Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Collaboration and license revenue	$ 3,550	$ 3,550	$ 10,649	$ 10,649
Operating expenses
Research and development	8,783	6,672	26,192	20,017
General and administrative	4,031	2,300	11,947	5,961
Total operating expenses	12,814	8,972	38,139	25,978
Loss from operations	(9,264)	(5,422)	(27,490)	(15,329)
Interest and other income, net	33	(40)	79	(22)
Change in fair value of redeemable convertible preferred stock call option liability				314
Net loss and comprehensive loss	(9,231)	(5,462)	(27,411)	(15,037)
Cumulative dividend relating to redeemable convertible preferred Stock		(1,900)		(4,530)
Accretion of redeemable convertible preferred stock to redemption value		(26)		(88)
Net loss attributable to common stockholders	$ (9,231)	$ (7,388)	$ (27,411)	$ (19,655)
Net loss per share attributable to common stockholders, basic and Diluted	$ (0.35)	$ (2.78)	$ (1.04)	$ (7.98)
Weighted-average number of common shares used to compute net loss per share, basic and diluted	26,470,298	2,654,365	26,331,852	2,463,876