB to purchase a number of shares of Series F redeemable convertible preferred stock at an exercise price of $2.045 per share equal to 2% of the aggregate amount of the advances made to the Company pursuant to the LSA, divided by the exercise price. Following the first and second tranches of the LSA, the warrant was exercisable to purchase an aggregate of 146,700 shares of Series F redeemable convertible preferred stock. Upon the completion of the Company’s IPO in April 2013, this warrant was converted into a warrant to purchase 41,323 shares of common stock at an exercise price of $7.26 per share. In May 2013, SVB exercised the warrant in full and it is no longer outstanding. Upon the completion of the Company’s IPO in April 2013, the Company’s outstanding shares of redeemable convertible preferred stock and dividends accrued on Series F redeemable convertible preferred stock were automatically converted into an aggregate of 15,556,091 shares of common stock. Warrants As discussed in Note 1, the warrants exercisable for the Company’s Series F preferred stock were classified as a liability and were required to be measured at fair value. Therefore, such warrants were recorded at the full fair value with the Company’s Series F preferred stock being recorded at the residual value at the time of issuance. At each reporting date prior to the Company’s IPO, the warrants exercisable for the Company’s Series F preferred stock were recorded to fair value which was charged to other income. For the years ended December 31, 2013 and 2012, the Company recorded expense of $6.6 million and $0.8 million, respectively, related to the valuation of the warrants. These amounts are stated as the fair value adjustments to warrant liability on the Consolidated Statements of Operations and Comprehensive Loss. Upon the completion of the Company’s IPO, all outstanding warrants to purchase redeemable convertible preferred stock were converted into warrants to purchase 1,613,395 shares of common stock and are no longer required to be measured at fair value. Warrants for the purchase of common stock of 726,601 and 1,337,845 were issued, outstanding and exercisable at December 31, 2014 and 2013, respectively.

Stockholders' Equity (Deficit)

12 Months Ended

Dec. 31, 2014

Stockholders' Equity Note [Abstract]

Stockholders' Equity (Deficit)

Stockholders’ Equity (Deficit)

Common Stock

The Company’s common stock consists of 200.0 million authorized shares at December 31, 2014 and 2013, and 41.0 million and 26.7 million shares issued and outstanding at December 31, 2014 and December 31, 2013, respectively.

On March 25, 2013, the Company’s board of directors approved and implemented a 3.55-for-1 reverse stock split of the Company’s outstanding common stock. The reverse stock split resulted in an adjustment to the preferred stock conversion price to reflect a proportional decrease in the number of shares of common stock to be issued upon conversion. The accompanying financial statements and notes to the financial statements give retroactive effect to the reverse stock split for all periods presented.

On April 10, 2013, the Company completed the initial public offering (IPO) of its common stock pursuant to a registration statement on Form S-1. In the IPO, the Company sold an aggregate of 7,320,000 shares of common stock under the registration statement at a public offering price of $14.00 per share. Net proceeds were approximately $93.3 million, after deducting underwriting discounts and commissions of $7.1 million and offering expenses of $2.1 million. Upon the completion of the IPO, all outstanding shares of the Company’s redeemable convertible preferred stock and dividends accrued on Series F redeemable convertible preferred stock were converted into 15,556,091 shares of common stock and all outstanding warrants to purchase redeemable convertible preferred stock were converted into warrants to purchase 1,613,395 shares of common stock. On April 16, 2013, the underwriters exercised the full over-allotment option pursuant to which the Company sold an additional 1,098,000 shares at $14.00 per share. Net proceeds from the over-allotment shares were approximately $14.3 million after deducting underwriting discounts and commissions of $1.1 million.

On October 23, 2013, the Company completed an underwritten secondary public offering of 2,476,995 shares of common stock held by certain of the Company’s existing stockholders. The Company did not issue any shares of common stock and received no proceeds in connection with such offering. The principal purposes of the offering were to facilitate an orderly distribution of shares and to increase the Company’s public float.

On May 27, 2014, the Company completed an underwritten public offering of 8,395,000 shares of common stock, including 1,095,000 shares sold pursuant to the full exercise of an over-allotment option previously granted to the underwriters. All of the shares were offered by the Company at a price to the public of $14.22 per share. Net proceeds to the Company from this offering, after deducting underwriting discounts and commissions and other offering expenses payable by the Company, were approximately $111.8 million. The securities described above were offered by the Company pursuant to a shelf registration statement declared effective by the SEC on May 16, 2014.

On November 5, 2014, the Company completed an underwritten public offering of 4,197,500 shares of common stock, including 547,500 shares sold pursuant to the full exercise of an option previously granted to the underwriters to purchase additional shares of common stock. All of the shares were offered by the Company at a price to the public of $29.00 per share. The net proceeds from this offering, after deducting underwriting discounts and commissions and other offering expenses payable by us, were approximately $114.0 million. The securities described above were offered by the Company pursuant to an automatic shelf registration statement which immediately became effective by rule of the SEC on October 29, 2014.

Shares Reserved for Future Issuance

The Company has reserved shares of common stock for future issuances as follows:



	December 31,
	2014	2013
For exercise of common stock warrants	726,601	1,337,845
For exercise of outstanding common stock options	1,859,970	1,946,823
For future equity awards under the 2013 Equity Incentive Plan	1,147,526	1,681,932
For future purchases under the 2013 Employee Stock Purchase Plan	944,599	704,225
Total shares of common stock reserved for future issuances	4,678,696	5,670,825

Stock Options

In connection with the Company’s IPO, the Company adopted the 2013 Equity Incentive Plan (the 2013 Plan). The 2013 Plan provides for the grant of incentive stock options (ISOs), nonstatutory stock options (NSOs), stock appreciation rights, restricted stock awards, restricted stock unit (RSU) awards, performance-based stock awards, and other forms of equity compensation (collectively, stock awards), all of which may be granted to employees, including officers, non-employee directors and consultants of the Company and its affiliates. Additionally, the 2013 Plan provides for the grant of performance cash awards. ISOs may be granted only to employees. All other awards may be granted to employees, including officers, and to non-employee directors and consultants. Initially, the aggregate number of shares of common stock that may be issued pursuant to stock awards under the 2013 Plan is the sum of (i) 1,408,450 shares, plus (ii) 244,717 shares, which was the number of shares reserved for issuance under the Company’s 2012 Equity Incentive Plan (the 2012 Plan) at the time the 2013 Plan became effective, plus (iii) any shares subject to outstanding stock options or other stock awards that would have otherwise returned to the 2012 Plan (such as upon the expiration or termination of a stock award prior to vesting). Additionally, the number of shares of common stock reserved for issuance under the 2013 Plan will automatically increase on January 1 of each year, beginning on January 1, 2014 and continuing through and including January 1, 2023, by 2.5% of the total number of shares of capital stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares determined by the Company’s board of directors. The maximum number of shares that may be issued upon the exercise of ISOs under the 2013 Plan is 2,816,901 shares. Following the effectiveness of the 2013 Plan in April 2013, no further grants were made under the 2012 Plan. At the Company's annual meeting held on June 20, 2014, shareholders approved a change to the annual automatic increase in the number of common shares to be reserved for issuance under the 2013 Plan by changing the percentage increase to 4.0%, or a lesser number of shares determined by the Company's board of directors.

The Company estimates the fair value of its share-based awards to employees, directors and consultants using the Black-Scholes option-pricing model. The Black-Scholes model requires the input of highly complex and subjective assumptions, including (a) the expected stock price volatility, (b) the calculation of expected term of the award, (c) the risk-free interest rate and (d) expected dividends. Due to the Company’s limited operating history and historical and implied volatility data, the Company has based its estimates of expected volatility on a blend of Company specific historical data and a group of similar public traded companies. When selecting these public companies on which it has based its expected stock price volatility, the Company selected companies with comparable characteristics to it, including enterprise value, risk profiles, positions within the industry, and with historical share price information sufficient to meet the expected life of its stock options. For employee stock options the Company uses the “simplified” method for estimating expected life, whereby, the expected life equals the arithmetic average of the vesting term and the original contractual term of the option due to its lack of sufficient historical data. The expected term for share-based compensation granted to non-employees is the contractual 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:



	Employees
	Years Ended December 31,
	2014			2013			2012
Expected volatility	71.81		%	76.40		%	80.55		%
Expected term (in years)	6.0			6.1			6.0
Weighted-average risk-free interest rate	1.92		%	1.49		%	0.86		%
Expected dividend yield	—		%	—		%	—		%
Weighted-average fair value per option	$	14.00		$	2.52		$	1.93



	Non-Employees
	Years Ended December 31,
	2014			2013			2012
Expected volatility	68.63		%	79.69		%	81.77		%
Expected term (in years)	5.7			6.2			5.8
Weighted-average risk-free interest rate	1.87		%	1.30		%	0.78		%
Expected dividend yield	—		%	—		%	—		%
Weighted-average fair value per option	$	14.13		$	5.01		$	3.48

The Company is also required to estimate forfeitures at the time of grant, and revise those estimates in subsequent periods if actual forfeitures differ from its estimates. The Company uses historical data to estimate pre-vesting option forfeitures and record share-based compensation expense only for those awards that are expected to vest. To the extent that actual forfeitures differ from the Company’s estimates, the difference is recorded as a cumulative adjustment in the period the estimates were revised. For the years ended December 31, 2014, 2013 and 2012, the Company applied a forfeiture rate based on the Company’s historical forfeitures.

