Redeemable Convertible Preferred Stock and Stockholders' Equity/(Deficit) - Accretion of Redeemable Convertible Preferred Stock (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	$ (175,915)	$ (55,502)	$ (102,224)
Series A-1
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	(4,904)	(1,618)	(2,674)
Series A-2
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	(123)	(40)	(67)
Series A-3
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	(10,192)	(3,360)	(5,650)
Series B
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	(40,336)	(13,416)	(22,561)
Series C
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	(37,062)	(12,333)	(20,390)
Series D
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	(31,717)	(10,555)	(17,450)
Series E
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	(12,884)	(4,086)	(7,353)
Series F
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	(20,184)	(6,025)	(12,494)
Series G
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	$ (18,513)	$ (4,069)	$ (13,585)