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 $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Derivative Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance at December 31, 2017	$ 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)
Preferred Stock Warrant Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Balance at December 31, 2017	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	2,709
Balance at September 30, 2018	$ 5,990