Condensed Consolidated Statements of Operations and Comprehensive Loss (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Grant revenues			$ 71	$ 367
Type of Revenue Extensible List			us-gaap:GrantMember	us-gaap:GrantMember
Operating expenses:
Research and development	$ 12,607	$ 10,285	$ 26,469	$ 17,540
General and administrative	4,322	3,060	8,313	6,216
Total operating expenses	16,929	13,345	34,782	23,756
Loss from operations	(16,929)	(13,345)	(34,711)	(23,389)
Other income (expense):
Interest income	260	960	816	1,970
Other income (expense), net	178	(42)	44	(24)
Total other income (expense), net	438	918	860	1,946
Net loss	(16,491)	(12,427)	(33,851)	(21,443)
Other comprehensive income (loss):
Unrealized gain (loss) on investment securities	(40)	89	(21)	173
Comprehensive loss	$ (16,531)	$ (12,338)	$ (33,872)	$ (21,270)
Net loss per share:
Net loss per share – basic and diluted	$ (0.53)	$ (0.51)	$ (1.21)	$ (0.89)
Weighted average shares outstanding – basic and diluted	31,409	24,161	27,948	24,128