A summary of activity related to the Company’s stock options is as follows:



	Number of Options Outstanding		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Life (in Years)
Balance, January 1, 2013	2,593,423		$	2.45	7.36
Granted	292,906		10.62		—
Exercised	(801,049	)	2.43		—
Expired or Canceled	(2,816	)	1.57		—
Forfeited	(135,641	)	4.55		—
Balance, December 31, 2013	1,946,823		$	3.54	7.03
Granted	1,385,370		21.57		—
Exercised	(1,287,883	)	3.57		—
Expired or Canceled	—		—		—
Forfeited	(184,340	)	15.30		—
Balance, December 31, 2014	1,859,970		$	15.79	8.46
Exercisable at December 31, 2014	589,141		$	9.03	7.17
Vested or expected to vest at December 31, 2014	1,816,056		$	15.73	8.45

At December 31, 2014, the aggregate intrinsic value of options outstanding and exercisable was $18.4 million. The total intrinsic value of options exercised was $30.4 million, $12.4 million, and $18,000 for the years ended December 31, 2014, 2013 and 2012, respectively.

As of December 31, 2014, there was approximately $13.9 million of total unrecognized compensation cost related to non-vested share-based compensation arrangements granted under the 2013 Plan. That compensation cost is expected to be recognized over a weighted-average period of approximately 3.14 years.

The following table summarizes, at December 31, 2014, 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 Plan, the number of stock option awards exercisable and their weighted-average exercise price:



	Outstanding				Exercisable
Range	Number	Weighted-Average Remaining Contractual Life (in years)	Weighted-Average Exercise Price		Number	Weighted-Average Exercise Price
$1.42 to 3.71	395,871	6.0	2.35		355,017	2.35
3.72 to 15.96	368,487	8.4	9.78		49,372	9.84
15.97 to 18.93	423,572	9.1	18.67		88,988	18.74
18.94 to 24.00	362,990	9.3	22.48		82,514	22.68
24.01 to 36.75	309,050	9.8	28.34		13,250	34.88
$1.42 to 36.75	1,859,970	8.5	$	15.79	589,141	$	9.03

In 2012, the Company modified option grants for four individuals. Three of the modifications extended the term to exercise the option resulting in $30,000 in additional compensation expense. One option was modified to continue vesting after the participant’s termination and to extend the time to exercise such option resulting in additional compensation expense of $0.3 million.

On April 9, 2014, the Company’s former President and Chief Executive Officer resigned. The Company entered into a severance agreement with the former officer that provides for severance benefits to him in connection with his resignation. Among other benefits, he received accelerated vesting of all of his outstanding stock options as if he had continued service for an additional 15 months period. In addition, his vested options were modified to extend his exercise period to December 31, 2014. The Company recorded a charge of $1.0 million to compensation expense on the date of his resignation related to the acceleration of vesting and the modification.

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. On January 1, 2014, the common stock reserved for future issuance under the ESPP was automatically increased by an additional 266,649 shares bringing the total number of shares of common stock that may be purchased under the ESPP to 970,874.

The Company has reserved a total of 970,874 shares of common stock to be purchased under the ESPP, of which 944,599 shares remain available for purchase at December 31, 2014. Eligible employees may authorize up to 15% of their salary to purchase common stock at the lower of 85% of the beginning price at the participants' plan entry date or 85% of the ending price at 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 26,275 shares of common stock pursuant to the ESPP for the year ended December 31, 2014. 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,
	2014			2013
Expected volatility	74.24		%	51.22		%
Expected term (in years)	0.8			1.3
Weighted-average risk-free interest rate	0.09		%	0.25		%
Expected dividend yield	—		%	—		%
Weighted-average option value per share	$	9.93		$	4.20

As of December 31, 2014, the Company had a liability of $296,000 representing employees' contributions to the ESPP.

Restricted Stock Units

In 2012 and 2013, the Company issued Restricted Stock Units (RSUs) to certain employees which vest based on specific performance criteria. By their terms, the RSUs became immediately vested upon the effective date of the registration statement for the Company’s common stock in connection with the IPO, subject to the continuous service with the Company at the vesting event. When vested, the RSU represented the right to be issued the number of shares of the Company’s common stock that is equal to the number of RSUs granted.

A summary of activity related to the Company’s RSUs is as follows:



	Number of Restricted Stock Units Outstanding
Balance, December 31, 2012	43,199
Granted	59,348
Share issuance	(102,547	)
Balance, December 31, 2013	—

In June 2014, the Company recorded an immaterial out of period adjustment of $1.4 million to properly state additional paid-in-capital with a resulting decrease in compensation expense related to the RSUs that vested in 2013 in connection with the IPO.

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,
	2014		2013		2012
Income Statement Classification:
Research and development expense:
Employee	$	1,085	$	1,905	$	336
Non-employee	—		67		80
General and administrative expense:
Employee	3,326		1,017		921
Non-employee	—		82		59
Total stock-based compensation expense	$	4,411	$	3,071	$	1,396

Cash received from exercises under all share-based payment arrangements for 2014 , 2013 and 2012 was $5.0 million, $1.9 million and $14,000, respectively. There was no actual tax benefit realized for the tax deductions from exercises of the share-based payment arrangements during 2014, 2013 or 2012.

The Company continues to account for stock options issued to non-employees using a fair value approach. The compensation costs of these arrangements are subject to re-measurement over the vesting terms as earned. Compensation cost for performance-based awards is recognized when it is probable that the performance criteria will be met.

Fair Value Estimate

Prior to the Company’s IPO, the Company was required to estimate the fair value of the common stock underlying share-based awards when performing the fair value calculations with the Black-Scholes option-pricing model. The fair value of the common stock underlying share-based awards was determined on each grant date by the Company’s board of directors, with input from management. All options to purchase shares of common stock were intended to be granted with an exercise price per share no less than the fair value per share of common stock underlying those options on the date of grant, based on the information known on the date of grant.

The Company was privately held with no active public market for its common stock. Therefore, management had for financial reporting purposes periodically determined the estimated per share fair value of the Company’s common stock and redeemable convertible preferred stock at various dates using contemporaneous valuations consistent with the American Institute of Certified Public Accountants Practice Aid, “Valuation of Privately-Held Company Equity Securities Issued as Compensation,” also known as the Practice Aid. These valuations were performed with the assistance of a third-party valuation specialist. The Company performed these contemporaneous valuations as of February 15, 2011, December 31, 2011, and September 30, 2012 and December 31, 2012. In conducting these contemporaneous valuations, management considered all objective and subjective factors that it believed to be relevant in each valuation conducted, including management’s best estimate of the Company’s business condition, prospects and operating performance at each valuation date. Within the contemporaneous valuations performed, a range of factors, assumptions and methodologies were used. The significant factors included external market conditions affecting the biotechnology industry, trends within the biotechnology industry, the prices at which the Company sold shares of preferred stock, the superior rights and preferences of the preferred stock relative to common stock at the time of each grant, the results of operations, financial position, status of research and development efforts, stage of development and business strategy, the lack of an active public market for the common and preferred stock, and the likelihood of achieving a liquidity event such as an IPO or sale of the Company in light of prevailing market conditions.

The dates of the Company’s contemporaneous valuations had not always coincided with the dates of its share-based compensation grants. In such instances, management’s estimates had been based on the most recent contemporaneous valuation of the Company’s shares of common stock and its assessment of additional objective and subjective factors management believed were relevant and which may have changed from the date of the most recent contemporaneous valuation through the date of the grant. In addition, the Company performed retrospective valuations as of certain key option grant dates using similar methodologies as were used in the contemporaneous valuations. As a result, the Company concluded certain options granted in 2012 had reassessed values different from the grant date. The reassessed values were used to determine share-based compensation expense for the year ended December 31, 2012.

