Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Collaboration and license revenue	$ 1,750	$ 1,750
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 23,142	$ 15,554
General and administrative	7,008	4,969
Total operating expenses	30,150	20,523
Loss from operations	(28,400)	(18,773)
Non-operating income (expense):
Interest and other income, net	647	1,534
Gain on deemed sale from equity method investee	482	0
Share of loss from equity method investee	(482)	(431)
Total non-operating income, net	647	1,103
Net loss	(27,753)	(17,670)
Other comprehensive income	224	136
Comprehensive loss	$ (27,529)	$ (17,534)
Net loss per share, basic and diluted	$ (0.63)	$ (0.41)
Weighted-average number of shares used to compute basic and diluted net loss per share	44,282,607	43,508,592