Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Grant and collaboration revenue	$ 0	$ 10
Operating expenses:
Research and development	14,724	7,353
General and administrative	4,098	4,513
Total operating expenses	18,822	11,866
Loss from operations	(18,822)	(11,856)
Investment income	240	277
Foreign currency transaction (loss), net	82	(30)
Interest expense	(273)	(396)
Change in fair value of warrant liabilities	(846)	0
Other (expense), net	(1)	(69)
Net loss	(19,620)	(12,074)
Stock‑based compensation expense	1,409	1,180
Other comprehensive loss:
Foreign currency translation adjustment	(60)	22
Unrealized gain on securities	0	2
Total comprehensive loss	$ (19,680)	$ (12,050)
Net loss per share:
Net loss per share attributable to common stockholders—basic and diluted (in dollars per share)	$ (0.21)	$ (0.31)
Weighted average common shares outstanding:
Basic and diluted (in shares)	94,723,513	38,447,319