Condensed Consolidated Statements of Mezzanine Equity and Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	PIPE Investment	Class A Common Stock Warrants	Common Stock	Common Stock PIPE Investment	Common Stock Class A Common Stock Warrants	Additional Paid-In Capital	Additional Paid-In Capital PIPE Investment	Additional Paid-In Capital Class A Common Stock Warrants	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Redeemable Convertible Preferred Stock	Redeemable Class A Common Stock	Previously Reported	Previously Reported Common Stock	Previously Reported Additional Paid-In Capital	Previously Reported Accumulated Other Comprehensive Income (Loss)	Previously Reported Accumulated Deficit	Previously Reported Redeemable Convertible Preferred Stock	Previously Reported Redeemable Class A Common Stock	Revision of Prior Period, Adjustment	Revision of Prior Period, Adjustment Common Stock	Revision of Prior Period, Adjustment Additional Paid-In Capital	Revision of Prior Period, Adjustment Redeemable Convertible Preferred Stock	Revision of Prior Period, Adjustment Redeemable Class A Common Stock
Mezzanine equity, beginning balance (in shares) at Dec. 31, 2019												84,514,191	737,058						186,573,651	1,627,132				(102,059,460)	(890,074)
Mezzanine equity, beginning balance at Dec. 31, 2019												$ 186,741	$ 4,500						$ 186,741	$ 4,500
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series D convertible preferred stock, net of issuance costs (in shares)												4,537,700
Issuance of Series D convertible preferred stock, net of issuance costs												$ 31,511
Conversion of convertible securities (in shares)													(737,058)
Conversion of convertible securities													$ (4,500)
Mezzanine equity, ending balance (in shares) at Mar. 31, 2020												89,051,891	0
Mezzanine equity, ending balance at Mar. 31, 2020												$ 218,252	$ 0
Beginning balance (in shares) at Dec. 31, 2019				51,588,612											113,887,093							(62,298,481)
Beginning balance at Dec. 31, 2019	$ (139,793)			$ 0			$ 13,383			$ 2	$ (153,178)			$ (139,793)	$ 0	$ 13,383	$ 2	$ (153,178)			$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of preferred stock to common stock (in shares)				737,058
Conversion of preferred stock to common stock	4,500						4,500
Exercise of vested stock options (in shares)				226
Early exercise of unvested stock options (in shares)				226
Vesting of early-exercised stock options	11						11
Forfeiture of unvested early exercised shares (in shares)				(46,242)
Stock-based compensation	1,404						1,404
Other comprehensive loss	(42)									(42)
Net loss	(6,033)										(6,033)
Ending balance (in shares) at Mar. 31, 2020				52,279,880
Ending balance at Mar. 31, 2020	(139,953)			$ 0			19,298			(40)	(159,211)
Mezzanine equity, beginning balance (in shares) at Dec. 31, 2019												84,514,191	737,058						186,573,651	1,627,132				(102,059,460)	(890,074)
Mezzanine equity, beginning balance at Dec. 31, 2019												$ 186,741	$ 4,500						$ 186,741	$ 4,500
Mezzanine equity, ending balance (in shares) at Sep. 30, 2020												92,850,809	0
Mezzanine equity, ending balance at Sep. 30, 2020												$ 245,168	$ 0
Beginning balance (in shares) at Dec. 31, 2019				51,588,612											113,887,093							(62,298,481)
Beginning balance at Dec. 31, 2019	(139,793)			$ 0			13,383			2	(153,178)			(139,793)	$ 0	13,383	2	(153,178)			0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive loss	(12)
Net loss	(12,885)
Ending balance (in shares) at Sep. 30, 2020				52,073,334
Ending balance at Sep. 30, 2020	$ (143,341)			$ 0			22,732			(10)	(166,063)
Mezzanine equity, beginning balance (in shares) at Dec. 31, 2019												84,514,191	737,058						186,573,651	1,627,132				(102,059,460)	(890,074)
Mezzanine equity, beginning balance at Dec. 31, 2019												$ 186,741	$ 4,500						$ 186,741	$ 4,500
Mezzanine equity, ending balance (in shares) at Dec. 31, 2020	93,328,118											93,328,118	0						206,031,290	0				(112,703,172)
Mezzanine equity, ending balance at Dec. 31, 2020	$ 249,962											$ 249,962	$ 0						$ 249,962	$ 0
Beginning balance (in shares) at Dec. 31, 2019				51,588,612											113,887,093							(62,298,481)
Beginning balance at Dec. 31, 2019	(139,793)			$ 0			13,383			2	(153,178)			(139,793)	$ 0	13,383	2	(153,178)			0
Ending balance (in shares) at Dec. 31, 2020				52,967,106											116,930,602							(63,963,496)