- Definition

The entire disclosure for shareholders' equity comprised of portions attributable to the parent entity and noncontrolling interest, including other comprehensive income. Includes, but is not limited to, balances of common stock, preferred stock, additional paid-in capital, other capital and retained earnings, accumulated balance for each classification of other comprehensive income and amount of comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(d),(e))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21484-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB TOPIC 4.E)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74512-122707

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21488-112644

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29-31)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-Subparagraph (SAB TOPIC 4.C)
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Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 30
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6405834&loc=d3e23285-112656

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644

Reference 11: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Preferred Stock
-URI http://asc.fasb.org/extlink&oid=6521494

Reference 12: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

Reference 13: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
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-Section 50
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21564-112644

Reference 14: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 08
-Paragraph d
-Article 4

Reference 15: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 29, 30, 31
-Article 5

Reference 16: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section E

Reference 17: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 08
-Article 4

Reference 18: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 4
-Section C

Reference 19: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 04
-Article 3

+ Details

Income Taxes

12 Months Ended

Dec. 31, 2014

Income Tax Disclosure [Abstract]

Income Taxes

No income tax expense or benefit has been recorded for the years ended December 31, 2014, 2013 or 2012. This is due to the establishment of a valuation allowance against the deferred tax assets generated during those periods. At December 31, 2014, 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, 2014, 2013, and 2012 (in thousands):



	2014					2013					2012
	Amount			% of Pretax Earnings		Amount			% of Pretax Earnings		Amount			% of Pretax Earnings
Income tax benefit at statutory rate	$	(20,166	)	34.0	%	$	(12,392	)	34.0	%	$	(1,499	)	34.0	%
State income taxes	(1,343		)	2.3	%	(852		)	2.3	%	(100		)	2.3	%
Research and development credits	(2,577		)	4.3	%	(2,265.0		)	6.2	%	—			—	%
Permanent items	1,525			(2.6	)%	3,060			(8.4	)%	868			(19.7	)%
Provision to return adjustments	64			(0.1	)%	320			(0.9	)%	2			—	%
Effect of change in state tax rate	2			—	%	(1		)	—	%	609			(13.9	)%
Increase in unrecognized tax benefits	425			(0.7	)%	566.0			(1.5	)%	—			—	%
Change in valuation allowance	22,070			(37.2	)%	11,564			(31.7	)%	120			(2.7	)%
Net benefit	$	—		—	%	$	—		—	%	$	—		—	%

The components of deferred tax assets and liabilities at December 31, 2014 and 2013 are as follows (in thousands):



	December 31,
	2014			2013
Deferred tax assets:
Net operating loss carryforwards	$	59,312		$	40,721
Research and development expenses	405			127
Capitalized Section 174 expenses	57			69
Research and development credits	4,572			2,640
Accrued bonuses	1,066			405
Share-based compensation	1,045			267
Other	135			230
Total gross deferred tax assets	66,592			44,459
Valuation allowance	(66,475		)	(44,405		)
Total deferred tax assets	117			54
Deferred tax liabilities:
Other	(117		)	(54		)
Total deferred tax liabilities	(117		)	(54		)
Total deferred tax assets and liabilities, net	$	—		$	—

At December 31, 2014, the Company has net operating loss carryforwards for federal and state tax purposes of approximately $192.6 million and $156.9 million, respectively. At December 31, 2013, the Company has net operating loss carryforwards for federal and state purposes of approximately $117.5 million and $106.0 million, respectively. The federal losses begin to expire in 2020 and the state losses begin to expire in 2018. The Company’s federal and state net operating loss carryforwards include approximately $31.3 million of excess tax benefits related to deductions from the exercise of stock options. The tax benefit of these deductions has not been recognized in deferred tax assets. If utilized, the benefits from these deductions will be recorded as adjustments to additional paid-in capital. In addition, the Company has tax credit carryforwards for federal tax purposes of approximately $6.1 million as of December 31, 2014, 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 during the year. However, the subsidiary has no activity as of December 31, 2014 and as such, has no undistributed earnings.

In general, if the Company experiences a greater than 50% aggregate change in ownership of certain significant stockholders over a three-year period (a Section 382 ownership change), utilization of its pre-change net operating loss carryforwards is subject to an annual limitation under Section 382 of the Internal Revenue Code (and similar state laws). The annual limitation generally is determined by multiplying the value of the Company’s stock at the time of such ownership change (subject to certain adjustments) by the applicable long-term tax-exempt rate. Such limitations may result in expiration of a portion of the net operating loss carryforwards before utilization and may be substantial. The ability of the Company to use its net operating loss carryforwards may be limited or lost if the Company experienced one or more Section 382 ownership changes in connection with past offerings of its stock 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 2014 and concluded that a Section 382 ownership change occurred in 2013 as a result of the initial public offering, the Company’s private placement and other transactions that have occurred since 2007. As such, losses incurred prior to that date are subject to an annual limitation of at least $6.7 million.

The Company had unrecognized tax benefits of approximately $0.2 million as of January 1, 2013. As the 2012 R&D tax credit was not yet enacted into current law, no R&D credit was reflected on the 2012 provision. As a result, the Company recognized an increase in the unrecognized tax benefit associated with the federal R&D credit carryover in the year ended December 31, 2013 related to the periods ending December 31, 2012 and 2013. As of December 31, 2014, the total unrecognized tax benefits were approximately $1.0 million and of this total, none would reduce the Company’s effective tax rate if recognized. The Company does not anticipate a significant change in total unrecognized tax benefits or the Company’s effective tax rate due to the settlement of audits or the expiration of statutes of limitations within the next twelve months. Furthermore, the Company does not expect any cash settlement with the taxing authorities as a result of these unrecognized tax benefits as the Company has sufficient unutilized carryforward attributes to offset the tax impact of these adjustments.

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 year ended December 31, 2014, as follows (in thousands):



Balance at December 31, 2012	$	207
Increases related to 2013	197
Increases related to prior periods	177
Balance at December 31, 2013	581
Increases related to 2014	425
Increases related to prior periods	—
Balance at December 31, 2014	$	1,006

The presentation of the above rollforward of the uncertain tax benefit was updated in the current year to include the benefit to the net operating loss if a portion of the federal R&D credit were to be disallowed. This change in presentation ties the current year increase to the amount included in the rate reconciliation. As this was merely a change in presentation there is no effect on prior year tax provision.

The Company has determined that it had no other material uncertain tax benefits for the year ended December 31, 2014. As of January 1, 2015, 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 2014. 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 January 1, 2014.

Significant Agreements	12 Months Ended
Significant Agreements	Dec. 31, 2014
Significant Agreements [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 and CMX157. The license agreement was amended in September 2002 in order to expand the scope of the license and again in December 2010 in order to modify certain financial terms. The agreement was amended a third time in September 2011 to add additional patents related to certain metabolically stable lipid-conjugate compounds. A fourth amendment was executed in July 2012 to alter the rights and obligations of the parties in light of the Company’s current business plans. As partial consideration for the rights granted to the Company under the license agreement, 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. 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. 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 addition, upon commercialization of any product utilizing the UC Patent Rights (which would include the commercialization of brincidofovir or CMX157), the Company will be required to pay low single digit royalties on net sales of such product. 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 size of any upfront payment 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. 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. As a result of meeting certain milestones and sublicense fees related to this license agreement, the Company recognized expenses of $0.1 million, $0 million, and $0.9 million for the year ended December 31, 2014, 2013, and 2012, respectively. 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 of approximately one year each, 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 first and second option segments of the contract during which the Company may receive up to a total of $5.3 million and $17.0 million in expense reimbursement and fees, respectively. The first option segment is expected to end on March 31, 2015 and the second option segment is scheduled to end on November 30, 2015. As of December 31, 2014, the Company has recognized revenue in aggregate of $36.7 million with respect to the base performance segment and first and second extension periods. ContraVir Pharmaceuticals On December 17, 2014, the Company entered into a license agreement with ContraVir Pharmaceuticals (NASDAQ: CTRV) for the development and commercialization of CMX157 for certain antiviral indications. Under the terms of the agreement, ContraVir has sole responsibility with respect to the control of the development and commercialization of CMX157. In exchange for the license to CMX157 rights, the Company received an upfront payment consisting of 120,000 shares of ContraVir Series B Convertible Preferred Stock with a stated value of $1.2 million. In addition, the Company is eligible to receive up to approximately $20 million in clinical, regulatory and initial commercial milestones in the United States and Europe, as well as royalties and additional milestones based on commercial sales in those territories. Either party may terminate the license agreement upon the occurrence of a material breach by the other party (subject to standard cure periods), or upon certain events involving the bankruptcy or insolvency of the other party. ContraVir may also terminate the license agreement without cause on a country-by-country basis upon sixty (60) days’ prior written notice. The upfront payment of 120,000 shares of ContraVir Series B Convertible Preferred Stock was valued at $1.5 million at the time of the agreement. The Company recorded this amount as a long-term investment and deferred revenue, which is included in accrued liabilities in the Consolidated Balance Sheets. Upon completion of the transfer of the IND and technical know-how related to CMX157, the Company will recognize the upfront payment as revenue. As of December 31, 2014, this transfer had not been completed. As of December 31, 2014, the fair value of the investment was $1.8 million.

