Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Collaboration and license revenue	$ 1,750	$ 1,250	$ 3,500	$ 2,500
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operation expenses:
Research and development	$ 24,999	$ 13,699	$ 40,553	$ 25,352
General and administrative	5,911	3,450	10,879	6,379
Total operating expenses	30,910	17,149	51,432	31,731
Loss from operations	(29,160)	(15,899)	(47,932)	(29,231)
Non-operating income (expense):
Interest and other income, net	1,482	1,288	3,016	1,891
Gain on deemed sale from equity method investee	0	1,229	0	1,229
Share of loss from equity method investee	(412)	(151)	(844)	(377)
Total non-operating income, net	1,070	2,366	2,172	2,743
Net loss	(28,090)	(13,533)	(45,760)	(26,488)
Other comprehensive income (loss)	84	14	220	(41)
Comprehensive loss	$ (28,006)	$ (13,519)	$ (45,540)	$ (26,529)
Net loss per share, basic and diluted	$ (0.64)	$ (0.32)	$ (1.05)	$ (1.01)
Weighted-average number of shares used to compute basic and diluted net loss per share	43,797,718	42,533,641	43,653,325	26,236,007