MEZZANINE EQUITY, COMMON STOCK AND STOCKHOLDERS' (DEFICIT) EQUITY - NARRATIVE (Details)		1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 29, 2021 USD ($)	Jun. 30, 2021 shares	May 31, 2021 $ / shares shares	Mar. 31, 2021 employee $ / shares	Sep. 30, 2021 USD ($) vote class $ / shares shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2021 USD ($) vote class $ / shares shares	Sep. 30, 2020 USD ($) shares	Dec. 31, 2020 $ / shares shares	Dec. 31, 2019 $ / shares shares	Aug. 02, 2021 class $ / shares shares	Feb. 28, 2021 USD ($) shares	Jun. 30, 2020 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Redeemable convertible preferred stock, authorized (in shares)					0		0		414,033,220
Redeemable convertible preferred stock, issued (in shares)					0		0		412,742,897
Redeemable convertible preferred stock, outstanding (in shares)					0		0		412,742,897
Conversion of preferred stock to common stock | $					$ (2,179,739,000)		$ (2,179,739,000)
Preferred stock, shares outstanding (in shares)					0		0		0
Number of classes of common stock | class					3		3
Share-based compensation | $					$ 1,244,314,000	$ 1,571,000	$ 1,254,448,000	$ 5,348,000
Capitalized share-based compensation expense | $					19,900,000	0	20,000,000	600,000
Unrecognized compensation cost | $					$ 2,170,000,000		$ 2,170,000,000
Unrecognized compensation cost related to outstanding stock options, weighted-average period							2 years 6 months 21 days
Tranche 1 Convertible Note Holders
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum amount of all warrants | $												$ 379,800,000
IPO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of classes of common stock | class											3
2013 and 2020 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of common stock authorized (in shares)					316,700,000		316,700,000
Shares issued under plans (in shares)							62,900,000
Common stock reserved for issuance (in shares)							127,900,000
2020 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares remaining available for issuance (in shares)					125,900,000		125,900,000
2021 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares reserved for issuance under equity incentive plan							14.00%
Annual increase as a percentage of outstanding shares							5.00%
Conversion of Class B Common Stock into Class A Common
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, conversion basis (in shares)					1		1
Conversion basis, outstanding shares, percentage					80.00%		80.00%
Conversion basis, percentage of aggregate shares outstanding							5.00%
Conversion basis, approval period, death or total disability of founders							9 months
Conversion basis, approval period delay (up to)							18 months
Conversion basis, final conversion date							15 years
Conversion of Class B Common Stock into Class A Common | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversion basis, outstanding shares, fixed days							61 days
Conversion basis, aggregate outstanding shares, fixed days							6100.00%
Conversion of Class B Common Stock into Class A Common | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversion basis, outstanding shares, fixed days							180 days
Conversion basis, aggregate outstanding shares, fixed days							18000.00%
Conversion of Class C Common Stock into Class A Common
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, conversion basis (in shares)					1		1
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting rate, percentage							25.00%
Vesting period							4 years
Exercisable, period (up to)							10 years
Share-based compensation | $					$ 1,800,000	0	$ 1,800,000	0
Stock options | Share-based Payment Arrangement, Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period							1 year
Time-Based RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period				1 year
Unvested restricted stock (in shares)					50,642,853		50,642,853		47,711,649
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares							$ 24.37
Granted (in shares)							28,635,488
Granted, Weighted-average grant date fair value (in dollars per share) | $ / shares							$ 40.76
Vested (in shares)							23,261,472
Share-based compensation | $					$ 907,300,000	0	$ 907,300,000	0
Number of employees affected | employee				500
Modified grant date fair value (in dollars per share) | $ / shares				$ 39.75	$ 32.20		$ 32.20		$ 10.84
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period			8 years
Exercisable, period (up to)							7 years
Unvested restricted stock (in shares)										27,700,000
Requisite service period										3 months
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares										$ 0.29
Weighted-average incremental fair value (in dollars per share) | $ / shares										$ 21.01
Granted (in shares)			35,500,000
Granted, Weighted-average grant date fair value (in dollars per share) | $ / shares			$ 22.68
