Redeemable Convertible Preferred Stock and Stockholders' Equity - Accretion of Redeemable Convertible Preferred Stock (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	$ 0	$ (40,224)	$ 0	$ (175,915)
Series A-1
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(1,119)		(4,904)
Series A-2
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(28)		(123)
Series A-3
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(2,349)		(10,192)
Series B
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(9,284)		(40,336)
Series C
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(8,530)		(37,062)
Series D
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(7,300)		(31,717)
Series E
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(2,948)		(12,884)
Series F
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(4,541)		(20,184)
Series G
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		$ (4,125)		$ (18,513)