Ending balance at Dec. 31, 2020	(146,874)			$ 5			24,424			(11)	(171,292)			(146,874)	$ 0	24,429	(11)	(171,292)			0	$ 5	$ (5)
Mezzanine equity, beginning balance (in shares) at Mar. 31, 2020												89,051,891	0
Mezzanine equity, beginning balance at Mar. 31, 2020												$ 218,252	$ 0
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series D convertible preferred stock, net of issuance costs (in shares)												868,768
Issuance of Series D convertible preferred stock, net of issuance costs												$ 6,028
Exercise of Series C convertible preferred stock warrants (in shares)												583,632
Exercise of Series C convertible preferred stock warrants												$ 3,852
Mezzanine equity, ending balance (in shares) at Jun. 30, 2020												90,504,291	0
Mezzanine equity, ending balance at Jun. 30, 2020												$ 228,132	$ 0
Beginning balance (in shares) at Mar. 31, 2020				52,279,880
Beginning balance at Mar. 31, 2020	(139,953)			$ 0			19,298			(40)	(159,211)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of vested stock options (in shares)				15,043
Exercise of vested stock options	27						27
Early exercise of unvested stock options (in shares)				10,438
Vesting of early-exercised stock options	9						9
Forfeiture of unvested early exercised shares (in shares)				(509,602)
Stock-based compensation	1,925						1,925
Other comprehensive loss	24									24
Net loss	(981)										(981)
Ending balance (in shares) at Jun. 30, 2020				51,795,759
Ending balance at Jun. 30, 2020	(138,949)			$ 0			21,259			(16)	(160,192)
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series D convertible preferred stock, net of issuance costs (in shares)												2,065,594
Issuance of Series D convertible preferred stock, net of issuance costs												$ 14,361
Exercise of Series C convertible preferred stock warrants (in shares)												280,924
Exercise of Series C convertible preferred stock warrants												$ 2,675
Mezzanine equity, ending balance (in shares) at Sep. 30, 2020												92,850,809	0
Mezzanine equity, ending balance at Sep. 30, 2020												$ 245,168	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of Class A common stock warrants (in shares)				143,452
Exercise of Class A common stock warrants	8						8
Exercise of vested stock options (in shares)				122,810
Exercise of vested stock options	44						44
Early exercise of unvested stock options (in shares)				11,313
Vesting of early-exercised stock options	7						7
Stock-based compensation	1,414						1,414
Other comprehensive loss	6									6
Net loss	(5,871)										(5,871)
Ending balance (in shares) at Sep. 30, 2020				52,073,334
Ending balance at Sep. 30, 2020	$ (143,341)			$ 0			22,732			(10)	(166,063)
Mezzanine equity, beginning balance (in shares) at Dec. 31, 2020	93,328,118											93,328,118	0						206,031,290	0				(112,703,172)
Mezzanine equity, beginning balance at Dec. 31, 2020	$ 249,962											$ 249,962	$ 0						$ 249,962	$ 0
Increase (Decrease) in Temporary Equity [Roll Forward]
Pre-closing stock repurchase, net of exercise of vested options (in shares)												(206,511)
Pre-closing stock repurchase, net of exercise of vested options												$ (125)
Conversion of convertible securities (in shares)												(93,121,607)
Conversion of convertible securities												$ (249,837)
Mezzanine equity, ending balance (in shares) at Mar. 31, 2021												0	0
Mezzanine equity, ending balance at Mar. 31, 2021												$ 0	$ 0
Beginning balance (in shares) at Dec. 31, 2020				52,967,106											116,930,602							(63,963,496)
Beginning balance at Dec. 31, 2020	(146,874)			$ 5			24,424			(11)	(171,292)			(146,874)	$ 0	24,429	(11)	(171,292)			0	$ 5	(5)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Pre-closing stock repurchase, net of exercise of vested options (in shares)				(1,817,519)
Pre-closing stock repurchase, net of exercise of vested options	(21,902)						(21,902)
Conversion of preferred stock to common stock (in shares)				93,121,607		1,867,292
Conversion of preferred stock to common stock	249,837		$ 21,678	$ 9			249,828		$ 21,678
Repayment of related-party promissory notes associated with vested shares	854						854
Forfeiture of related-party promissory notes (in shares)				(370,734)
Conversion of Series D preferred stock warrants to Class A common warrants	1,160						1,160
Issuance of common stock upon Merger, net of transaction costs and merger warrant liability (in shares)				23,892,244
