Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Collaboration revenue	$ 30,434	$ 26,117	$ 16,479
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 89,115	$ 67,647	$ 31,840
General and administrative	32,189	28,025	16,798
Total operating expenses	121,304	95,672	48,638
Operating loss	(90,870)	(69,555)	(32,159)
Interest income	5,527	2,012	525
Net loss	$ (85,343)	$ (67,543)	$ (31,634)
Net loss per share, basic and diluted	$ (1.98)	$ (1.88)	$ (1.42)
Weighted average shares outstanding, basic and diluted	43,069	36,006	22,222
Other comprehensive loss:
Unrealized loss on marketable securities	$ (28)
Comprehensive loss	$ (85,371)	$ (67,543)	$ (31,634)