Condensed Consolidated Statements of Changes in Redeemable Convertible Preferred Stock and Stockholders’ Deficit - USD ($) $ in Thousands	Total	Common Stock	Common Stock Class A and Class B Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning balance, temporary equity (in shares) at Dec. 31, 2019	42,769,000
Beginning balance, temporary equity at Dec. 31, 2019	$ 262,849
Ending balance, temporary equity (in shares) at Mar. 31, 2020	42,769,000
Ending balance, temporary equity at Mar. 31, 2020	$ 262,849
Beginning balance (in shares) at Dec. 31, 2019		52,627,000
Beginning balance at Dec. 31, 2019	(189,453)	$ 5		$ 79,991	$ 22	$ (269,471)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises (in shares)		29,000
Stock option exercises	128			128
Stock-based compensation	649			649
Other comprehensive loss	(261)				(261)
Net loss	2,778					2,778
Ending balance (in shares) at Mar. 31, 2020		52,656,000
Ending balance at Mar. 31, 2020	$ (186,159)	$ 5		80,768	(239)	(266,693)
Beginning balance, temporary equity (in shares) at Dec. 31, 2019	42,769,000
Beginning balance, temporary equity at Dec. 31, 2019	$ 262,849
Ending balance, temporary equity (in shares) at Sep. 30, 2020	54,042,000
Ending balance, temporary equity at Sep. 30, 2020	$ 506,535
Beginning balance (in shares) at Dec. 31, 2019		52,627,000
Beginning balance at Dec. 31, 2019	$ (189,453)	$ 5		79,991	22	(269,471)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Tender offer repurchase and share retirement (in shares)	0
Net loss	$ (51,619)
Ending balance (in shares) at Sep. 30, 2020		53,596,000
Ending balance at Sep. 30, 2020	$ (195,492)	$ 5		125,650	(56)	(321,091)
Beginning balance, temporary equity (in shares) at Mar. 31, 2020	42,769,000
Beginning balance, temporary equity at Mar. 31, 2020	$ 262,849
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Preferred Stock, net of issuance costs (in shares)	6,413,000
Issuance of Preferred stock, net of issuance costs	$ 124,717
Ending balance, temporary equity (in shares) at Jun. 30, 2020	49,182,000
Ending balance, temporary equity at Jun. 30, 2020	$ 387,566
Beginning balance (in shares) at Mar. 31, 2020		52,656,000
Beginning balance at Mar. 31, 2020	(186,159)	$ 5		80,768	(239)	(266,693)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation	724			724
Other comprehensive loss	116				116
Net loss	(12,786)					(12,786)
Ending balance (in shares) at Jun. 30, 2020		52,656,000
Ending balance at Jun. 30, 2020	$ (198,105)	$ 5		81,492	(123)	(279,479)
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Preferred Stock, net of issuance costs (in shares)	4,860,000
Issuance of Preferred stock, net of issuance costs	$ 118,969
Ending balance, temporary equity (in shares) at Sep. 30, 2020	54,042,000
Ending balance, temporary equity at Sep. 30, 2020	$ 506,535
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises (in shares)		597,000
Stock option exercises	836			836
Stock-based compensation	42,377			42,377
Proceeds from repayment of related party loans (in shares)		343,000
Proceeds from repayment of related party loans	945			945
Other comprehensive loss	67				67
Net loss	(41,612)					(41,612)
Ending balance (in shares) at Sep. 30, 2020		53,596,000
Ending balance at Sep. 30, 2020	$ (195,492)	$ 5		125,650	(56)	(321,091)
Beginning balance, temporary equity (in shares) at Dec. 31, 2020	54,041,904
Beginning balance, temporary equity at Dec. 31, 2020	$ 506,510
Increase (Decrease) in Temporary Equity [Roll Forward]
Stock repurchases, temporary equity (in shares)	(220,000)
Stock repurchases, temporary equity	$ (790)
Ending balance, temporary equity (in shares) at Mar. 31, 2021	53,822,000
Ending balance, temporary equity at Mar. 31, 2021	$ 505,720
Beginning balance (in shares) at Dec. 31, 2020		53,944,000
Beginning balance at Dec. 31, 2020	(198,097)	$ 5		127,179	109	(325,390)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises (in shares)		64,000
