Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Collaboration revenue	$ 490	$ 2,243	$ 1,334
Other revenue	500	0	0
Total revenue	990	2,243	1,334
Operating expenses:
Research and development	26,336	26,703	13,788
General and administrative	9,850	8,471	4,024
Total operating expenses	36,186	35,174	17,812
Loss from operations	(35,196)	(32,931)	(16,478)
Other income (expense):
Other income (expense), net	3,139	(849)	(147)
Interest expense	(1,395)	(376)	(1,400)
Total other income (expense), net	1,744	(1,225)	(1,547)
Net loss and comprehensive loss	(33,452)	(34,156)	(18,025)
Cumulative preferred stock dividends and accretion of preferred stock discount	0	(519)	(3,857)
Net loss applicable to common stockholders	$ (33,452)	$ (34,675)	$ (21,882)
Net loss per share applicable to common stockholders—basic and diluted (in dollars per share)	$ (1.76)	$ (2.37)	$ (16.18)
Weighted-average number of common shares used in net loss per share applicable to common stockholders—basic and diluted	18,993	14,644	1,352