Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Collaboration revenue	$ 300
Operating expenses:
Research and development	45,118	$ 53,249	$ 41,078
General and administrative	20,123	13,743	14,339
Total operating expenses	65,241	66,992	55,417
Loss from operations	(64,941)	(66,992)	(55,417)
Other income (expense), net:
Changes in fair value of warrant liabilities	(16,240)	(20,926)
Other income (expense), net	421	(517)	1,780
Total other income (expense), net	(15,819)	(21,443)	1,780
Loss before provision for (benefit from) income taxes, net	(80,760)	(88,435)	(53,637)
Provision for (benefit from) income taxes, net	142	(160)	(302)
Net loss and comprehensive loss	$ (80,902)	$ (88,275)	$ (53,335)
Net loss per common share, basic and diluted	$ (7.70)	$ (30.30)	$ (30.16)
Weighted-average shares used in computing net loss per common share, basic and diluted	10,506,739	2,913,487	1,768,480