Consecutive trading days			60 days
RSUs | Share-based Payment Arrangement, Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested shares upon achievement of price targets (in shares)			4,500,000							5,600,000
Price targets, vested (in dollars per share) | $ / shares			$ 120							$ 30.45
RSUs | Share-based Payment Arrangement, Tranche Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested shares upon achievement of price targets (in shares)			5,300,000							8,300,000
Price targets, vested (in dollars per share) | $ / shares			$ 150							$ 50.75
RSUs | Share-based Payment Arrangement, Tranche Two | IPO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested shares upon achievement of price targets (in shares)										4,000,000
RSUs | Share-based Payment Arrangement, Tranche Three
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested shares upon achievement of price targets (in shares)										13,800,000
Price targets, vested (in dollars per share) | $ / shares			180							$ 101.50
RSUs | Share-based Payment Arrangement, Tranche Four
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Price targets, vested (in dollars per share) | $ / shares			210
RSUs | Share-based Payment Arrangement, Tranche Five
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Price targets, vested (in dollars per share) | $ / shares			240
RSUs | Share-based Payment Arrangement, Tranche Six
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Price targets, vested (in dollars per share) | $ / shares			270
RSUs | Share-based Payment Arrangement, Tranche Seven
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Price targets, vested (in dollars per share) | $ / shares			$ 300
2021 Market-Based RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested (in shares)							0
Market-Based RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unvested restricted stock (in shares)					59,034,110		59,034,110		27,663,658
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares							$ 0
Granted (in shares)							35,520,000
Granted, Weighted-average grant date fair value (in dollars per share) | $ / shares							$ 22.68
Vested (in shares)							4,149,548		0
Share-based compensation | $					$ 360,900,000	$ 0	$ 360,900,000	$ 0
Modified grant date fair value (in dollars per share) | $ / shares					$ 23.46		$ 23.46		$ 0.29
Employee Stock | 2021 ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual increase as a percentage of outstanding shares		1.00%
Shares of common stock authorized (in shares)		200,000,000
Maximum payroll deduction for ESPP, percentage		15.00%
ESPP purchase price discount, percentage		85.00%
Percentage of shares outstanding after close of offering		2.00%
Number of share purchased (in shares)							0
Share available for future issuance under the ESPP (in shares)					17,000,000		17,000,000
Series G-1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Redeemable convertible preferred stock, authorized (in shares)												244,300,000
Redeemable convertible preferred stock, issued (in shares)												0
Redeemable convertible preferred stock, outstanding (in shares)												0
Common Class A
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Conversion basis	1
Number of voting rights per share | vote					1		1
Common Class A | IPO
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate warrants exercisable (in shares)											14,300,000
Exercise price (in dollars per share) | $ / shares											$ 26.60
Redeemable convertible preferred stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Redeemable convertible preferred stock, issued (in shares)					0		0
Redeemable convertible preferred stock, outstanding (in shares)		412,742,897			0	412,742,897	0	412,742,897	412,742,897	321,626,778			366,266,778
Conversion of preferred stock to common stock | $	$ 2,180,000,000				$ 2,179,739,000		$ 2,179,739,000
Common Class B
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of voting rights per share | vote					10		10
Series F
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Redeemable convertible preferred stock, authorized (in shares)									48,000,000
Redeemable convertible preferred stock, issued (in shares)									48,000,000
Redeemable convertible preferred stock, outstanding (in shares)									48,000,000
Per Share Liquidation Preference (in dollars per share) | $ / shares									$ 12.5000
Series A
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Redeemable convertible preferred stock, authorized (in shares)									131,913,460
Redeemable convertible preferred stock, issued (in shares)									131,913,460
Redeemable convertible preferred stock, outstanding (in shares)									131,913,460
Per Share Liquidation Preference (in dollars per share) | $ / shares									$ 0.1954