Statements of Operations and Comprehensive Loss (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Collaboration revenue	$ 2,243	$ 1,334
Operating expenses:
Research and development	26,703	13,788	15,263
General and administrative	8,471	4,024	4,213
Total operating expenses	35,174	17,812	19,476
Loss from operations	(32,931)	(16,478)	(19,476)
Other expense:
Other expense, net	(849)	(147)	(13)
Interest expense	(376)	(1,400)	(168)
Total other expense, net	(1,225)	(1,547)	(181)
Net loss and comprehensive loss	(34,156)	(18,025)	(19,657)
Cumulative preferred stock dividends and accretion of preferred stock discount	(519)	(3,857)	(3,111)
Net loss applicable to common stockholders	$ (34,675)	$ (21,882)	$ (22,768)
Net loss per share applicable to common stockholders-basic and diluted	$ (2.37)	$ (16.18)	$ (22.93)
Weighted-average number of common shares used in net loss per share applicable to common stockholders-basic and diluted	14,644	1,352	993