Fair Value Measurements - Schedule of Redeemable Convertible Preferred Stock Warrant Liability Measured and Recorded at Fair Value (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Redeemable Convertible Preferred Stock Warrant Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance as of December 31, 2018	$ 1,618
Addition	0
Change in fair value	1,517
Conversion of redeemable convertible preferred stock warrants into Class B common stock warrants	(3,135)
Balance as of September 30, 2019	0
Level III
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Redeemable convertible preferred stock warrant liability, before reclassification	3,135
Reclassification to stockholders' equity (deficit)	(3,135)
Redeemable convertible preferred stock warrant liability	0	$ 1,618
Long-Term Liability [Member] | Level III
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Redeemable convertible preferred stock warrant liability, before reclassification	3,135
Reclassification to stockholders' equity (deficit)	(3,135)
Redeemable convertible preferred stock warrant liability	$ 0	$ 1,618