Selected Quarterly Financial Data (Unaudited) (USD $) In Thousands, except Share data, unless otherwise specified	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
Operating Expenses [Abstract]
Research and development	$ 6,200	$ 7,174	$ 6,528	$ 5,400	$ 7,338	$ 3,483	$ 3,100	$ 2,093	$ 25,302	$ 16,014	$ 7,998
General and administrative	3,180	2,526	2,726	2,490	2,398	1,924	1,172	1,251	10,922	6,745	2,206
Total operating expenses	9,380	9,700	9,254	7,890	9,736	5,407	4,272	3,344	36,224	22,759	10,204
Loss from operations	(9,380)	(9,700)	(9,254)	(7,890)	(9,736)	(5,407)	(4,272)	(3,344)	(36,224)	(22,759)	(10,204)
Interest expense.	(134)	(135)	(1)				(108)	(828)	(270)	(936)	(1,486)
Change in fair value of warrant liability							(2,545)	(42)		(2,587)	32
Other income (expense), net	57	29	17	16	46	169	4	(25)	119	194	(84)
Net loss	$ (9,457)	$ (9,806)	$ (9,238)	$ (7,874)	$ (9,690)	$ (5,238)	$ (6,921)	$ (4,239)	$ (36,375)	$ (26,088)	$ (11,742)
Net loss per common share (basic and diluted) (in dollars per share)	$ (0.49)	$ (0.64)	$ (0.60)	$ (0.51)	$ (0.63)	$ (0.34)	$ (19.82)	$ (12.24)	$ (2.22)	$ (3.31)	$ (36.31)
Weighted-average shares outstanding (basic and diluted) (in shares)	19,276,639	15,432,641	15,399,018	15,369,055	15,340,713	15,253,704	349,170	346,478	16,374,102	7,885,921	323,382