Fair Value of Financial Assets and Liabilities -Aggregate Fair Value of Warrant and Derivative Liabilities (Details) - Level 3 - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Preferred Stock Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 4,947	$ 1,245
Issuance of warrants to purchase shares of Series B convertible preferred stock		304
Initial fair value of derivative liability in connection with the Hercules Loan Agreement		0
Change in fair value	288	3,398
Conversion of convertible preferred stock warrant into common stock warrant in connection with Merger	(5,235)
Ending balance	0	4,947
Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	201	94
Issuance of warrants to purchase shares of Series B convertible preferred stock		0
Initial fair value of derivative liability in connection with the Hercules Loan Agreement		18
Change in fair value	(183)	89
Conversion of convertible preferred stock warrant into common stock warrant in connection with Merger	0
Ending balance	$ 18	$ 201