Convertible Notes, Convertible Preferred Stock and Common and Preferred Stock Warrants - Summary of Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Shares authorized (in shares)	80,461,552	80,461,552
Shares issued (in shares)	0	71,740,162
Shares outstanding (in shares)	0	71,740,162
Carrying value at conversion	$ 0	$ 1,465,841
Class B common stock
Temporary Equity [Line Items]
Shares outstanding (in shares)	71,740,162
Convertible preferred stock
Temporary Equity [Line Items]
Shares authorized (in shares)	80,461,552
Shares issued (in shares)	71,740,162
Carrying value at conversion	$ 1,465,841
Series A preferred
Temporary Equity [Line Items]
Shares authorized (in shares)	9,374,101
Shares issued (in shares)	9,374,101
Shares outstanding (in shares)	9,374,101
Carrying value at conversion	$ 8,956
Series B preferred
Temporary Equity [Line Items]
Shares authorized (in shares)	7,868,854
Shares issued (in shares)	7,868,856
Shares outstanding (in shares)	7,868,854
Carrying value at conversion	$ 11,941
Series C preferred
Temporary Equity [Line Items]
Shares authorized (in shares)	5,979,062
Shares issued (in shares)	5,979,069
Shares outstanding (in shares)	5,979,062
Carrying value at conversion	$ 44,928
Series D preferred
Temporary Equity [Line Items]
Shares authorized (in shares)	6,443,818
Shares issued (in shares)	6,443,831
Shares outstanding (in shares)	6,443,818
Carrying value at conversion	$ 102,648
Series E preferred
Temporary Equity [Line Items]
Shares authorized (in shares)	9,486,362
Shares issued (in shares)	9,486,398
Shares outstanding (in shares)	9,486,362
Carrying value at conversion	$ 198,264
Series F preferred
Temporary Equity [Line Items]
Shares authorized (in shares)	14,597,248
Shares issued (in shares)	13,885,893
Shares outstanding (in shares)	13,885,893
Carrying value at conversion	$ 376,962
Series G preferred
Temporary Equity [Line Items]
Shares authorized (in shares)	26,712,107
Shares issued (in shares)	18,702,014
Shares outstanding (in shares)	18,702,072
Carrying value at conversion	$ 722,142