Selected Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2014

Selected Quarterly Financial Information [Abstract]

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 2014 and 2013 are as follows (in thousands, except per share data):



	2014 Quarters
	Fourth			Third			Second			First
Total revenues	$	1,156		$	1,185		$	919		$	780
Operating loss	(20,226		)	(16,860		)	(11,596		)	(10,184		)
Net loss	(20,247		)	(16,951		)	(11,734		)	(10,380		)
Net loss per common share, basic and diluted	$	(0.52	)	$	(0.47	)	$	(0.39	)	$	(0.39	)
Weighted-average shares outstanding, basic and diluted	39,128,297			35,845,792			30,111,380			26,762,264



	2013 Quarters
	Fourth			Third			Second			First
Total revenues	$	879		$	912		$	808		$	1,771
Operating loss	(7,979		)	(6,436		)	(7,656		)	(6,548		)
Net loss	(8,174		)	(6,706		)	(12,459		)	(9,107		)
Net loss per common share, basic and diluted	$	(0.31	)	$	(0.26	)	$	(0.91	)	$	(22.58	)
Weighted-average shares outstanding, basic and diluted	26,416,787			25,866,109			23,067,201			1,534,016

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 common share is identical to basic net loss per common share for all quarters of 2014 and 2013.

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2014
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, 2014, and events which occurred subsequently but were not recognized in the financial statements.

The Business and Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2014

Accounting Policies [Abstract]

Basis of Presentation

Reclassifications

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 accounts.

Investments

Concentration of Credit Risk

Accounts Receivable

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.

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.

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, 2014
	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	$	125,606	$	125,606	$	—	$	—
Certificates of deposit	480		—		480		—
Total cash equivalents	126,086		125,606		480		—
Short-term investments
Certificates of deposit	16,981		—		16,981		—
Corporate bonds	69,892		—		69,892		—
Commercial paper	11,240		—		11,240		—
U.S. Treasury securities	8,000		8,000		—		—
Total short-term investments	106,114		8,000		98,114		—
Long-term investments
Certificates of deposit	10,996		—		10,996		—
U.S. Treasury securities	40,197		40,197		—		—
Preferred stock of U.S. corporation	1,781		—		—		1,781
Total long-term investments	52,973		40,197		10,996		1,781
Total assets	$	285,173	$	173,803	$	109,590	$	1,781



			Fair Value Measurements
			December 31, 2013
	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	$	107,349	$	107,349	$	—	$	—
Total cash equivalents	$	107,349	$	107,349	$	—	$	—



	Fair Value Measurements (Level 3)
Redeemable convertible preferred stock warrant liability:
Fair value at January 1, 2012	$	6,491
Issuance	174
Fair value increase recorded in other expense	847
Fair value at December 31, 2012	7,512
Fair value increase recorded in other expense	6,590
Reclassification of warrant liability to additional paid-in capital	(14,102		)
Fair value at December 31, 2013	$	—

Preferred stock of U.S. corporation:
Fair value at January 1, 2014	$	—
Investment acquired	1,545
Fair value increase recorded in other comprehensive loss	236
Fair value at December 31, 2014	$	1,781

Property and Equipment

Impairment of Long-Lived Assets

Deferred Lease Obligation

Redeemable Convertible Preferred Stock Warrant Liability

Redeemable Convertible Preferred Stock

Revenue Recognition

Research and Development Prepaids and Accruals

Research and Development Expenses

Interest Income (Expense), Net

Income Taxes

Share-Based Compensation

Basic and Dilutive Net Loss Per Share of Common Stock

Segments

Impact of Recently Issued Accounting Standards

The Business and Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2014

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, 2014
	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	$	125,606	$	125,606	$	—	$	—
Certificates of deposit	480		—		480		—
Total cash equivalents	126,086		125,606		480		—
Short-term investments
Certificates of deposit	16,981		—		16,981		—
Corporate bonds	69,892		—		69,892		—
Commercial paper	11,240		—		11,240		—
U.S. Treasury securities	8,000		8,000		—		—
Total short-term investments	106,114		8,000		98,114		—
Long-term investments
Certificates of deposit	10,996		—		10,996		—
U.S. Treasury securities	40,197		40,197		—		—
Preferred stock of U.S. corporation	1,781		—		—		1,781
Total long-term investments	52,973		40,197		10,996		1,781
Total assets	$	285,173	$	173,803	$	109,590	$	1,781



			Fair Value Measurements
			December 31, 2013
	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	$	107,349	$	107,349	$	—	$	—
Total cash equivalents	$	107,349	$	107,349	$	—	$	—

Reconciliation of Liabilities Measured at Fair Value on a Recurring Basis



	Fair Value Measurements (Level 3)
Redeemable convertible preferred stock warrant liability:
Fair value at January 1, 2012	$	6,491
Issuance	174
Fair value increase recorded in other expense	847
Fair value at December 31, 2012	7,512
Fair value increase recorded in other expense	6,590
Reclassification of warrant liability to additional paid-in capital	(14,102		)
Fair value at December 31, 2013	$	—

Preferred stock of U.S. corporation:
Fair value at January 1, 2014	$	—
Investment acquired	1,545
Fair value increase recorded in other comprehensive loss	236
Fair value at December 31, 2014	$	1,781

Schedule of Prepaid and Other Current Assets

Prepaid expenses and other current assets consist of the following (in thousands):



	December 31,
	2014		2013
Prepaid research and development expenses	$	1,685	$	2,433
Interest receivable	553		—
Prepaid insurance	342		176
Other prepaid expenses and current assets	195		176
Total prepaid expenses and other current assets	$	2,775	$	2,785

Schedule of Accrued Liabilities

Accrued liabilities consist of the following (in thousands):



	December 31,
	2014		2013
Accrued compensation	$	3,057	$	1,779
Deferred revenue	1,545		—
Accrued research and development expenses	1,116		350
Other accrued liabilities	1,115		291
Total accrued liabilities	$	6,833	$	2,420

Investments (Tables)

12 Months Ended

Dec. 31, 2014

Investments [Abstract]

Summary of Available-for-Sale Securities

The following table summarizes the Company's short-term and long-term investments as of December 31, 2014 (in thousands):



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Corporate bonds	$	69,947	$	—	$	(56	)	$	69,891
Certificates of deposit	28,039		1		(62		)	27,978
U.S. Treasury securities	48,279		—		(82		)	48,197
Commercial paper	11,242		—		(2		)	11,240
Preferred stock of U.S. corporation	$	1,545	$	236	$	—		$	1,781
Total investments	$	159,052	$	237	$	(202	)	$	159,087

Summary of the scheduled maturity of Company investments

The following table summarizes the scheduled maturity for the Company's investments at December 31, 2014 (in thousands):



	December 31, 2014
Maturing in one year or less	$	106,114
Maturing after one year through two years	52,973
Total investments	$	159,087

- 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

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2014

Property, Plant and Equipment [Abstract]

Property and Equipment

Property and equipment, net of accumulated depreciation consists of the following (in thousands):



	December 31,
	2014			2013
Lab equipment	$	1,049		$	873
Leasehold improvements	777			78
Computer equipment	512			324
Office furniture and equipment	343			222
Property and equipment	2,681			1,497
Less accumulated depreciation	(1,371		)	(1,159		)
Property and equipment, net of accumulated depreciation	$	1,310		$	338

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2014

Commitments and Contingencies Disclosure [Abstract]

Schedule of Future Minimum Rental Payments for Operating Leases

The Company has the following minimum rental payments under noncancelable operating lease obligations that existed at December 31, 2014 (in thousands):



Years Ending December 31,	Minimal Rental Payment
2015	$	541
2016	511
2017	516
2018	128
Total future minimum rental payments	$	1,696

Stockholders' Equity (Deficit) (Tables)

12 Months Ended

Dec. 31, 2014

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,
	2014	2013
For exercise of common stock warrants	726,601	1,337,845
For exercise of outstanding common stock options	1,859,970	1,946,823
For future equity awards under the 2013 Equity Incentive Plan	1,147,526	1,681,932
For future purchases under the 2013 Employee Stock Purchase Plan	944,599	704,225
Total shares of common stock reserved for future issuances	4,678,696	5,670,825

Schedule of Fair Value Assumptions

The following table illustrates the assumptions for the Black-Scholes model used in determining the fair value of the stock options granted:



	Employees
	Years Ended December 31,
	2014			2013			2012
Expected volatility	71.81		%	76.40		%	80.55		%
Expected term (in years)	6.0			6.1			6.0
Weighted-average risk-free interest rate	1.92		%	1.49		%	0.86		%
Expected dividend yield	—		%	—		%	—		%
Weighted-average fair value per option	$	14.00		$	2.52		$	1.93



	Non-Employees
	Years Ended December 31,
	2014			2013			2012
Expected volatility	68.63		%	79.69		%	81.77		%
Expected term (in years)	5.7			6.2			5.8
Weighted-average risk-free interest rate	1.87		%	1.30		%	0.78		%
Expected dividend yield	—		%	—		%	—		%
Weighted-average fair value per option	$	14.13		$	5.01		$	3.48

Summary of Activity Related to Stock Options

A summary of activity related to the Company’s stock options is as follows:



