Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Collaboration and license revenue	$ 4,291	$ 163	$ 6,791	$ 163
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operation expenses:
Research and development	$ 12,859	$ 21,435	$ 38,210	$ 35,072
General and administrative	3,577	1,862	9,956	5,188
Total operating expenses	16,436	23,297	48,166	40,260
Loss from operations	(12,145)	(23,134)	(41,375)	(40,097)
Interest and other income, net	1,333	12	4,076	225
Net loss	(10,812)	(23,122)	(37,299)	(39,872)
Other comprehensive income (loss)	(25)	14	(66)	17
Comprehensive loss	$ (10,837)	$ (23,108)	$ (37,365)	$ (39,855)
Net loss per share, basic and diluted	$ (0.25)	$ (11.86)	$ (1.16)	$ (23.47)
Weighted-average number of shares used to compute basic and diluted net loss per share	42,838,098	1,949,258	32,056,675	1,699,045