Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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	$ 16,869	$ 14,806	$ 31,431	$ 29,195
General and administrative	4,691	3,816	9,151	7,084
Total operating expenses	21,560	18,622	40,582	36,279
Loss from operations	(21,560)	(18,622)	(40,582)	(36,279)
Other income (expense):
Interest income	161	619	461	1,126
Other expense, net	(19)	(16)	(25)	(33)
Total other income	142	603	436	1,093
Net loss	$ (21,418)	$ (18,019)	$ (40,146)	$ (35,186)
Net loss per share, basic and diluted	$ (0.40)	$ (0.55)	$ (0.93)	$ (1.14)
Weighted-average common shares outstanding, basic and diluted	52,941,603	32,840,357	43,019,670	30,936,623