	Number of Options Outstanding		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Life (in Years)
Balance, January 1, 2013	2,593,423		$	2.45	7.36
Granted	292,906		10.62		—
Exercised	(801,049	)	2.43		—
Expired or Canceled	(2,816	)	1.57		—
Forfeited	(135,641	)	4.55		—
Balance, December 31, 2013	1,946,823		$	3.54	7.03
Granted	1,385,370		21.57		—
Exercised	(1,287,883	)	3.57		—
Expired or Canceled	—		—		—
Forfeited	(184,340	)	15.30		—
Balance, December 31, 2014	1,859,970		$	15.79	8.46
Exercisable at December 31, 2014	589,141		$	9.03	7.17
Vested or expected to vest at December 31, 2014	1,816,056		$	15.73	8.45

Schedule of Share-based Compensation, Shares Authorized



	Outstanding				Exercisable
Range	Number	Weighted-Average Remaining Contractual Life (in years)	Weighted-Average Exercise Price		Number	Weighted-Average Exercise Price
$1.42 to 3.71	395,871	6.0	2.35		355,017	2.35
3.72 to 15.96	368,487	8.4	9.78		49,372	9.84
15.97 to 18.93	423,572	9.1	18.67		88,988	18.74
18.94 to 24.00	362,990	9.3	22.48		82,514	22.68
24.01 to 36.75	309,050	9.8	28.34		13,250	34.88
$1.42 to 36.75	1,859,970	8.5	$	15.79	589,141	$	9.03

Fair Value of Option Vested and Share-Based Compensation Expense

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,
	2014			2013
Expected volatility	74.24		%	51.22		%
Expected term (in years)	0.8			1.3
Weighted-average risk-free interest rate	0.09		%	0.25		%
Expected dividend yield	—		%	—		%
Weighted-average option value per share	$	9.93		$	4.20

Summary of Activity Related to Restricted Stock Units

A summary of activity related to the Company’s RSUs is as follows:



	Number of Restricted Stock Units Outstanding
Balance, December 31, 2012	43,199
Granted	59,348
Share issuance	(102,547	)
Balance, December 31, 2013	—

Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs



	Years Ended December 31,
	2014		2013		2012
Income Statement Classification:
Research and development expense:
Employee	$	1,085	$	1,905	$	336
Non-employee	—		67		80
General and administrative expense:
Employee	3,326		1,017		921
Non-employee	—		82		59
Total stock-based compensation expense	$	4,411	$	3,071	$	1,396

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2014

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



	2014					2013					2012
	Amount			% of Pretax Earnings		Amount			% of Pretax Earnings		Amount			% of Pretax Earnings
Income tax benefit at statutory rate	$	(20,166	)	34.0	%	$	(12,392	)	34.0	%	$	(1,499	)	34.0	%
State income taxes	(1,343		)	2.3	%	(852		)	2.3	%	(100		)	2.3	%
Research and development credits	(2,577		)	4.3	%	(2,265.0		)	6.2	%	—			—	%
Permanent items	1,525			(2.6	)%	3,060			(8.4	)%	868			(19.7	)%
Provision to return adjustments	64			(0.1	)%	320			(0.9	)%	2			—	%
Effect of change in state tax rate	2			—	%	(1		)	—	%	609			(13.9	)%
Increase in unrecognized tax benefits	425			(0.7	)%	566.0			(1.5	)%	—			—	%
Change in valuation allowance	22,070			(37.2	)%	11,564			(31.7	)%	120			(2.7	)%
Net benefit	$	—		—	%	$	—		—	%	$	—		—	%

Components of Deferred Tax Assets and Liabilities

The components of deferred tax assets and liabilities at December 31, 2014 and 2013 are as follows (in thousands):



	December 31,
	2014			2013
Deferred tax assets:
Net operating loss carryforwards	$	59,312		$	40,721
Research and development expenses	405			127
Capitalized Section 174 expenses	57			69
Research and development credits	4,572			2,640
Accrued bonuses	1,066			405
Share-based compensation	1,045			267
Other	135			230
Total gross deferred tax assets	66,592			44,459
Valuation allowance	(66,475		)	(44,405		)
Total deferred tax assets	117			54
Deferred tax liabilities:
Other	(117		)	(54		)
Total deferred tax liabilities	(117		)	(54		)
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 year ended December 31, 2014, as follows (in thousands):



Balance at December 31, 2012	$	207
Increases related to 2013	197
Increases related to prior periods	177
Balance at December 31, 2013	581
Increases related to 2014	425
Increases related to prior periods	—
Balance at December 31, 2014	$	1,006

Selected Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2014

Selected Quarterly Financial Information [Abstract]

Schedule of Quarterly Financial Information



	2014 Quarters
	Fourth			Third			Second			First
Total revenues	$	1,156		$	1,185		$	919		$	780
Operating loss	(20,226		)	(16,860		)	(11,596		)	(10,184		)
Net loss	(20,247		)	(16,951		)	(11,734		)	(10,380		)
Net loss per common share, basic and diluted	$	(0.52	)	$	(0.47	)	$	(0.39	)	$	(0.39	)
Weighted-average shares outstanding, basic and diluted	39,128,297			35,845,792			30,111,380			26,762,264



	2013 Quarters
	Fourth			Third			Second			First
Total revenues	$	879		$	912		$	808		$	1,771
Operating loss	(7,979		)	(6,436		)	(7,656		)	(6,548		)
Net loss	(8,174		)	(6,706		)	(12,459		)	(9,107		)
Net loss per common share, basic and diluted	$	(0.31	)	$	(0.26	)	$	(0.91	)	$	(22.58	)
Weighted-average shares outstanding, basic and diluted	26,416,787			25,866,109			23,067,201			1,534,016

The Business and Summary of Significant Accounting Policies (Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	$ 159,087
Total investments	159,087
Fair value, measurements, recurring
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	126,086	$ 107,349
Estimated Fair Value	106,114
Total investments	52,973
Assets, fair value	285,173
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 and Cash Equivalents, Fair Value Disclosure	125,606	107,349
Estimated Fair Value	8,000
Total investments	40,197
Assets, fair value	173,803
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 and Cash Equivalents, Fair Value Disclosure	480	0
Estimated Fair Value	98,114
Total investments	10,996
Assets, fair value	109,590
Fair value, measurements, recurring \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Estimated Fair Value	0
Total investments	1,781
Assets, fair value	1,781
Fair value, measurements, recurring \| Money market funds
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	125,606	107,349
Fair value, measurements, recurring \| Money market funds \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	125,606	107,349
Fair value, measurements, recurring \| Money market funds \| Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Fair value, measurements, recurring \| Money market funds \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0	$ 0
Fair value, measurements, recurring \| Certificates of deposit
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	480
Estimated Fair Value	16,981
Total investments	10,996
Fair value, measurements, recurring \| Certificates of deposit \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0
Estimated Fair Value	0
Total investments	0
Fair value, measurements, recurring \| Certificates of deposit \| Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	480
Estimated Fair Value	16,981
Total investments	10,996
Fair value, measurements, recurring \| Certificates of deposit \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	0
Estimated Fair Value	0
Total investments	0
Fair value, measurements, recurring \| Corporate bonds
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	69,892
Fair value, measurements, recurring \| Corporate bonds \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	0
Fair value, measurements, recurring \| Corporate bonds \| Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	69,892
Fair value, measurements, recurring \| Corporate bonds \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	0
Fair value, measurements, recurring \| Commercial paper
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	11,240
Fair value, measurements, recurring \| Commercial paper \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	0
Fair value, measurements, recurring \| Commercial paper \| Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	11,240
Fair value, measurements, recurring \| Commercial paper \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	0
Fair value, measurements, recurring \| U.S. Treasury securities
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	8,000
Total investments	40,197
Fair value, measurements, recurring \| U.S. Treasury securities \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	8,000
Total investments	40,197
Fair value, measurements, recurring \| U.S. Treasury securities \| Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	0
Total investments	0
Fair value, measurements, recurring \| U.S. Treasury securities \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Estimated Fair Value	0
Total investments	0
Fair value, measurements, recurring \| Preferred stock of U.S. corporation
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total investments	1,781
Fair value, measurements, recurring \| Preferred 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]
Total investments	0
Fair value, measurements, recurring \| Preferred stock of U.S. corporation \| Significant Other Observable Inputs (Level 2)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total investments	0
Fair value, measurements, recurring \| Preferred stock of U.S. corporation \| Significant Unobservable Inputs (Level 3)
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Total investments	$ 1,781

The Business and Summary of Significant Accounting Policies (Reconciliation of Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Issuances	1,545
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	236
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	1,781	$ 0
Significant Unobservable Inputs (Level 3)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning of period	$ 0	7,512	$ 6,491
Issuance			174
Fair value increase recorded in other expense		6,590	847
Reclassification of warrant liability to additional paid-in capital		(14,102)
Ending period		$ 0	$ 7,512

The Business and Summary of Significant Accounting Policies (Schedule of Prepaid and Other Current Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Accounting Policies [Abstract]
Prepaid research and development expenses	$ 1,685	$ 2,433
Interest receivable	553	0
Prepaid insurance	342	176
Other prepaid expenses and current assets	195	176
Total prepaid expenses and other current assets	$ 2,775	$ 2,785

