Fair Value Measurement - Activity for Preferred Stock Warrant Liability and Derivative Liability Measured At Fair Value Using Significant Unobservable Level 3 Inputs (Details) - Level 3 - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Derivative Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 3,028
Issuance of warrants or derivative		$ 2,971
Changes in fair value reflected as change in fair value of liability	676	57
Extinguishment of derivative upon conversion of convertible promissory notes	(3,704)
Ending balance		3,028
Preferred Stock Warrant Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	1,656	880
Issuance of warrants or derivative	2,822	1,795
Exercise of warrants	(1,197)
Changes in fair value reflected as change in fair value of liability	(11)	(1,019)
Conversion to common stock warrants upon IPO	$ (3,270)
Ending balance		$ 1,656