Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Collaboration and license revenue	$ 1,750	$ 4,291	$ 5,250	$ 6,791
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operation expenses:
Research and development	$ 17,241	$ 12,859	$ 57,795	$ 38,210
General and administrative	7,758	3,577	18,637	9,956
Total operating expenses	24,999	16,436	76,432	48,166
Loss from operations	(23,249)	(12,145)	(71,182)	(41,375)
Non-operating income (expense):
Interest and other income, net	1,254	1,519	4,272	3,410
Gain on deemed sale from equity method investee	0	0	0	1,229
Share of loss from equity method investee	(357)	(186)	(1,202)	(563)
Total non-operating income, net	897	1,333	3,070	4,076
Net loss	(22,352)	(10,812)	(68,112)	(37,299)
Other comprehensive income (loss)	(59)	(25)	160	(66)
Comprehensive loss	$ (22,411)	$ (10,837)	$ (67,952)	$ (37,365)
Net loss per share, basic and diluted	$ (0.51)	$ (0.25)	$ (1.56)	$ (1.16)
Weighted-average number of shares used to compute basic and diluted net loss per share	43,939,281	42,838,098	43,750,154	32,056,675