Consolidated Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue:
Collaboration revenue - related party	$ 27,188,000	$ 26,917,000	$ 32,100,000
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Grant revenue	$ 1,102,000	$ 1,350,000
Collaboration revenue	6,215,000
Total revenue	34,505,000	28,267,000	$ 32,100,000
Operating expenses:
Research and development expenses	80,141,000	95,955,000	89,455,000
General and administrative expenses	24,748,000	32,596,000	34,040,000
Restructuring expenses	1,492,000	0	0
Total operating expenses	106,381,000	128,551,000	123,495,000
Loss from operations	(71,876,000)	(100,284,000)	(91,395,000)
Other income (expense):
Interest income	1,033,000	1,172,000	1,590,000
Interest expense	(502,000)
Other income	1,066,000	170,000	425,000
Total other income (expense), net	1,597,000	1,342,000	2,015,000
Net loss	$ (70,279,000)	$ (98,942,000)	$ (89,380,000)
Net loss per share attributable to common stockholders, basic and diluted	$ (1.24)	$ (2.43)	$ (2.21)
Weighted average common shares outstanding, basic and diluted	56,649,220	40,743,492	40,449,410
Other comprehensive income (loss):
Unrealized gain (loss) on investments, net of tax of $0		$ 146,000	$ 3,000
Total other comprehensive income (loss)		146,000	3,000
Comprehensive loss	$ (70,279,000)	$ (98,796,000)	$ (89,377,000)