Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Income Statement [Abstract]
Collaboration revenue	$ 107
Operating expenses:
Research and development	8,679	$ 5,671
General and administrative	3,341	2,152
Total operating expenses	12,020	7,823
Loss from operations	(11,913)	(7,823)
Other income (expense), net:
Interest expense, net	(308)	(1,245)
Change in fair value of preferred stock warrant liabilities		2,036
Net loss and comprehensive loss	$ (12,221)	$ (7,032)
Net loss per share, basic and diluted	$ (0.65)	$ (17.43)
Weighted-average common shares outstanding, basic and diluted	18,815,689	403,539