CONDENSED CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		76,821,000
Beginning balance (in shares) at Dec. 31, 2019	153,938,000
Beginning balance at Dec. 31, 2019	$ 1,334,572
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable convertible preferred stock	$ 371,522
Ending balance (in shares) at Sep. 30, 2020	153,938,000
Ending balance at Sep. 30, 2020	$ 1,706,094
Beginning balance at Dec. 31, 2019	(1,122,721)	$ 1	$ 0	$ 139	$ (1,122,861)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accretion of redeemable convertible preferred stock	(371,522)		(43,401)		(328,121)
Issuance of common stock upon exercise of stock options (in shares)		392,000
Issuance of common stock upon exercise of stock options	121		121
Stock-based compensation	43,280		43,280
Unrealized (loss) gain on marketable securities	615			615
Net loss	(55,759)				(55,759)
Ending balance (in shares) at Sep. 30, 2020		77,213,000
Ending balance at Sep. 30, 2020	$ (1,505,986)	$ 1	0	754	(1,506,741)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		76,857,000
Beginning balance (in shares) at Jun. 30, 2020	153,938,000
Beginning balance at Jun. 30, 2020	$ 1,436,736
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable convertible preferred stock	$ 269,358
Ending balance (in shares) at Sep. 30, 2020	153,938,000
Ending balance at Sep. 30, 2020	$ 1,706,094
Beginning balance at Jun. 30, 2020	(1,237,631)	$ 1	0	1,248	(1,238,880)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accretion of redeemable convertible preferred stock	(269,358)		(110)		(269,248)
Issuance of common stock upon exercise of stock options (in shares)		356,000
Issuance of common stock upon exercise of stock options	110	$ 0	110
Unrealized (loss) gain on marketable securities	(494)			(494)
Net loss	1,387				1,387
Ending balance (in shares) at Sep. 30, 2020		77,213,000
Ending balance at Sep. 30, 2020	$ (1,505,986)	$ 1	0	754	(1,506,741)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		77,213,000
Beginning balance (in shares)		77,619,000
Beginning balance (in shares) at Dec. 31, 2020	153,937,730
Beginning balance at Dec. 31, 2020	$ 2,895,096
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable convertible preferred stock	$ 2,646,662
Conversion of redeemable convertible preferred stock into common stock upon initial public offering (in shares)	(153,938,000)
Conversion of redeemable convertible preferred stock into common stock upon initial public offering	$ (5,541,758)
Ending balance (in shares) at Sep. 30, 2021	0
Ending balance at Sep. 30, 2021	$ 0
Beginning balance at Dec. 31, 2020	(2,696,741)	$ 1	0	411	(2,697,153)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accretion of redeemable convertible preferred stock	(2,646,662)		(2,264,869)		(381,793)
Conversion of redeemable convertible preferred stock (in shares)		153,938,000
Conversion of redeemable convertible preferred stock into common stock upon initial public offering	5,541,758	$ 2	5,541,756
Issuance of common stock upon initial public offering, net of underwriting discounts and offering expenses		31,350,000
Issuance of common stock upon initial public offering, net of underwriting discounts and offering expenses	$ 1,062,453		1,062,453
Issuance of common stock upon exercise of stock options (in shares)	210,000	210,000
Issuance of common stock upon exercise of stock options	$ 43		43
Stock-based compensation	124,717		124,717
Unrealized (loss) gain on marketable securities	(431)			(431)
Net loss	(117,259)				(117,259)
Ending balance (in shares) at Sep. 30, 2021		263,117,000
Ending balance at Sep. 30, 2021	$ 1,267,878	$ 3	4,464,100	(20)	(3,196,205)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		77,756,000
Beginning balance (in shares) at Jun. 30, 2021	153,938,000
Beginning balance at Jun. 30, 2021	$ 3,276,920
Increase (Decrease) in Temporary Equity [Roll Forward]
Accretion of redeemable convertible preferred stock	$ 2,264,838
Conversion of redeemable convertible preferred stock into common stock upon initial public offering (in shares)	(153,938,000)
Conversion of redeemable convertible preferred stock into common stock upon initial public offering	$ (5,541,758)
Ending balance (in shares) at Sep. 30, 2021	0
Ending balance at Sep. 30, 2021	$ 0
Beginning balance at Jun. 30, 2021	(3,088,791)	$ 1	0	(2)	(3,088,790)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accretion of redeemable convertible preferred stock	(2,264,838)		(2,264,838)		0
Conversion of redeemable convertible preferred stock (in shares)		153,938,000
Conversion of redeemable convertible preferred stock into common stock upon initial public offering	5,541,758	$ 2	5,541,756
Issuance of common stock upon initial public offering, net of underwriting discounts and offering expenses		31,350,000
Issuance of common stock upon initial public offering, net of underwriting discounts and offering expenses	1,062,453		1,062,453
Issuance of common stock upon exercise of stock options (in shares)		73,000
Issuance of common stock upon exercise of stock options	12		12
Stock-based compensation	124,717		124,717
Unrealized (loss) gain on marketable securities	(18)			(18)
Net loss	(107,415)				(107,415)
Ending balance (in shares) at Sep. 30, 2021		263,117,000
Ending balance at Sep. 30, 2021	$ 1,267,878	$ 3	$ 4,464,100	$ (20)	$ (3,196,205)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)		263,117,000