Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Type of revenue [extensible list]	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember	us-gaap:GrantMember
Operating expenses:
Research and development	$ 12,451	$ 17,839	$ 43,882	$ 47,034
General and administrative	5,672	4,307	14,823	11,391
Total operating expenses	18,123	22,146	58,705	58,425
Loss from operations	(18,123)	(22,146)	(58,705)	(58,425)
Other income (expense):
Interest income	88	585	549	1,711
Other income (expense), net	2	(12)	(23)	(45)
Total other income	90	573	526	1,666
Net loss	$ (18,033)	$ (21,573)	$ (58,179)	$ (56,759)
Net loss per share, basic and diluted	$ (0.29)	$ (0.66)	$ (1.18)	$ (1.80)
Weighted-average common shares outstanding, basic and diluted	62,240,412	32,894,205	49,473,350	31,596,321