CONVERTIBLE PREFERRED STOCK - Narrative (Details)							1 Months Ended													2 Months Ended	3 Months Ended				9 Months Ended				12 Months Ended		16 Months Ended		45 Months Ended
		Jul. 23, 2021 shares	Dec. 31, 2020 USD ($) $ / shares shares		Dec. 24, 2020 USD ($) shares	Dec. 22, 2020 USD ($) shares	Apr. 30, 2021 USD ($) $ / shares shares	Mar. 31, 2021 USD ($)	Feb. 28, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares		Sep. 30, 2020 USD ($) condition $ / shares shares		Aug. 31, 2020 USD ($) $ / shares shares	Jun. 30, 2020 USD ($) shares		Mar. 31, 2020 USD ($) shares	Oct. 31, 2019 USD ($)	Apr. 30, 2019 USD ($)	Apr. 30, 2021 USD ($) $ / shares shares	Sep. 30, 2021 USD ($) $ / shares shares		Sep. 30, 2020 USD ($) condition $ / shares shares		Sep. 30, 2021 USD ($) $ / shares shares		Sep. 30, 2020 USD ($) condition $ / shares shares		Dec. 31, 2019 USD ($) $ / shares shares		Dec. 31, 2019 USD ($) $ / shares shares		Sep. 30, 2021 USD ($) $ / shares shares		Jul. 22, 2021 shares	Jun. 30, 2021 USD ($) shares		Dec. 31, 2018 USD ($)
Class of Stock [Line Items]
Convertible preferred stock, shares outstanding (in shares) | shares			957,159,704	[1]						957,159,704	[1]	825,191,683	[1]		667,574,747	[1]					0	[1]	825,191,683	[1]	0	[1]	825,191,683	[1]	502,582,534	[1]	502,582,534	[1]	0	[1]	437,182,072	1,155,909,367	[1]
Stock converted, reverse recapitalization (in shares) | shares		1,193,226,511
Temporary equity, carrying amount, attributable to parent			$ 2,494,076,000							$ 2,494,076,000		$ 1,992,106,000			$ 1,513,573,000						$ 0		$ 1,992,106,000		$ 0		$ 1,992,106,000		$ 1,074,010,000		$ 1,074,010,000		$ 0			$ 5,836,785,000
Number of milestone conditions required to receive funding | condition												2											2				2
Contingent forward contract liability, fair value loss																									454,546,000		$ 8,719,000
Stock-based compensation expense																					$ 236,956,000		$ 1,276,000		$ 366,200,000		3,257,000
Common stock, shares authorized (in shares) | shares			1,189,800,259						1,316,758,889	1,189,800,259											15,000,000,000				15,000,000,000								15,000,000,000
Convertible preferred stock, shares authorized (in shares) | shares			1,058,949,780						1,155,909,398	1,058,949,780											0				0								0
Convertible preferred stock, par value (in dollars per share) | $ / shares			$ 0.0001							$ 0.0001											$ 0.0001				$ 0.0001								$ 0.0001
Number of common shares issuable upon conversion of preferred shares (in shares) | shares																					1				1								1
Convertible stock, percentage of holders required to trigger conversion																					66.67%				66.67%								66.67%
Convertible stock. pre-offering market capitalization threshold to trigger conversion																					$ 2,500,000,000				$ 2,500,000,000								$ 2,500,000,000
Convertible stock, IPO gross proceeds required to trigger conversion																					200,000,000				200,000,000								200,000,000
Eligible Holders
Class of Stock [Line Items]
Aggregate value of convertible preferred shares approved for issuance									$ 71,000,000
Series E Contingent Forward Contract Liability
Class of Stock [Line Items]
Contingent forward contract, fair value			$ 110,500,000		$ 110,500,000		$ 722,400,000		1,400,000,000	$ 110,500,000		$ 800,000								$ 722,400,000	$ 0		$ 800,000		0		800,000						0
Contingent forward contract liability, fair value loss										$ 109,700,000															$ 454,500,000
Series A
Class of Stock [Line Items]
Convertible preferred stock, shares outstanding (in shares) | shares			32,045,280							32,045,280
Temporary equity, carrying amount, attributable to parent			$ 11,925,000							$ 11,925,000
Convertible preferred stock, shares authorized (in shares) | shares			32,045,280							32,045,280
Convertible preferred stock, dividend rate (in dollars per share) | $ / shares																									$ 0.03
Convertible preferred stock, liquidation preference, rate																									1
Series B
Class of Stock [Line Items]
Convertible preferred stock, shares outstanding (in shares) | shares			24,677,332							24,677,332
Repurchase of convertible preferred shares (in shares) | shares	[1]																								3,525,365
Payments for repurchase of convertible preferred stock																									$ 3,000,000		$ 0
Temporary equity, carrying amount, attributable to parent			$ 23,740,000							$ 23,740,000
Convertible preferred stock, shares authorized (in shares) | shares			24,677,332							24,677,332
Convertible preferred stock, dividend rate (in dollars per share) | $ / shares																									$ 0.09
Convertible preferred stock, liquidation preference, rate																									1
Series B | Fifth Company Repurchase
Class of Stock [Line Items]
Number of shares authorized to be repurchased (in shares) | shares						3,525,332
Payments for repurchase of convertible preferred stock						$ 3,000,000
Temporary equity, carrying amount, attributable to parent						4,000,000
Cumulative adjustments to additional paid in capital for share repurchases						$ 1,000,000
Convertible preferred shares subject to repurchase, mandatory redemption term			45 days
Series C
Class of Stock [Line Items]
Convertible preferred stock, shares outstanding (in shares) | shares			59,575,253							59,575,253
Repurchase of convertible preferred shares (in shares) | shares																							9,656,589	[1]			9,656,589	[1]			11,331,430
Repurchase of convertible preferred shares																															$ 60,000,000.0
Price of stock repurchased (in dollars per share) | $ / shares																															$ 5.30
Payments for repurchase of convertible preferred stock																									$ 0		$ 9,861,000