The Business and Summary of Significant Accounting Policies (Schedule of Accrued Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Accounting Policies [Abstract]
Accrued compensation	$ 3,057	$ 1,779
Deferred revenue	1,545	0
Accrued research and development expenses	1,116	350
Other accrued liabilities	1,115	291
Total accrued liabilities	$ 6,833	$ 2,420

The Business and Summary of Significant Accounting Policies (Narrative) (Details)				12 Months Ended
	Dec. 17, 2014 $ / shares	Apr. 10, 2013 $ / shares	Mar. 25, 2013	Dec. 31, 2014 USD ($) segment $ / shares shares	Dec. 31, 2013 USD ($) $ / shares shares	Dec. 31, 2012 $ / shares shares	May 27, 2014 $ / shares
Class of Stock [Line Items]
Share Price					$ 14.00
Stockholders' Equity, Reverse Stock Split			the Company’s board of directors approved and implemented a 3.55-for-1 reverse stock split of the Company’s outstanding common stock
Fair Value of Financial Instruments
Volatility				68.63%	79.69%	81.77%
Expected term				5 years 8 months 12 days	6 years 2 months 12 days	5 years 9 months 18 days
Risk free interest rate				1.87%	1.30%	0.78%
Dividend yield				0.00%	0.00%	0.00%
Fair Value Assumptions, Exercise Price	$ 1.79			$ 2.22
Fair Value Assumptions, Expected Dividend Rate	0.00%			0.00%
Fair Value Assumptions, Expected Term	1 year 6 months			1 year 6 months
Fair Value Assumptions, Risk Free Interest Rate	0.43%			0.44%
Fair Value Assumptions, Expected Volatility Rate	66.00%			65.00%
Property and Equipment
Property and equipment, useful life, leasehold improvements				amortized over the shorter of the useful life of the asset or the term of the related lease
Impairment of long-lived assets \| $				$ 0
Per share information:
Anti-dilutive securities excluded from the calculation loss per share \| shares				2,170,660	7,550,950	15,694,862
Segments
Number of operating segments \| segment				1
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities \| $				$ 0	$ 0
Reverse stock split, conversion ratio			0.2816901408000	0.2816901408000
Maximum
Property and Equipment
Property and equipment, useful life				5 years
Minimum
Property and Equipment
Property and equipment, useful life				3 years
Warrant
Fair Value of Financial Instruments
Volatility		79.00%				79.00%
Expected term		2 years 6 months
Risk free interest rate		0.24%
Dividend yield		0.00%
Warrant \| Maximum
Fair Value of Financial Instruments
Expected term						5 years
Expected liquidity						31.00%
Warrant \| Minimum
Fair Value of Financial Instruments
Expected term						8 months
Expected liquidity						2.00%
Series F
Fair Value of Financial Instruments
Fair value		$ 3.94
Series F \| Maximum
Fair Value of Financial Instruments
Fair value						$ 2.85
Series F \| Minimum
Fair Value of Financial Instruments
Fair value						$ 2.19
Secondary offering
Class of Stock [Line Items]
Share Price							$ 14,220

Investments - Summary of Available-for-Sale Securities (Details) $ in Thousands	Dec. 31, 2014 USD ($)
Schedule of Investments [Line Items]
Amortized Cost	$ 159,052
Gross Unrealized Gains	237
Gross Unrealized Losses	(202)
Estimated Fair Value	159,087
Corporate bonds
Schedule of Investments [Line Items]
Amortized Cost	69,947
Gross Unrealized Gains	0
Gross Unrealized Losses	(56)
Estimated Fair Value	69,891
Certificates of deposit
Schedule of Investments [Line Items]
Amortized Cost	28,039
Gross Unrealized Gains	1
Gross Unrealized Losses	(62)
Estimated Fair Value	27,978
U.S. Treasury securities
Schedule of Investments [Line Items]
Amortized Cost	48,279
Gross Unrealized Gains	0
Gross Unrealized Losses	(82)
Estimated Fair Value	48,197
Commercial paper
Schedule of Investments [Line Items]
Amortized Cost	11,242
Gross Unrealized Gains	0
Gross Unrealized Losses	(2)
Estimated Fair Value	11,240
Preferred stock of U.S. corporation
Schedule of Investments [Line Items]
Amortized Cost	1,545
Gross Unrealized Gains	236
Gross Unrealized Losses	0
Estimated Fair Value	$ 1,781

Investments - Available for Sale Securities (Details) $ in Thousands	Dec. 31, 2014 USD ($)
Investments [Abstract]
Maturing in one year or less	$ 106,114
Maturing after one year through two years	52,973
Total investments	$ 159,087

Property and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 2,681	$ 1,497
Less accumulated depreciation	(1,371)	(1,159)
Property and equipment, net	1,310	338
Lab equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	1,049	873
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	777	78
Computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	512	324
Office furniture and equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 343	$ 222

Loan Payable (Narrative) (Details) - USD ($)		1 Months Ended		12 Months Ended
Loan Payable (Narrative) (Details) - USD ($)	Jan. 27, 2012	Sep. 30, 2012	Jan. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 10, 2013
Line of Credit Facility [Line Items]
Loan and security agreement, maximum capacity	$ 15,000,000.0
Loan payment				$ 4,400,000
Warrant strike price							$ 7.26
Warrants to purchase common stock							41,323
First tranche
Line of Credit Facility [Line Items]
Loan and security agreement, outstanding	3,000,000.0
Loan payment	2,600,000
Deferred financing costs	$ 100,000
Interest only period	12 months
Principal and interest amortization period	30 months
Loan and security agreement, interest rate	8.25%
Second tranche
Line of Credit Facility [Line Items]
Loan and security agreement, remaining capacity	$ 12,000,000.0	$ 12,000,000
Interest only period		6 months
Principal and interest amortization period		32 months
Loan and security agreement
Line of Credit Facility [Line Items]
Interest expense				$ 100,000	$ 100,000	$ 100,000
Series F \| First tranche
Line of Credit Facility [Line Items]
Warrants to purchase common stock	29,340
Series F \| Second tranche
Line of Credit Facility [Line Items]
Warrants to purchase common stock		117,360
Silicon Valley Bank \| Series F
Line of Credit Facility [Line Items]
Warrant strike price	$ 2.045		$ 2.045
Aggregate amount of advances, percentage	2.00%		2.00%

Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commitments and Contingencies Disclosure [Abstract]
Operating leases, rent expense	$ 0.4	$ 0.5	$ 0.4

Commitments and Contingencies (Schedule of Future Minimum Rental Payments for Operating Leases) (Details) $ in Thousands	Dec. 31, 2014 USD ($)
Commitments and Contingencies Disclosure [Abstract]
2015	$ 541
2016	511
2017	516
2018	128
Operating leases, future minimum payments due, total	$ 1,696

Redeemable Convertible Preferred Stock (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended		12 Months Ended
	Apr. 10, 2013	Jan. 27, 2012	Jan. 31, 2012	Feb. 28, 2011	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Temporary Equity [Line Items]
Redeemable convertible preferred stock issued					0	0
Redeemable convertible preferred stock, warrants issued	1,613,395
Warrants exercise price	$ 7.26
Fair value adjustment of warrants					$ 0	$ 6,590	$ 847
Warrants, issued					726,601	1,337,845
Warrants, outstanding					726,601	1,337,845
Warrants, exercisable					726,601	1,337,845
Series F
Temporary Equity [Line Items]
Redeemable convertible preferred stock issued				22,004,895
Redeemable convertible preferred stock issued, par value per share				$ 0.001
Redemption price per share (in USD per share)				$ 2.045
Redeemable convertible preferred stock, warrants issued				5,501,215
Proceeds from issuance of redeemable preferred stock				$ 45,000
Issuance of common stock, issuance costs				$ 200
Aggregate number of common shares converted from redeemable convertible preferred stock	15,556,091
Series F \| Silicon Valley Bank
Temporary Equity [Line Items]
Redeemable convertible preferred stock, warrants issued			146,700
Warrants exercise price		$ 2.045	$ 2.045
Aggregate amount of advances, percentage		2.00%	2.00%
Common Stock
Temporary Equity [Line Items]
Redeemable convertible preferred stock, warrants issued				1,549,628
Warrant \| Silicon Valley Bank
Temporary Equity [Line Items]
Redeemable convertible preferred stock, warrants issued	41,323
Warrants exercise price			$ 7.26
Initial public offering completion \| Common Stock
Temporary Equity [Line Items]
Warrants exercise price				$ 7.26

Stockholders' Equity (Deficit) (Schedule of Shares Reserved for Future Issuance) (Details) - shares	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	4,678,696	5,670,825
The 2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	1,147,526	1,681,932
The 2013 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	944,599	704,225
Convertible common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	726,601	1,337,845
Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance	1,859,970	1,946,823

