Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Operating expenses:
Research and development	$ 9,646	$ 10,244	$ 32,945	$ 28,589
General and administrative	2,689	2,566	9,286	7,736
Total operating expenses	12,335	12,810	42,231	36,325
Loss from operations	(12,335)	(12,810)	(42,231)	(36,325)
Other income (expense):
Interest income	13	5	36	13
Other income (expense), net	22	7	31	0
Revaluation of future purchase rights liabilities	0	0	0	2,600
Total other income (expense)	35	12	67	2,613
Net loss	(12,300)	(12,798)	(42,164)	(33,712)
Amortization of deemed dividend	0	0	0	(4,744)
Accretion to redemption value of redeemable convertible preferred stock	0	0	0	(4,410)
Net loss attributable to common stockholders	$ (12,300)	$ (12,798)	$ (42,164)	$ (42,866)
Net loss per share attributable to common stockholders, basic and diluted (usd per share)	$ (0.51)	$ (0.59)	$ (1.76)	$ (3.18)
Weighted-average common shares outstanding, basic and diluted (in shares)	24,025,481	21,759,061	23,998,396	13,483,813