Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 6,677	$ 903	$ 207
Operating expenses:
Research and development	42,743	47,687	45,165
General and administrative	16,389	18,238	18,826
Total operating expenses	59,132	65,925	63,991
Loss from operations	(52,455)	(65,022)	(63,784)
Investment income	834	1,050	617
Loss on extinguishment of debt	0	0	(673)
Foreign currency transaction (loss), net	(47)	120	(123)
Interest expense	(1,519)	(1,494)	(1,206)
Change in fair value of warrant liabilities	(857)	0	0
Other (expense), net	(1,306)	10	(152)
Net loss	(55,350)	(65,336)	(65,321)
Other comprehensive loss:
Foreign currency translation adjustment	34	(153)	78
Unrealized gain on securities	0	16	20
Total comprehensive loss	$ (55,316)	$ (65,473)	$ (65,223)
Net loss per share:
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (1.22)	$ (2.92)	$ (3.20)
Weighted average common shares outstanding:
Basic and diluted (in shares)	45,548,511	22,389,286	20,425,050