Redeemable Convertible Preferred Stock and Stockholders' Equity - Accretion of Redeemable Convertible Preferred Stock (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock	$ 0	$ (110,921)	$ 0	$ (135,690)
Series A-1
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(3,085)		(3,785)
Series A-2
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(77)		(95)
Series A-3
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(6,478)		(7,843)
Series B
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(25,601)		(31,052)
Series C
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(23,523)		(28,531)
Series D
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(20,131)		(24,417)
Series E
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(8,130)		(9,936)
Series F
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		(12,522)		(15,643)
Series G
Temporary Equity [Line Items]
Accretion of redeemable convertible preferred stock		$ (11,374)		$ (14,388)