Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue:
Collaboration revenue - related party	$ 26,917	$ 32,100	$ 21,766
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Grant revenue	$ 1,350
Total revenue	28,267	$ 32,100	$ 21,766
Operating expenses:
Research and development expenses	95,955	89,455	81,989
General and administrative expenses	32,596	34,040	32,616
Total operating expenses	128,551	123,495	114,605
Loss from operations	(100,284)	(91,395)	(92,839)
Other income (expense):
Interest income (expense), net	1,172	1,590	1,260
Other income	170	425
Total other income (expense), net	1,342	2,015	1,260
Net loss	$ (98,942)	$ (89,380)	$ (91,579)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.43)	$ (2.21)	$ (2.30)
Weighted average common shares outstanding, basic and diluted	40,743,492	40,449,410	39,846,928
Other comprehensive income (loss):
Unrealized gain (loss) on investments, net of tax of $0	$ 146	$ 3	$ (179)
Total other comprehensive income (loss)	146	3	(179)
Comprehensive loss	$ (98,796)	$ (89,377)	$ (91,758)