Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue:
Grant revenue	$ 371		$ 917
Total revenue	9,055	$ 23,015	17,638	$ 29,044
Operating expenses:
Research and development expenses	23,675	22,210	71,188	65,413
General and administrative expenses	7,591	8,119	25,063	25,251
Total operating expenses	31,266	30,329	96,251	90,664
Loss from operations	(22,211)	(7,314)	(78,613)	(61,620)
Other income (expense):
Interest income (expense), net	262	379	958	1,193
Total other income (expense), net	262	379	958	1,193
Net loss	$ (21,949)	$ (6,935)	$ (77,655)	$ (60,427)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.54)	$ (0.17)	$ (1.91)	$ (1.49)
Weighted average common shares outstanding, basic and diluted	40,806,413	40,494,049	40,699,422	40,419,522
Other comprehensive income:
Unrealized gain on investments, net of tax of $0	$ 20	$ 77	$ 137	$ 50
Total other comprehensive income	20	77	137	50
Comprehensive loss	(21,929)	(6,858)	(77,518)	(60,377)
Collaboration Revenue - Related Party [Member]
Revenue:
Collaboration revenue - related party	$ 8,684	$ 23,015	$ 16,721	$ 29,044