Stockholders' Equity (Deficit) (Assumption Used to Determine the Fair Value of Options Granted) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility	68.63%	79.69%	81.77%
Expected term	5 years 8 months 12 days	6 years 2 months 12 days	5 years 9 months 18 days
Risk free interest rate	1.87%	1.30%	0.78%
Dividend yield	0.00%	0.00%	0.00%
Weighted-average fair value per option (in USD per share)	$ 14.13	$ 5.01	$ 3.48
Employee stock option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility	71.81%	76.40%	80.55%
Expected term	6 years	6 years 1 month 6 days	6 years
Risk free interest rate	1.92%	1.49%	0.86%
Dividend yield	0.00%	0.00%	0.00%
Weighted-average fair value per option (in USD per share)	$ 14.00	$ 2.52	$ 1.93

Stockholders' Equity (Deficit) (Summary of Activity Related to Stock Options) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of options outstanding, Beginning balance	1,946,823	2,593,423
Number of options outstanding, Granted	1,385,370	292,906
Number of options outstanding, Exercised	(1,287,883)	(801,049)
Number of options outstanding, Expired or Canceled	0	(2,816)
Number of options outstanding, Forfeited	(184,340)	(135,641)
Number of options outstanding, Ending balance	1,859,970	1,946,823	2,593,423
Number of options outstanding, Exercisable	589,141
Number of options outstanding, Vested and expected to vest	1,816,056
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Weighted-average exercise price, Beginning balance (in USD per share)	$ 3.54	$ 2.45
Weighted-average exercise price, Granted (in USD per share)	21.57	10.62
Weighted-average exercise price, Exercised (in USD per share)	3.57	2.43
Weighted-average exercise price, Expired or Canceled (in USD per share)	0.00	1.57
Weighted-average exercise price, Forfeited (in USD per share)	15.30	4.55
Weighted-average exercise price, Ending balance (in USD per share)	15.79	$ 3.54	$ 2.45
Weighted-average exercise price, Exercisable (in USD per share)	9.03
Weighted-average exercise price, Vested or expected to vest (in USD per share)	$ 15.73
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	8 years 5 months 16 days	7 years 11 days	7 years 4 months 10 days
SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2	8 years 5 months 16 days	7 years 11 days	7 years 4 months 10 days
Weighted-average remaining contractual life (in years), Exercisable	7 years 2 months 1 day
Weighted-average remaining contractual life (in years), Vested or expected to vest	8 years 5 months 12 days

Stockholders' Equity (Deficit) Stockholders' Equity (Deficit) (Schedule of Share-based Compensation, Shares Authorized) (Details)	12 Months Ended
	Dec. 31, 2014 $ / shares shares
Exercise price range, $1.42 to 3.71
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options \| shares	395,871
Share-based compensation, outstanding options, weighted average remaining contractual term	6 years 15 days
Share-based compensation, outstanding options, weighted average exercise price (in USD per share) \| $ / shares	$ 2.35
Share-based compensation, number of exercisable options \| shares	355,017
Share-based compensation, exercisable options, weighted average exercise price (in USD per share) \| $ / shares	$ 2.35
Exercise price range, $3.72 to 15.96
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options \| shares	368,487
Share-based compensation, outstanding options, weighted average remaining contractual term	8 years 5 months 9 days
Share-based compensation, outstanding options, weighted average exercise price (in USD per share) \| $ / shares	$ 9.78
Share-based compensation, number of exercisable options \| shares	49,372
Share-based compensation, exercisable options, weighted average exercise price (in USD per share) \| $ / shares	$ 9.84
Exercise price range, $15.97 to 18.93
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options \| shares	423,572
Share-based compensation, outstanding options, weighted average remaining contractual term	9 years 22 days
Share-based compensation, outstanding options, weighted average exercise price (in USD per share) \| $ / shares	$ 18.67
Share-based compensation, number of exercisable options \| shares	88,988
Share-based compensation, exercisable options, weighted average exercise price (in USD per share) \| $ / shares	$ 18.74
Exercise price range, $8.94 to 24.00
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options \| shares	362,990
Share-based compensation, outstanding options, weighted average remaining contractual term	9 years 3 months 26 days
Share-based compensation, outstanding options, weighted average exercise price (in USD per share) \| $ / shares	$ 22.48
Share-based compensation, number of exercisable options \| shares	82,514
Share-based compensation, exercisable options, weighted average exercise price (in USD per share) \| $ / shares	$ 22.68
Exercise price range, $24.00 to 36.75
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options \| shares	309,050
Share-based compensation, outstanding options, weighted average remaining contractual term	9 years 9 months
Share-based compensation, outstanding options, weighted average exercise price (in USD per share) \| $ / shares	$ 28.34
Share-based compensation, number of exercisable options \| shares	13,250
Share-based compensation, exercisable options, weighted average exercise price (in USD per share) \| $ / shares	$ 34.88
Exercise price range, $1.42 to 36.75
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, number of outstanding options \| shares	1,859,970
Share-based compensation, outstanding options, weighted average remaining contractual term	8 years 5 months 16 days
Share-based compensation, outstanding options, weighted average exercise price (in USD per share) \| $ / shares	$ 15.79
Share-based compensation, number of exercisable options \| shares	589,141
Share-based compensation, exercisable options, weighted average exercise price (in USD per share) \| $ / shares	$ 9.03

Stockholders' Equity (Deficit) Stockholders' Equity (Deficit) (Schedule of Share-Based Compensation) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, fair value assumptions, expected volatility rate	68.63%	79.69%	81.77%
Share-based Compensation, fair value assumptions, expected term	5 years 8 months 12 days	6 years 2 months 12 days	5 years 9 months 18 days
Share-based Compensation, fair value assumptions, risk free interest rate	1.87%	1.30%	0.78%
Share-based Compensation, fair value assumptions, expected dividend rate	0.00%	0.00%	0.00%
Share-based Compensation, weighted average grant date fair value (in USD per share)	$ 14.13	$ 5.01	$ 3.48
Stock compensation plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, fair value assumptions, expected volatility rate	74.24%	51.22%
Share-based Compensation, fair value assumptions, expected term	9 months 18 days	1 year 3 months
Share-based Compensation, fair value assumptions, risk free interest rate	0.09%	0.25%
Share-based Compensation, fair value assumptions, expected dividend rate	0.00%	0.00%
Share-based Compensation, weighted average grant date fair value (in USD per share)	$ 9.93	$ 4.20

Stockholders' Equity (Deficit) (Summary of Activity Related to Restricted Stock Units) (Details) - Restricted stock units (RSUs)	12 Months Ended
	Dec. 31, 2013 shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Balance, December 31, 2012	43,199
Granted	59,348
Shares issuance	(102,547)
Balance, December 31, 2013	0

Stockholders' Equity (Deficit) (Schedule of Allocation of Recognized Period Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 4,411	$ 3,071	$ 1,396
Research and development expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	0	67	80
Research and development expense \| Stock compensation plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	1,085	1,905	336
General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	0	82	59
General and administrative expense \| Stock compensation plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense	$ 3,326	$ 1,017	$ 921

Stockholders' Equity (Deficit) (Narrative) (Details) - USD ($)									1 Months Ended			3 Months Ended			12 Months Ended
	Nov. 05, 2014	Jun. 20, 2014	May 27, 2014	Apr. 09, 2014	Jan. 01, 2014	Oct. 23, 2013	Apr. 16, 2013	Apr. 10, 2013	Apr. 30, 2013	Feb. 28, 2013	Feb. 28, 2011	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 10, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares authorized															200,000,000	200,000,000
Common stock, shares issued															41,031,770	26,664,972
Common stock, shares outstanding															41,031,770	26,664,972	1,533,996
Proceeds from initial public offering															$ 225,939,000	$ 107,634,000	$ 0
Warrants to purchase common stock								41,323
Stock issued during period, value																$ 107,634,000
Number of shares outstanding under the plan															1,859,970	1,946,823	2,593,423
Aggregate intrinsic value of options outstanding															$ 18,400,000
Aggregate intrinsic value of options exercisable																$ 20,800,000
Intrinsic value of options exercised															30,400,000	12,400,000	$ 18,000
Share-based compensation, award vesting period				15 months
Tax benefit from stock options exercised															0		0
Compensation expense, allocated															$ 4,411,000	$ 3,071,000	1,396,000
Common stock, capital shares reserved for future issuance															4,678,696	5,670,825
Employees' contribution liabilities, ESPP															$ 296,000
Restricted stock expense															1,400,000
Share-based compensation, cash received from exercise of stock options												$ 5,000,000	$ 1,900,000	$ 14,000
The 2012 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized to be granted								244,717
The 2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized to be granted								1,408,450
Share-based compensation, equity increase		4.00%							2.50%
Share-based compensation, nonvested awards, compensation not yet recognized, stock options															$ 13,900,000
Unrecognized compensation costs, period of recognition															3 years 1 month 21 days
Common stock, capital shares reserved for future issuance															1,147,526	1,681,932
The 2013 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized to be granted					970,874					704,225					944,599
Increase in number of shares authorized to be granted					266,649
Number of shares authorized, increase amount										422,535
Percentage of total number of shares of capital stock outstanding										1.00%
Common stock, capital shares reserved for future issuance															944,599	704,225
Share-based compensation, maximum employee subscription rate															15.00%
Share-based compensation, shares issued in period															26,275
Modified grants \| Three individuals
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional compensation expense																	30,000
Modified grants \| One individual
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Additional compensation expense																	$ 300,000
Increase in compensation expense															$ 1,000,000
Maximum \| The 2013 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized to be granted															2,816,901
Maximum \| The 2013 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Discount from market price, entry date															85.00%
Discount from market price, purchase date															85.00%
Over-allotment option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New issues of stock during period	547,500						1,098,000
Offering price per share	$ 29.00						$ 14.00
Public offering underwriting discounts and commissions							$ 1,100,000
Stock issued during period, value							$ 14,300,000
Secondary offering
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New issues of stock during period	4,197,500					2,476,995
Series F
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issuance costs											$ 200,000
Shares converted in stock conversion								15,556,091
Warrant
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Warrants to purchase common stock								1,613,395
Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Offering price per share			$ 14.22
Proceeds from initial public offering	$ 114,000,000
Stock issued during period, value																$ 23,000
Common stock, capital shares reserved for future issuance															1,859,970	1,946,823
Over-allotment option \| Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New issues of stock during period			1,095,000
IPO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Offering price per share																		$ 14.00
Proceeds from initial public offering								$ 93,300,000
Public offering underwriting discounts and commissions								7,100,000
Stock issuance costs								$ 2,100,000
IPO \| Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
New issues of stock during period			8,395,000					7,320,000
Proceeds from initial public offering			$ 111,800,000
Common stock \| The 2013 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, capital shares reserved for future issuance															970,874

