Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Collaboration and license revenue									$ 33,558	$ 11,442	$ 41,971
Operating expenses:
Research and development, net									68,774	48,136	36,215
General and administrative									38,435	21,973	16,289
Total operating expenses	$ 30,281	$ 26,867	$ 26,130	$ 23,931	$ 13,700	$ 20,245	$ 18,771	$ 17,393	107,209	70,109	52,504
Loss from operations									(73,651)	(58,667)	(10,533)
Interest and other income, net									5,953	1,657	153
Net loss									(67,698)	(57,010)	(10,380)
Other comprehensive income (loss)									125	(200)	8
Comprehensive loss									$ (67,573)	$ (57,210)	$ (10,372)
Net loss per share, basic and diluted	$ (0.39)	$ (0.39)	$ (0.49)	$ (0.50)	$ (0.28)	$ (0.50)	$ (0.50)	$ (0.47)	$ (1.76)	$ (1.74)	$ (0.38)
Weighted average number of shares used to compute net loss per share, basic and diluted	40,259,575	40,116,644	37,440,024	35,827,235	35,684,201	33,525,567	31,200,773	31,089,310	38,386,906	32,832,514	27,475,792