Condensed Consolidated Statements of Operations and Comprehensive Loss (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Consolidated Statements of Comprehensive Loss
Contract revenue		$ 44us-gaap_ContractsRevenue	$ 6,098us-gaap_ContractsRevenue
Operating expenses:
Research and development	26,573us-gaap_ResearchAndDevelopmentExpense	32,640us-gaap_ResearchAndDevelopmentExpense	24,519us-gaap_ResearchAndDevelopmentExpense
General and administrative	10,145us-gaap_GeneralAndAdministrativeExpense	8,313us-gaap_GeneralAndAdministrativeExpense	5,061us-gaap_GeneralAndAdministrativeExpense
Total operating expenses	36,718us-gaap_OperatingExpenses	40,953us-gaap_OperatingExpenses	29,580us-gaap_OperatingExpenses
Loss from operations	(36,718)us-gaap_OperatingIncomeLoss	(40,909)us-gaap_OperatingIncomeLoss	(23,482)us-gaap_OperatingIncomeLoss
Other (expense) income:
Interest income	88us-gaap_InvestmentIncomeInterest	86us-gaap_InvestmentIncomeInterest	44us-gaap_InvestmentIncomeInterest
Interest expense	(1,214)us-gaap_InterestExpense	(1,086)us-gaap_InterestExpense	(184)us-gaap_InterestExpense
Other (expense) income, net	(154)us-gaap_OtherNonoperatingIncomeExpense	(39)us-gaap_OtherNonoperatingIncomeExpense	113us-gaap_OtherNonoperatingIncomeExpense
Net loss	(37,998)us-gaap_NetIncomeLoss	(41,948)us-gaap_NetIncomeLoss	(23,509)us-gaap_NetIncomeLoss
Other comprehensive (loss) income:
Net unrealized (loss) gain on marketable securities	(11)	(1)	5
Comprehensive loss	$ (38,009)us-gaap_ComprehensiveIncomeNetOfTax	$ (41,949)us-gaap_ComprehensiveIncomeNetOfTax	$ (23,504)us-gaap_ComprehensiveIncomeNetOfTax
Basic and diluted net loss per common share (in dollars per share)	$ (1.15)us-gaap_EarningsPerShareBasicAndDiluted	$ (1.73)us-gaap_EarningsPerShareBasicAndDiluted	$ (11.22)us-gaap_EarningsPerShareBasicAndDiluted
Weighted average common shares outstanding used to calculate basic and diluted net loss per common share (in shares)	32,979,288	24,270,407	2,095,950

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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