Redeemable Convertible Preferred Stock and Common Stock - Summary of Activity for Each Series of Outstanding Redeemable Convertible Preferred Stock (Details) - USD ($) $ in Thousands			1 Months Ended			12 Months Ended
	Dec. 11, 2018	Aug. 10, 2016	Sep. 30, 2018	May 31, 2018	Feb. 28, 2018	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	448,686,791
Balance at beginning of period							$ 1,176,661
Conversion of redeemable convertible preferred stock into common stock							$ 1,833,561
Balance at end of period (in shares)	508,539,515					0	0
Balance at end of period	$ 1,833,561
Issuance costs							$ 41,322
Series A Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							42,000,000
Balance at beginning of period							$ 182
Balance at end of period (in shares)	42,000,000
Balance at end of period	$ 182
Series B Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							122,296,280
Balance at beginning of period							$ 770
Balance at end of period (in shares)	122,296,280
Balance at end of period	$ 770
Series C Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							85,669,774
Balance at beginning of period							$ 36,238
Balance at end of period (in shares)	85,669,774
Balance at end of period	$ 36,238
Series D Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							63,291,156
Balance at beginning of period							$ 164,059
Repurchase of redeemable convertible preferred stock (in shares)			(269,180)
Repurchase of redeemable convertible preferred stock							$ (2,009)
Balance at end of period (in shares)	63,021,976
Balance at end of period	$ 163,355
Series E Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							81,428,340
Balance at beginning of period							$ 501,880
Repurchase of redeemable convertible preferred stock (in shares)			(544,100)
Repurchase of redeemable convertible preferred stock			$ (8,200)				$ (2,118)
Balance at end of period (in shares)	80,884,240
Balance at end of period	$ 498,525
Series F Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)							54,001,241
Balance at beginning of period							$ 473,532
Balance at end of period (in shares)	54,001,241
Balance at end of period	$ 473,532
Issuance costs		$ 600
Series G Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock, net of issuance costs							51
Balance at end of period (in shares)	55,666,004
Balance at end of period	$ 549,413
Issuance costs					$ 10,500	$ 10,517	10,517
Series H Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at end of period (in shares)	5,000,000
Balance at end of period	$ 111,546
Issuance costs				$ 400		$ 474	$ 474
Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	448,686,791
Balance at beginning of period						$ 0	$ 1,176,661
Conversion of redeemable convertible preferred stock into common stock (in shares)							(508,539,515)
Conversion of redeemable convertible preferred stock into common stock							$ (1,833,561)
Balance at end of period (in shares)						0	0
Balance at end of period						$ 0	$ 0
Redeemable Convertible Preferred Stock | Series A Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	42,000,000
Balance at beginning of period						$ 0	$ 182
Conversion of redeemable convertible preferred stock into common stock (in shares)							(42,000,000)
Conversion of redeemable convertible preferred stock into common stock							$ (182)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series B Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	122,296,280
Balance at beginning of period						$ 0	$ 770
Conversion of redeemable convertible preferred stock into common stock (in shares)							(122,296,280)
Conversion of redeemable convertible preferred stock into common stock							$ (770)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series C Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	85,669,774
Balance at beginning of period						$ 0	$ 36,238
Conversion of redeemable convertible preferred stock into common stock (in shares)							(85,669,774)
Conversion of redeemable convertible preferred stock into common stock							$ (36,238)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series D Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	63,291,156
Balance at beginning of period						$ 0	$ 164,059
Repurchase of redeemable convertible preferred stock (in shares)							(269,180)
Repurchase of redeemable convertible preferred stock							$ (704)
Conversion of redeemable convertible preferred stock into common stock (in shares)							(63,021,976)
Conversion of redeemable convertible preferred stock into common stock							$ (163,355)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series E Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	81,428,340
Balance at beginning of period						$ 0	$ 501,880
Repurchase of redeemable convertible preferred stock (in shares)							(544,100)
Repurchase of redeemable convertible preferred stock							$ (3,355)
Conversion of redeemable convertible preferred stock into common stock (in shares)							(80,884,240)
Conversion of redeemable convertible preferred stock into common stock							$ (498,525)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series F Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	54,001,241
Balance at beginning of period						$ 0	$ 473,532
Conversion of redeemable convertible preferred stock into common stock (in shares)							(54,001,241)
Conversion of redeemable convertible preferred stock into common stock							$ (473,532)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series G Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	0
Balance at beginning of period						$ 0	$ 0
Issuance of redeemable convertible preferred stock, net of issuance costs (in shares)							55,666,004
Issuance of redeemable convertible preferred stock, net of issuance costs							$ 549,413
Conversion of redeemable convertible preferred stock into common stock (in shares)							(55,666,004)
Conversion of redeemable convertible preferred stock into common stock							$ (549,413)
Balance at end of period (in shares)							0
Balance at end of period							$ 0
Redeemable Convertible Preferred Stock | Series H Redeemable Convertible Preferred Stock
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance at beginning of period (in shares)						0	0
Balance at beginning of period						$ 0	$ 0
Issuance of redeemable convertible preferred stock, net of issuance costs (in shares)							5,000,000
Issuance of redeemable convertible preferred stock, net of issuance costs							$ 111,546
Conversion of redeemable convertible preferred stock into common stock (in shares)							(5,000,000)
Conversion of redeemable convertible preferred stock into common stock							$ (111,546)
Balance at end of period (in shares)							0
Balance at end of period							$ 0