Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development	$ 53,249	$ 41,078	$ 30,157
General and administrative	13,743	14,339	12,462
Total operating expenses	66,992	55,417	42,619
Loss from operations	(66,992)	(55,417)	(42,619)
Other income (expense), net:
Changes in fair value of warrant liabilities	(20,926)
Other income (expense), net	(517)	1,780	760
Total other income (expense)	(21,443)	1,780	760
Loss before provision for (benefit from) income taxes, net	(88,435)	(53,637)	(41,859)
Provision for (benefit from) income taxes, net	(160)	(302)	156
Net loss and comprehensive loss	$ (88,275)	$ (53,335)	$ (42,015)
Net loss per common share, basic and diluted	$ (30.30)	$ (30.16)	$ (33.68)
Weighted-average shares used in computing net loss per common share, basic and diluted	2,913,487	1,768,480	1,247,482