Redeemable Convertible Preferred Stock and Common Stock - Schedule of Redeemable Convertible Preferred Stock (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 11, 2018	May 07, 2018	Dec. 31, 2017
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			509,352,795	509,352,795	448,686,791
Redeemable Convertible Preferred Shares Issued	0	0	508,539,515		448,686,791
Redeemable Convertible Preferred Shares Outstanding	0	0	508,539,515		448,686,791
Carrying Value			$ 1,833,561		$ 1,176,661
Liquidation Preference			$ 1,903,887		$ 1,209,940
Series A redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			42,000,000		42,000,000
Redeemable Convertible Preferred Shares Issued			42,000,000		42,000,000
Redeemable Convertible Preferred Shares Outstanding			42,000,000		42,000,000
Carrying Value			$ 182		$ 182
Liquidation Preference			$ 2,859		$ 2,701
Series B redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			122,296,280		122,296,280
Redeemable Convertible Preferred Shares Issued			122,296,280		122,296,280
Redeemable Convertible Preferred Shares Outstanding			122,296,280		122,296,280
Carrying Value			$ 770		$ 770
Liquidation Preference			$ 12,493		$ 11,801
Series C redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			85,669,774		85,669,774
Redeemable Convertible Preferred Shares Issued			85,669,774		85,669,774
Redeemable Convertible Preferred Shares Outstanding			85,669,774		85,669,774
Carrying Value			$ 36,238		$ 36,238
Liquidation Preference			$ 41,750		$ 39,676
Series D redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			63,291,156		63,291,156
Redeemable Convertible Preferred Shares Issued			63,021,976		63,291,156
Redeemable Convertible Preferred Shares Outstanding			63,021,976		63,291,156
Carrying Value			$ 163,355		$ 164,059
Liquidation Preference			$ 188,840		$ 179,462
Series E redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			81,428,340		81,428,340
Redeemable Convertible Preferred Shares Issued			80,884,240		81,428,340
Redeemable Convertible Preferred Shares Outstanding			80,884,240		81,428,340
Carrying Value			$ 498,525		$ 501,880
Liquidation Preference			$ 498,814		$ 502,169
Series F redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			54,001,241		54,001,241
Redeemable Convertible Preferred Shares Issued			54,001,241		54,001,241
Redeemable Convertible Preferred Shares Outstanding			54,001,241		54,001,241
Carrying Value			$ 473,532		$ 473,532
Liquidation Preference			$ 474,131		$ 474,131
Series G redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			55,666,004
Redeemable Convertible Preferred Shares Issued			55,666,004
Redeemable Convertible Preferred Shares Outstanding			55,666,004
Carrying Value			$ 549,413
Liquidation Preference			$ 560,000
Series H redeemable convertible preferred stock
Temporary Equity [Line Items]
Redeemable Convertible Preferred Shares Authorized			5,000,000
Redeemable Convertible Preferred Shares Issued			5,000,000
Redeemable Convertible Preferred Shares Outstanding			5,000,000
Carrying Value			$ 111,546
Liquidation Preference			$ 125,000