Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Collaboration and license revenue	$ 39,199	$ 14,199	$ 5,916
Operating expenses:
Research and development	36,215	28,393	18,296
General and administrative	16,289	9,019	4,838
Total operating expenses	52,504	37,412	23,134
Loss from operations	(13,305)	(23,213)	(17,218)
Interest and other income, net	153	(47)	2
Change in fair value of redeemable convertible preferred stock call option liability		314	387
Net loss	(13,152)	(22,946)	(16,829)
Other comprehensive income	8
Comprehensive loss	(13,144)	(22,946)	(16,829)
Cumulative dividend relating to redeemable convertible preferred stock		(5,151)	(2,864)
Accretion of redeemable convertible preferred stock to redemption value		(98)	(158)
Net loss attributable to common stockholders	$ (13,152)	$ (28,195)	$ (19,851)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.48)	$ (4.48)	$ (11.30)
Weighted average number of shares used to compute net loss per share attributable to common stockholders, basic and diluted	27,475,792	6,292,800	1,756,900