Summary of Significant Accounting Policies - Consolidated Statements of Operations and Comprehensive Losses (Details) - USD ($) $ / shares in Units, $ 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
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration and license revenue	$ 12,400	$ 9,188	$ 6,639	$ 5,331	$ 2,954	$ 3,077	$ 3,001	$ 2,410		$ 11,442	$ 41,971
Operating expenses:
Research and development									$ 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, gain (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
As Originally Reported | Accounting Standards Update 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration and license revenue										$ 22,500	$ 39,199
Operating expenses:
Research and development										48,136	36,215
General and administrative										21,973	16,289
Total operating expenses										70,109	52,504
Loss from operations										(47,609)	(13,305)
Interest and other income, net										1,657	153
Net loss										(45,952)	(13,152)
Other comprehensive, gain (loss)										(200)	8
Comprehensive loss										$ (46,152)	$ (13,144)
Net loss per share, basic and diluted										$ (1.40)	$ (0.48)
Weighted average number of shares used to compute net loss per share, basic and diluted										32,832,514	27,475,792
Effect of Change | Accounting Standards Update 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration and license revenue										$ (11,058)	$ 2,772
Operating expenses:
Loss from operations										(11,058)	2,772
Net loss										(11,058)	2,772
Comprehensive loss										$ (11,058)	$ 2,772
Net loss per share, basic and diluted										$ (0.34)	$ 0.10