Issuance of common stock upon Merger, net of transaction costs of $16.2 million	129,659			$ 2			129,657
Issuance of PIPE shares (in shares)					7,500,000
Issuance of PIPE shares		$ 75,000			$ 1			$ 74,999
Issuance of earn-out shares to common stockholders (in shares)				14,153,520
Issuance of earn-out shares to common stockholders	1			$ 1
Exercise of vested stock options (in shares)				37,887
Exercise of vested stock options	80						80
Vesting of early-exercised stock options	54						54
Warrant expense in connection with Merger	154						154
Stock-based compensation	34,230						34,230
Other comprehensive loss	(61)									(61)
Net loss	(51,404)										(51,404)
Ending balance (in shares) at Mar. 31, 2021				191,351,403
Ending balance at Mar. 31, 2021	$ 292,466			$ 18			515,216			(72)	(222,696)
Mezzanine equity, beginning balance (in shares) at Dec. 31, 2020	93,328,118											93,328,118	0						206,031,290	0				(112,703,172)
Mezzanine equity, beginning balance at Dec. 31, 2020	$ 249,962											$ 249,962	$ 0						$ 249,962	$ 0
Mezzanine equity, ending balance (in shares) at Sep. 30, 2021	0											0	0
Mezzanine equity, ending balance at Sep. 30, 2021	$ 0											$ 0	$ 0
Beginning balance (in shares) at Dec. 31, 2020				52,967,106											116,930,602							(63,963,496)
Beginning balance at Dec. 31, 2020	(146,874)			$ 5			24,424			(11)	(171,292)			$ (146,874)	$ 0	$ 24,429	$ (11)	$ (171,292)			$ 0	$ 5	$ (5)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Other comprehensive loss	(41)
Net loss	(76,498)
Ending balance (in shares) at Sep. 30, 2021				203,817,249
Ending balance at Sep. 30, 2021	355,153			$ 20			602,975			(52)	(247,790)
Mezzanine equity, beginning balance (in shares) at Mar. 31, 2021												0	0
Mezzanine equity, beginning balance at Mar. 31, 2021												$ 0	$ 0
Mezzanine equity, ending balance (in shares) at Jun. 30, 2021												0	0
Mezzanine equity, ending balance at Jun. 30, 2021												$ 0	$ 0
Beginning balance (in shares) at Mar. 31, 2021				191,351,403
Beginning balance at Mar. 31, 2021	292,466			$ 18			515,216			(72)	(222,696)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of preferred stock to common stock (in shares)						88
Conversion of preferred stock to common stock			1						1
Issuance of common stock upon vesting of RSUs, net of tax withholdings (in shares)				725,740
Issuance of common stock upon vesting of RSUs, net of tax withholdings	(1,958)			$ 1			(1,959)
Issuance of common stock upon Merger, net of transaction costs and merger warrant liability (in shares)				250,000
Issuance of common stock for acquisition of business (in shares)				624,880
Issuance of common stock for acquisition of business	1,949						1,949
Exercise of vested stock options (in shares)				294,374
Exercise of vested stock options	178						178
Vesting of early exercised stock options, net of cancellations (in shares)				(2,643)
Vesting of early-exercised stock options	48						48
Stock-based compensation	9,491						9,491
Other comprehensive loss	32									32
Net loss	(9,153)										(9,153)
Ending balance (in shares) at Jun. 30, 2021				193,243,842
Ending balance at Jun. 30, 2021	$ 293,054			$ 19			524,924			(40)	(231,849)
Mezzanine equity, ending balance (in shares) at Sep. 30, 2021	0											0	0
Mezzanine equity, ending balance at Sep. 30, 2021	$ 0											$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of preferred stock to common stock (in shares)						1,958,615
Conversion of preferred stock to common stock			$ 16,967						$ 16,967
Issuance of common stock upon vesting of RSUs, net of tax withholdings (in shares)				159,247
Issuance of common stock upon vesting of RSUs, net of tax withholdings	(651)						(651)
Issuance of common stock for acquisition of business (in shares)				8,074,935
Issuance of common stock for acquisition of business	50,665			$ 1			50,664
Exercise of vested stock options (in shares)				380,779
Exercise of vested stock options	313						313
Vesting of early exercised stock options, net of cancellations (in shares)				(169)
Vesting of early-exercised stock options	41						41
Stock-based compensation	10,717						10,717
Other comprehensive loss	(12)									(12)
Net loss	(15,941)										(15,941)
Ending balance (in shares) at Sep. 30, 2021				203,817,249
Ending balance at Sep. 30, 2021	$ 355,153			$ 20			$ 602,975			$ (52)	$ (247,790)