Fair Value Measurement - Activity for Preferred Stock Warrant Liability and Derivative Liability Measured At Fair Value Using Significant Unobservable Level 3 Inputs (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 3,028
Changes in fair value reflected as change in fair value of warrant liability	676
Extinguishment of derivative upon conversion of convertible promissory notes	3,704
Ending balance	0
Preferred Stock Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1,656
Issuance of warrants	2,822
Exercise of warrants	1,197
Changes in fair value reflected as change in fair value of warrant liability	(11)
Conversion to common stock warrants upon IPO	3,270
Ending balance	$ 0