Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Collaboration revenue	$ 7,677	$ 5,917	$ 15,146	$ 12,132
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Operating expenses:
Research and development	$ 23,467	$ 15,565	$ 45,960	$ 28,996
General and administrative	7,805	6,369	15,211	12,101
Total operating expenses	31,272	21,934	61,171	41,097
Operating loss	(23,595)	(16,017)	(46,025)	(28,965)
Interest income	1,376	424	2,450	741
Net loss	$ (22,219)	$ (15,593)	$ (43,575)	$ (28,224)
Net loss per share, basic and diluted	$ (0.52)	$ (0.45)	$ (1.03)	$ (0.81)
Weighted average shares outstanding, basic and diluted	42,836	34,916	42,441	34,820