Condensed 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
Operating expenses:
Research and development	$ 25,302	$ 16,014	$ 7,998
General and administrative	10,922us-gaap_GeneralAndAdministrativeExpense	6,745us-gaap_GeneralAndAdministrativeExpense	2,206us-gaap_GeneralAndAdministrativeExpense
Total operating expenses	36,224us-gaap_OperatingExpenses	22,759us-gaap_OperatingExpenses	10,204us-gaap_OperatingExpenses
Loss from operations	(36,224)us-gaap_OperatingIncomeLoss	(22,759)us-gaap_OperatingIncomeLoss	(10,204)us-gaap_OperatingIncomeLoss
Interest expense.	(270)us-gaap_InterestExpense	(936)us-gaap_InterestExpense	(1,486)us-gaap_InterestExpense
Change in fair value of warrant liability		(2,587)us-gaap_FairValueAdjustmentOfWarrants	32us-gaap_FairValueAdjustmentOfWarrants
Other income (expense), net	119us-gaap_OtherNonoperatingIncomeExpense	194us-gaap_OtherNonoperatingIncomeExpense	(84)us-gaap_OtherNonoperatingIncomeExpense
Net loss	(36,375)us-gaap_NetIncomeLoss	(26,088)us-gaap_NetIncomeLoss	(11,742)us-gaap_NetIncomeLoss
Net loss per common share (basic and diluted) (in dollars per share)	$ (2.22)us-gaap_EarningsPerShareBasicAndDiluted	$ (3.31)us-gaap_EarningsPerShareBasicAndDiluted	$ (36.31)us-gaap_EarningsPerShareBasicAndDiluted
Weighted-average shares outstanding (basic and diluted)	16,374,102	7,885,921	323,382
Other comprehensive loss:
Unrealized loss on investments	(56)	(3)
Total comprehensive loss	$ (36,431)us-gaap_ComprehensiveIncomeNetOfTax	$ (26,091)us-gaap_ComprehensiveIncomeNetOfTax	$ (11,742)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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