Stock option exercises	157			157
Stock repurchases (in shares)		(27,000)
Stock repurchases	(5,274)					(5,274)
Stock-based compensation	1,261			1,261
Other comprehensive loss	(224)				(224)
Net loss	3,010					3,010
Ending balance (in shares) at Mar. 31, 2021		53,981,000
Ending balance at Mar. 31, 2021	$ (199,167)	$ 5		128,597	(115)	(327,654)
Beginning balance, temporary equity (in shares) at Dec. 31, 2020	54,041,904
Beginning balance, temporary equity at Dec. 31, 2020	$ 506,510
Ending balance, temporary equity (in shares) at Sep. 30, 2021	0
Ending balance, temporary equity at Sep. 30, 2021	$ 0
Beginning balance (in shares) at Dec. 31, 2020		53,944,000
Beginning balance at Dec. 31, 2020	$ (198,097)	$ 5		127,179	109	(325,390)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises (in shares)	3,302,973
Net loss	$ (98,368)
Ending balance (in shares) at Sep. 30, 2021			111,392,000
Ending balance at Sep. 30, 2021	$ 291,556		$ 11	738,834	50	(447,339)
Beginning balance, temporary equity (in shares) at Mar. 31, 2021	53,822,000
Beginning balance, temporary equity at Mar. 31, 2021	$ 505,720
Increase (Decrease) in Temporary Equity [Roll Forward]
Stock repurchases, temporary equity (in shares)	(46,000)
Stock repurchases, temporary equity	$ (716)
Tender offer repurchase and share retirement, temporary equity (in shares)	(477,000)
Tender offer repurchase and share retirement, temporary equity	$ (3,561)
Ending balance, temporary equity (in shares) at Jun. 30, 2021	53,299,000
Ending balance, temporary equity at Jun. 30, 2021	$ 501,443
Beginning balance (in shares) at Mar. 31, 2021		53,981,000
Beginning balance at Mar. 31, 2021	(199,167)	$ 5		128,597	(115)	(327,654)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises (in shares)		402,000
Stock option exercises	$ 1,212			1,212
Restricted stock unit releases (in shares)	73,000
Stock repurchases (in shares)		(37,000)
Stock repurchases	$ (1,305)					(1,305)
Stock-based compensation	10,409			10,409
Proceeds from repayment of related party loans (in shares)		12,000
Proceeds from repayment of related party loans	44			44
Tender offer repurchase and share retirement (in shares)		(362,000)
Tender offer repurchase and share retirement	(17,002)					(17,002)
Other comprehensive loss	294				294
Net loss	(10,305)					(10,305)
Ending balance (in shares) at Jun. 30, 2021		54,069,000
Ending balance at Jun. 30, 2021	$ (215,820)	$ 5		140,262	179	(356,266)
Increase (Decrease) in Temporary Equity [Roll Forward]
Stock option and warrant exercises, temporary equity (in shares)	22,000
Stock option and warrant exercises, temporary equity	$ 75
Conversion of preferred stock, temporary equity (in shares)	(53,321,000)
Conversion of preferred stock, temporary equity	$ (501,518)
Ending balance, temporary equity (in shares) at Sep. 30, 2021	0
Ending balance, temporary equity at Sep. 30, 2021	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock option exercises (in shares)		465,000	189,000
Stock option exercises	10,326			10,326
Restricted stock unit releases (in shares)		55,000	937,000
Stock-based compensation	64,332			64,332
Proceeds from repayment of related party loans (in shares)		2,172,000
Proceeds from repayment of related party loans	14,644			14,644
Non-cash charitable contributions (in shares)		179,000
Non-cash charitable contributions	7,757			7,757
Other equity activity	0		$ 5
Conversion of common stock (in shares)		(56,940,000)	56,940,000
Conversion of common stock	0	$ (5)	$ 5
Conversion of preferred stock (in shares)			53,321,000
Conversion of preferred stock	501,519		$ 6	501,513
Other comprehensive loss	(129)				(129)
Net loss	(91,073)					(91,073)
Ending balance (in shares) at Sep. 30, 2021			111,392,000
Ending balance at Sep. 30, 2021	$ 291,556		$ 11	$ 738,834	$ 50	$ (447,339)