Statements of Operations and Comprehensive Loss (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Operating expenses
General and administrative	$ (8,023)us-gaap_GeneralAndAdministrativeExpense	$ (3,612)us-gaap_GeneralAndAdministrativeExpense
Research and development	(11,618)us-gaap_ResearchAndDevelopmentExpense	(5,370)us-gaap_ResearchAndDevelopmentExpense
Loss from operations	(19,641)us-gaap_OperatingIncomeLoss	(8,982)us-gaap_OperatingIncomeLoss
Other income (expense), net	2,402us-gaap_OtherNonoperatingIncomeExpense	2,311us-gaap_OtherNonoperatingIncomeExpense
Net loss	(17,239)us-gaap_NetIncomeLoss	(6,671)us-gaap_NetIncomeLoss
Net loss attributable to common stockholders—basic and diluted	(17,239)	(6,671)
Net loss per share attributable to common stockholders—basic and diluted	$ (0.70)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.28)us-gaap_EarningsPerShareBasicAndDiluted
Weighted average number of common shares outstanding—basic and diluted	24,602,668	23,717,393
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	49	(21)
Comprehensive loss	$ (17,190)us-gaap_ComprehensiveIncomeNetOfTax	$ (6,692)us-gaap_ComprehensiveIncomeNetOfTax

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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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Average number of shares or units issued and outstanding that are used in calculating basic and diluted earnings per share (EPS).

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