Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Income Statement [Abstract]
Collaboration and license revenue	$ 1,750	$ 1,250
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operation expenses:
Research and development	$ 15,554	$ 11,652
General and administrative	4,969	2,929
Total operating expenses	20,523	14,581
Loss from operations	(18,773)	(13,331)
Non-operating income (expense):
Interest and other income, net	1,534	603
Share of loss from equity method investee	(431)	(226)
Total non-operating income, net	1,103	377
Net loss	(17,670)	(12,954)
Other comprehensive income (loss)	136	(55)
Comprehensive loss	$ (17,534)	$ (13,009)
Net loss per share, basic and diluted	$ (0.41)	$ (1.37)
Weighted-average number of shares used to compute basic and diluted net loss per share	43,508,592	9,488,352