Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue:
Collaboration revenue - related party	$ 4,840,000	$ 8,684,000	$ 21,909,000	$ 16,721,000
Type of Revenue [Extensible List]	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Grant revenue	$ 85,000	$ 371,000	$ 791,000	$ 917,000
Collaboration revenue	2,106,000		4,183,000
Total revenue	7,031,000	9,055,000	26,883,000	17,638,000
Operating expenses:
Research and development expenses	18,317,000	23,675,000	59,109,000	71,188,000
General and administrative expenses	5,897,000	7,591,000	18,966,000	25,063,000
Restructuring expenses	0		1,492,000
Total operating expenses	24,214,000	31,266,000	79,567,000	96,251,000
Loss from operations	(17,183,000)	(22,211,000)	(52,684,000)	(78,613,000)
Other income (expense):
Interest income (expense), net	335,000	262,000	744,000	958,000
Other income	439,000		439,000
Total other income (expense), net	774,000	262,000	1,183,000	958,000
Net loss	$ (16,409,000)	$ (21,949,000)	$ (51,501,000)	$ (77,655,000)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.23)	$ (0.54)	$ (0.99)	$ (1.91)
Weighted average common shares outstanding, basic and diluted	69,944,068	40,806,413	52,143,492	40,699,422
Net loss	$ (16,409,000)	$ (21,949,000)	$ (51,501,000)	$ (77,655,000)
Other comprehensive income:
Unrealized gain on investments, net of tax of $0	7,000	20,000	7,000	137,000
Total other comprehensive income	7,000	20,000	7,000	137,000
Comprehensive loss	$ (16,402,000)	$ (21,929,000)	$ (51,494,000)	$ (77,518,000)