Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating expenses:
Research and development	$ 39,773	$ 39,479	$ 21,787
General and administrative	12,347	10,863	9,615
Total operating expenses	52,120	50,342	31,402
Loss from operations	(52,120)	(50,342)	(31,402)
Other income (expense):
Interest income	58	19	5
Other income (expense), net	39	56	(15)
Revaluation of future purchase rights liabilities	0	2,600	(1,306)
Total other income (expense)	97	2,675	(1,316)
Net loss	(52,023)	(47,667)	(32,718)
Amortization of deemed dividend	0	(4,744)	(64)
Accretion to redemption value of redeemable convertible preferred stock	0	(4,410)	(6,303)
Net loss attributable to common stockholders	$ (52,023)	$ (56,821)	$ (39,085)
Net loss per share attributable to common stockholders, basic and diluted (usd per share)	$ (2.07)	$ (3.54)	$ (74.86)
Weighted-average common shares outstanding, basic and diluted (in shares)	25,152,948	16,054,452	522,102