Stockholders' Equity (Deficit) Stockholders' Equity (Deficit) (Phantom) (Details)		12 Months Ended
	Mar. 25, 2013	Dec. 31, 2014 $ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reverse stock split, conversion ratio	0.2816901408000	0.2816901408000
Exercise price range, $1.42 to 3.71 \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit		$ 1.42
Exercise price range, $1.42 to 3.71 \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit		3.71
Exercise price range, $3.72 to 15.96 \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit		3.72
Exercise price range, $3.72 to 15.96 \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit		15.96
Exercise price range, $15.97 to 18.93 \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit		15.97
Exercise price range, $15.97 to 18.93 \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit		18.93
Exercise price range, $8.94 to 24.00 \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit		18.94
Exercise price range, $8.94 to 24.00 \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit		24.00
Exercise price range, $24.00 to 36.75 \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit		24.01
Exercise price range, $24.00 to 36.75 \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit		36.75
Exercise price range, $1.42 to 36.75 \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, lower range limit		1.42
Exercise price range, $1.42 to 36.75 \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, upper range limit		$ 36.75

Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
Income Taxes (Narrative) (Details) - USD ($)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2007	Dec. 31, 2002	Dec. 31, 2012
Operating Loss Carryforwards [Line Items]
Loss limitation ownership change benchmark amount			$ 762,000	$ 64,000
Unrecognized tax benefits, gross	$ 1,006,000	$ 581,000			$ 207,000
Annual limitation of operating loss carryforwards	0.50	6,700,000
Employee stock option
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	31,300,000
Domestic tax authority
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	192,600,000	117,500,000
Tax credit carryforward	6,100,000
State and local jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 156,900,000	$ 106,000,000

Income Taxes (Reconciliation of the Difference between the Benefit for Income Taxes and Income Taxes at the Statutory U.S. Federal Income Tax Rate) (Alternate) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income tax benefit at statutory rate	$ (20,166,000)	$ (12,392,000)	$ (1,499,000)
State income taxes	(1,343,000)	(852,000)	(100,000)
Research and development credits	(2,577,000)	(2,265,000)	0
Effective Income Tax Reconciliation Tax Permanent Items	1,525,000	3,060,000	868,000
Provision to return adjustments	64,000	320,000	2,000
Effect of change in state tax rate	2,000	(1,000)	609,000
Increase in unrecognized tax benefits	425,000	566,000	0
Change in valuation allowance	22,070,000	11,564,000	120,000
Net income tax benefit	$ 0	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax benefit at statutory rate, tax rate	34.00%	34.00%	34.00%
State income taxes, tax rate	2.30%	2.30%	2.30%
Research and development credits, tax rate	4.30%	6.20%	0.00%
Effective Income Tax Rate Reconciliation, Deduction, Percent	(2.60%)	(8.40%)	(19.70%)
Provision to return adjustments, tax rate	(0.10%)	(0.90%)	0.00%
Effect of change in state tax rate, tax rate	0.00%	0.00%	(13.90%)
Increase in unrecognized tax benefits, tax rate	(0.70%)	(1.50%)	0.00%
Valuation allowance, tax rate	(37.20%)	(31.70%)	(2.70%)
Net income tax benefit, tax rate	0.00%	0.00%	0.00%

Income Taxes (Components of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2014	Dec. 31, 2013
Deferred Tax Assets, Gross [Abstract]
Net operating loss carryforwards	$ 59,312	$ 40,721
Research and development expenses	405	127
Capitalized Section 174 expenses	57	69
Research and development credits	4,572	2,640
Accrued bonuses	1,066	405
Share-based compensation	1,045	267
Other	135	230
Total gross deferred tax assets	66,592	44,459
Valuation allowance	(66,475)	(44,405)
Deferred tax assets, net of valuation allowance, total	117	54
Deferred Tax Liabilities, Gross [Abstract]
Other	(117)	(54)
Total deferred tax liabilities	(117)	(54)
Net deferred tax assets	$ 0	$ 0

Income Taxes (Summary of Uncertain Tax Benefit) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 581	$ 207
Increase related to current year tax positions	425	197
Increase related to prior periods	0	177
Ending balance	$ 1,006	$ 581

Significant Agreements (Details) $ in Thousands		1 Months Ended	12 Months Ended
Significant Agreements (Details) $ in Thousands	Dec. 17, 2014 USD ($) shares	May 31, 2002 USD ($) shares	Dec. 31, 2014 USD ($) contract	Dec. 31, 2013 USD ($) shares	Dec. 31, 2012 USD ($)
Long-term Purchase Commitment [Line Items]
Shares issued in consideration for rights granted under license agreement \| shares				8,418,000
Contract revenue			$ 4,040	$ 4,370	$ 16,275
Preferred stock value			0	0
Deferred revenue			$ 1,545	0
Regents of University of California
Long-term Purchase Commitment [Line Items]
Shares issued in consideration for rights granted under license agreement \| shares		64,788
Sublicense fee percentage		50.00%
Royalties percentage			50.00%
Expenses recognized			$ 100	$ 0	$ 875
Research and development arrangement
Long-term Purchase Commitment [Line Items]
Other segments, period			1 year
Maximum \| CMX157
Long-term Purchase Commitment [Line Items]
Milestone payments		$ 3,400
Maximum \| Research and development arrangement
Long-term Purchase Commitment [Line Items]
Number of extension periods \| contract			4
Reimbursement revenue			$ 75,800
Contract revenue			5,300
BARDA
Long-term Purchase Commitment [Line Items]
Revenue recognized			36,700
BARDA \| First option segment
Long-term Purchase Commitment [Line Items]
Reimbursement revenue			5,300
BARDA \| Second option segment
Long-term Purchase Commitment [Line Items]
Reimbursement revenue			17,000
Royalty arrangement \| ContraVir Pharmaceuticals
Long-term Purchase Commitment [Line Items]
Deferred revenue	$ 20,000
Preferred stock, ContraVir Series B \| ContraVir Pharmaceuticals
Long-term Purchase Commitment [Line Items]
Shares received as consideration for license agreement \| shares	120,000
Value of shares received as consideration for license agreement	$ 1,500
Preferred stock value	$ 1,200
Deferred revenue			$ 1,800

Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 1,156	$ 1,185	$ 919	$ 780	$ 879	$ 912	$ 808	$ 1,771	$ 4,040	$ 4,370	$ 33,720
Operating loss	(20,226)	(16,860)	(11,596)	(10,184)	(7,979)	(6,436)	(7,656)	(6,548)	(58,866)	(28,619)	(2,783)
Net loss	$ (20,247)	$ (16,951)	$ (11,734)	$ (10,380)	$ (8,174)	$ (6,706)	$ (12,459)	$ (9,107)	$ (59,312)	$ (36,445)	$ (4,406)
Net loss, basic and diluted (in USD per share)	$ (0.52)	$ (0.47)	$ (0.39)	$ (0.39)	$ (0.31)	$ (0.26)	$ (0.91)	$ (22.58)	$ (1.80)	$ (3.65)	$ (5.75)
Weighted-average shares outstanding, basic and diluted	39,128,297	35,845,792	30,111,380	26,762,264	26,416,787	25,866,109	23,067,201	1,534,016	33,003,714	19,307,422	1,524,628