CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Unaudited) - USD ($) $ in Thousands	Total	IPO	Follow-On Public Offering	Redeemable Convertible Preferred Stock	Common Stock	Common Stock Restricted Stock Units	Common Stock IPO	Common Stock Follow-On Public Offering	Additional Paid-in Capital	Additional Paid-in Capital IPO	Additional Paid-in Capital Follow-On Public Offering	Accumulated Deficit
Temporary Equity, Balance at Dec. 31, 2019				$ 874,332
Temporary Equity, Balance, shares at Dec. 31, 2019				83,568,628
Balance at Dec. 31, 2019	$ (573,852)				$ 8							$ (573,860)
Balance (in shares) at Dec. 31, 2019					8,650,922
Stock-based compensation	600								$ 600
Exercise of stock options	6								6
Exercise of stock options (in shares)					2,774
Repurchase of common stock	(1,818)								(606)			(1,212)
Repurchase of common stock (in shares)					(200,000)
Issuance of Series H redeemable convertible preferred stock, net of issuance costs				$ 26,714
Issuance of Series H redeemable convertible preferred stock, net of issuance costs, shares				1,964,766
Net loss	(41,059)											(41,059)
Balance at Mar. 31, 2020	(616,123)				$ 8							(616,131)
Temporary Equity, Balance (in shares) at Mar. 31, 2020				85,533,394
Temporary Equity, Balance at Mar. 31, 2020				$ 901,046
Balance (in shares) at Mar. 31, 2020					8,453,696
Temporary Equity, Balance at Dec. 31, 2019				$ 874,332
Temporary Equity, Balance, shares at Dec. 31, 2019				83,568,628
Balance at Dec. 31, 2019	(573,852)				$ 8							(573,860)
Balance (in shares) at Dec. 31, 2019					8,650,922
Net loss	(142,137)
Balance at Sep. 30, 2020	1,277,850				$ 130				1,994,929			(717,209)
Balance (in shares) at Sep. 30, 2020					130,230,591
Temporary Equity, Balance at Mar. 31, 2020				$ 901,046
Temporary Equity, Balance, shares at Mar. 31, 2020				85,533,394
Balance at Mar. 31, 2020	(616,123)				$ 8							(616,131)
Balance (in shares) at Mar. 31, 2020					8,453,696
Issuance of common stock	2,127	$ 496,510					$ 24		2,127	$ 496,486
Issuance of common stock (in shares)					183,870		24,437,500
Conversion of redeemable convertible preferred stock to common stock	901,046				$ 86				900,960
Temporary equity, Conversion of redeemable convertible preferred stock to common stock (in shares)				(85,533,394)
Temporary equity, Conversion of redeemable convertible preferred stock to common stock				$ (901,046)
Conversion of redeemable convertible preferred stock to common stock (in shares)					85,533,394
Conversion of redeemable convertible preferred stock warrant to common stock warrant	21,873								21,873
Stock-based compensation	4,100								4,100
Exercise of stock options	7								7
Exercise of stock options (in shares)					500
Exercise of common stock warrants	1				$ 1
Exercise of common stock warrants (in shares)					636,112
Vesting of restricted stock units (in shares)						133,334
Common stock shares withheld to satisfy employee tax withholding obligations	(878)								(878)
Common stock shares withheld to satisfy employee tax withholding obligations (in shares)					(41,818)
Net loss	(63,228)											(63,228)
Balance at Jun. 30, 2020	745,435				$ 119				1,424,675			(679,359)
Balance (in shares) at Jun. 30, 2020					119,336,588
Issuance of common stock			$ 567,952					$ 11			$ 567,941
Issuance of common stock (in shares)								10,800,000
Stock-based compensation	4,230								4,230
Exercise of stock options	120								120
Exercise of stock options (in shares)					31,100
Vesting of restricted stock units (in shares)					104,000
Common stock shares withheld to satisfy employee tax withholding obligations	(2,037)								(2,037)
Common stock shares withheld to satisfy employee tax withholding obligations (in shares)					(41,097)
Net loss	(37,850)											(37,850)
Balance at Sep. 30, 2020	1,277,850				$ 130				1,994,929			(717,209)
Balance (in shares) at Sep. 30, 2020					130,230,591
Balance at Dec. 31, 2020	1,227,102				$ 132				2,004,841			(777,871)
Balance (in shares) at Dec. 31, 2020					134,043,969
Issuance of common stock for CarStory acquisition	39,030				$ 1				39,029
Issuance of common stock for CarStory acquisition (in shares)					1,072,117
Fair value of unvested stock options assumed in CarStory acquisition	1,017								1,017
Stock-based compensation	2,820								2,820
Exercise of stock options	2,821				$ 1				2,820
Exercise of stock options (in shares)					687,336
Vesting of restricted stock units (in shares)					499,879
Net loss	(77,189)											(77,189)
Balance at Mar. 31, 2021	1,195,601				$ 134				2,050,527			(855,060)
Balance (in shares) at Mar. 31, 2021					136,303,301
Balance at Dec. 31, 2020	1,227,102				$ 132				2,004,841			(777,871)
Balance (in shares) at Dec. 31, 2020					134,043,969
Net loss	(241,118)
Balance at Sep. 30, 2021	1,040,651				$ 135				2,059,505			(1,018,989)
Balance (in shares) at Sep. 30, 2021					136,897,954
Balance at Mar. 31, 2021	1,195,601				$ 134				2,050,527			(855,060)
Balance (in shares) at Mar. 31, 2021					136,303,301
Stock-based compensation	5,392								5,392
Exercise of stock options	1,560								1,560
Exercise of stock options (in shares)					395,491
Vesting of restricted stock units (in shares)					18,555
Net loss	(65,807)											(65,807)
Balance at Jun. 30, 2021	1,136,746				$ 134				2,057,479			(920,867)
Balance (in shares) at Jun. 30, 2021					136,717,347
Stock-based compensation	1,542								1,542
Exercise of stock options	704				$ 1				703
Exercise of stock options (in shares)					164,631
Vesting of restricted stock units (in shares)					21,649
Common stock shares cancelled to satisfy working capital adjustment related to the CarStory acquisition (in shares)					(5,673)
Common stock shares cancelled to satisfy working capital adjustment related to the CarStory acquisition	(219)								(219)
Net loss	(98,122)
Balance at Sep. 30, 2021	$ 1,040,651				$ 135				$ 2,059,505			$ (1,018,989)
Balance (in shares) at Sep. 30, 2021					136,897,954