Temporary equity, carrying amount, attributable to parent			$ 137,475,000							$ 137,475,000
Convertible preferred stock, shares authorized (in shares) | shares			82,414,075							82,414,075
Convertible preferred stock, dividend rate (in dollars per share) | $ / shares																									$ 0.19
Convertible preferred stock, liquidation preference, rate																									1
Series C | Third Company Repurchase
Class of Stock [Line Items]
Price of stock repurchased (in dollars per share) | $ / shares														$ 1.02
Number of shares authorized to be repurchased (in shares) | shares														9,656,589
Payments for repurchase of convertible preferred stock														$ 9,900,000
Temporary equity, carrying amount, attributable to parent			$ 20,400,000							$ 20,400,000
Cumulative adjustments to additional paid in capital for share repurchases			$ 10,500,000							$ 10,500,000
Series C | Fourth Company Repurchase
Class of Stock [Line Items]
Price of stock repurchased (in dollars per share) | $ / shares										$ 1.21
Number of shares authorized to be repurchased (in shares) | shares			1,850,800							1,850,800
Payments for repurchase of convertible preferred stock										$ 2,200,000
Temporary equity, carrying amount, attributable to parent												$ 4,500,000											$ 4,500,000				$ 4,500,000
Cumulative adjustments to additional paid in capital for share repurchases			$ 2,200,000							$ 2,200,000
Convertible preferred stock, fair value (in dollars per share) | $ / shares												$ 2.42											$ 2.42				$ 2.42
Series D
Class of Stock [Line Items]
Convertible preferred stock, shares outstanding (in shares) | shares			539,769,493							539,769,493
Temporary equity, carrying amount, attributable to parent			$ 1,311,548,000							$ 1,311,548,000
Convertible preferred stock issuable, value																	$ 400,000,000
Proceeds from issuance of convertible preferred shares															$ 200,000,000		$ 200,000,000	$ 400,000,000	$ 200,000,000						$ 3,000,000		$ 400,000,000		872,000,000
Conversion of Convertible Notes																													$ 272,000,000				$ 271,985,000
Issuance of convertible preferred shares (in shares) | shares															82,496,121		82,496,092										164,992,213	[1]	374,777,281
Preferred stock, price (in dollars per share) | $ / shares																													$ 2.33		$ 2.33
Convertible preferred stock, shares authorized (in shares) | shares			618,720,748							618,720,748
Convertible preferred stock, dividend rate, percentage																									8.00%
Convertible preferred stock, liquidation preference, rate																									1.5
Series D Preferred Stock, Tranche Three
Class of Stock [Line Items]
Convertible preferred stock issuable, value																																						$ 400,000,000
Series D Preferred Stock, Tranche Two
Class of Stock [Line Items]
Convertible preferred stock issuable, value																																						400,000,000
Series D Preferred Stock, Tranche One
Class of Stock [Line Items]
Convertible preferred stock issuable, value																																						$ 200,000,000
Series E
Class of Stock [Line Items]
Convertible preferred stock, shares outstanding (in shares) | shares			301,092,346							301,092,346
Temporary equity, carrying amount, attributable to parent			$ 1,009,388,000							$ 1,009,388,000
Proceeds from issuance of convertible preferred shares					$ 400,000,000		$ 92,900,000	$ 107,100,000	$ 400,000,000	$ 400,000,000		$ 500,000,000								$ 200,000,000					$ 600,000,000		$ 499,724,000
Issuance of convertible preferred shares (in shares) | shares					133,818,821		66,909,408		133,818,821			167,273,525											167,273,525	[1]	200,728,229	[1]	167,273,525	[1]
Preferred stock, price (in dollars per share) | $ / shares							$ 2.99		$ 2.99			$ 2.99								$ 2.99			$ 2.99				$ 2.99
Convertible preferred stock, additional shares approved for issuance (in shares) | shares									200,700,000
Stock-based compensation expense																									$ 123,600,000
Convertible preferred stock, shares authorized (in shares) | shares			301,092,345							301,092,345
Convertible preferred stock, dividend rate, percentage																									8.00%
Convertible preferred stock, liquidation preference, rate																									1.5
Convertible stock, percentage of holders required to trigger conversion																					50.00%				50.00%								50.00%
Series E | Eligible Holders
Class of Stock [Line Items]
Preferred stock, price (in dollars per share) | $ / shares									$ 2.99
Convertible preferred shares approved for issuance (in shares) | shares									23,737,221
Series E | Additional Purchasers
Class of Stock [Line Items]
Preferred stock, price (in dollars per share) | $ / shares									$ 2.99
Series E | Management
Class of Stock [Line Items]
Issuance of convertible preferred shares (in shares) | shares																				3,034,194
Series E | Director
Class of Stock [Line Items]
Issuance of convertible preferred shares (in shares) | shares																				1,658,705
Series E | Chief Executive Officer
Class of Stock [Line Items]
Issuance of convertible preferred shares (in shares) | shares							535,275
Legacy Preferred Shareholders
Class of Stock [Line Items]
Stock converted, reverse recapitalization (in shares) | shares		437,182,072

[1] The number of shares of convertible preferred stock and common stock issued and outstanding prior to the Merger have been retroactively adjusted by the Exchange Ratio to give effect to the reverse recapitalization treatment of the Merger. See Note 1 - Description of Business and Note 3 - Reverse Capitalization for more information.