CONVERTIBLE PREFERRED STOCK - Schedule of Convertible Preferred Shares (Details) $ / shares in Units, $ in Thousands	Sep. 30, 2021 USD ($) shares		Jul. 22, 2021 shares	Jun. 30, 2021 USD ($) shares		Feb. 28, 2021 shares	Dec. 31, 2020 USD ($) $ / shares shares		Sep. 30, 2020 USD ($) shares		Jun. 30, 2020 USD ($) shares		Dec. 31, 2019 USD ($) shares
Class of Stock [Line Items]
Convertible preferred stock, shares authorized (in shares) | shares	0					1,155,909,398	1,058,949,780
Convertible preferred stock, shares outstanding (in shares) | shares	0	[1]	437,182,072	1,155,909,367	[1]		957,159,704	[1]	825,191,683	[1]	667,574,747	[1]	502,582,534	[1]
Temporary equity, carrying amount, attributable to parent | $	$ 0			$ 5,836,785			$ 2,494,076		$ 1,992,106		$ 1,513,573		$ 1,074,010
Convertible preferred stock, liquidation preference amount | $	$ 0						$ 3,497,913
Series A
Class of Stock [Line Items]
Convertible preferred stock, shares authorized (in shares) | shares							32,045,280
Convertible preferred stock, shares outstanding (in shares) | shares							32,045,280
Temporary equity, carrying amount, attributable to parent | $							$ 11,925
Conversion Per Share to Common Stock							0.38
Temporary equity, liquidation preference per share (in dollars per share) | $ / shares							$ 0.38
Convertible preferred stock, liquidation preference amount | $							$ 12,120
Series B
Class of Stock [Line Items]
Convertible preferred stock, shares authorized (in shares) | shares							24,677,332
Convertible preferred stock, shares outstanding (in shares) | shares							24,677,332
Temporary equity, carrying amount, attributable to parent | $							$ 23,740
Conversion Per Share to Common Stock							1.13
Temporary equity, liquidation preference per share (in dollars per share) | $ / shares							$ 1.13
Convertible preferred stock, liquidation preference amount | $							$ 28,000
Convertible preferred stock, shares cancelled (in shares) | shares							3,525,332
Convertible preferred stock, shares cancelled, fair value | $							$ 3,000
Series C
Class of Stock [Line Items]
Convertible preferred stock, shares authorized (in shares) | shares							82,414,075
Convertible preferred stock, shares outstanding (in shares) | shares							59,575,253
Temporary equity, carrying amount, attributable to parent | $							$ 137,475
Conversion Per Share to Common Stock							2.42
Temporary equity, liquidation preference per share (in dollars per share) | $ / shares							$ 2.42
Convertible preferred stock, liquidation preference amount | $							$ 144,432
Series D
Class of Stock [Line Items]
Convertible preferred stock, shares authorized (in shares) | shares							618,720,748
Convertible preferred stock, shares outstanding (in shares) | shares							539,769,493
Temporary equity, carrying amount, attributable to parent | $							$ 1,311,548
Conversion Per Share to Common Stock							2.33
Temporary equity, liquidation preference per share (in dollars per share) | $ / shares							$ 3.64
Convertible preferred stock, liquidation preference amount | $							$ 1,963,912
Series E
Class of Stock [Line Items]
Convertible preferred stock, shares authorized (in shares) | shares							301,092,345
Convertible preferred stock, shares outstanding (in shares) | shares							301,092,346
Temporary equity, carrying amount, attributable to parent | $							$ 1,009,388
Conversion Per Share to Common Stock							2.99
Temporary equity, liquidation preference per share (in dollars per share) | $ / shares							$ 4.48
Convertible preferred stock, liquidation preference amount | $							$ 1,349,449

[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.