Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Collaboration and license revenue	$ 8,353	$ 1,413
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operation expenses:
Research and development	$ 49,646	$ 47,218	$ 14,247
General and administrative	13,566	7,636	3,935
Total operating expenses	63,212	54,854	18,182
Loss from operations	(54,859)	(53,441)	(18,182)
Non-operating income (expense):
Interest and other income (expense), net	4,922	775	212
Gain on deemed sale from equity method investee	1,229	0	0
Share of loss from equity method investee	(886)	(416)	0
Total non-operating income, net	5,265	359	212
Net loss	(49,594)	(53,082)	(17,970)
Other comprehensive loss	(65)	(16)	(26)
Comprehensive loss	$ (49,659)	$ (53,098)	$ (17,996)
Net loss per share, basic and diluted	$ (1.43)	$ (29.03)	$ (20.80)
Weighted-average number of shares used to compute basic and diluted net loss per share	34,618,237	1,828,262	863,983