CONDENSED CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Conversion of Class B to Class A common stock	Additional paid-in capital	Accumulated deficit	Accumulated other comprehensive income (loss)	Redeemable convertible preferred stock	Class A common stock	Class A common stock Common stock	Class A common stock Common stock Conversion of Class B to Class A common stock	Class B common stock	Class B common stock Common stock	Class B common stock Common stock Conversion of Class B to Class A common stock
Beginning balance (in shares) at Dec. 31, 2018						165,658,000
Beginning balance at Dec. 31, 2018						$ 331,521
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)						(165,658,000)
Conversion of redeemable convertible preferred stock to common stock upon initial offering						$ (331,521)
Ending balance (in shares) at Sep. 30, 2019						0
Ending balance at Sep. 30, 2019						$ 0
Beginning balance (in shares) at Dec. 31, 2018								0			91,542,000
Beginning balance at Dec. 31, 2018	$ (113,505)		$ 82,345	$ (195,878)	$ (57)			$ 0			$ 85
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon initial public offering, net of underwriting discounts and issuance costs (in shares)								40,250,000
Issuance of common stock upon initial public offering, net of underwriting discounts and issuance costs	565,041		565,001					$ 40
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)								31,381,000			134,277,000
Conversion of redeemable convertible preferred stock to common stock upon initial offering	331,521		331,355					$ 31			$ 135
Conversion of redeemable convertible preferred stock warrants into common stock warrants and issuance of common stock upon net exercise of common stock warrants (in shares)											174,000
Conversion of redeemable convertible preferred stock warrants into common stock warrants and issuance of common stock upon net exercise of common stock warrants	3,135		3,135
Issuance of common stock in connection with acquisition (in shares)											7,000
Issuance of common stock in connection with acquisition	18		18
Issuance of common stock upon exercise of stock options (in shares)								9,000			1,689,000
Issuance of common stock upon exercise of stock options	2,902		2,899					$ 1			$ 2
Repurchases of unvested common stock (in shares)											(73,000)
Issuance of common stock related to early exercised stock options (in shares)											891,000
Vesting of shares issued upon early exercise of stock options	2,389		2,389
Conversion of Class B to Class A common stock (in shares)									(15,199,000)			(15,199,000)
Conversion of Class B to Class A common stock		$ 0							$ 15			$ (15)
Stock-based compensation	26,440		26,440
Net loss	(77,673)			(77,673)				$ (3,588)			$ (74,085)
Other comprehensive income (loss)	130				130
Ending balance (in shares) at Sep. 30, 2019								86,839,000			213,308,000
Ending balance at Sep. 30, 2019	740,398		1,013,582	(273,551)	73			$ 87			$ 207
Beginning balance (in shares) at Dec. 31, 2018						165,658,000
Beginning balance at Dec. 31, 2018						$ 331,521
Ending balance (in shares) at Dec. 31, 2019						0
Ending balance at Dec. 31, 2019						$ 0
Beginning balance (in shares) at Dec. 31, 2018								0			91,542,000
Beginning balance at Dec. 31, 2018	(113,505)		82,345	(195,878)	(57)			$ 0			$ 85
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of redeemable convertible preferred stock warrants into common stock warrants and issuance of common stock upon net exercise of common stock warrants (in shares)										174,347
Ending balance (in shares) at Dec. 31, 2019							87,071,783	87,072,000		213,101,364	213,101,000
Ending balance at Dec. 31, 2019	725,828		1,027,179	(301,706)	61			$ 87			$ 207
Beginning balance (in shares) at Jun. 30, 2019						165,658,000
Beginning balance at Jun. 30, 2019						$ 331,521
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)						(165,658,000)
Conversion of redeemable convertible preferred stock to common stock upon initial offering						$ (331,521)
Ending balance (in shares) at Sep. 30, 2019						0
Ending balance at Sep. 30, 2019						$ 0
Beginning balance (in shares) at Jun. 30, 2019								0			92,686,000
Beginning balance at Jun. 30, 2019	(145,435)		87,111	(232,698)	66			$ 0			$ 86
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon initial public offering, net of underwriting discounts and issuance costs (in shares)								40,250,000
Issuance of common stock upon initial public offering, net of underwriting discounts and issuance costs	565,041		565,001					$ 40
Conversion of redeemable convertible preferred stock to common stock upon initial public offering (in shares)								31,381,000			134,277,000
Conversion of redeemable convertible preferred stock to common stock upon initial offering	331,521		331,355					$ 31			$ 135
Conversion of redeemable convertible preferred stock warrants into common stock warrants and issuance of common stock upon net exercise of common stock warrants (in shares)											174,000
Conversion of redeemable convertible preferred stock warrants into common stock warrants and issuance of common stock upon net exercise of common stock warrants	3,135		3,135
Issuance of common stock upon exercise of stock options (in shares)								9,000			1,108,000
Issuance of common stock upon exercise of stock options	1,835		1,833	0				$ 1			$ 1
Repurchases of unvested common stock (in shares)											(33,000)
Issuance of common stock related to early exercised stock options (in shares)											295,000
Vesting of shares issued upon early exercise of stock options	951		951
Conversion of Class B to Class A common stock (in shares)									(15,199,000)			(15,199,000)
Conversion of Class B to Class A common stock		0							$ 15			$ (15)
Stock-based compensation	24,196		24,196
Net loss	(40,853)			(40,853)				$ (4,573)			$ (36,280)
Other comprehensive income (loss)	7				7
Ending balance (in shares) at Sep. 30, 2019								86,839,000			213,308,000
Ending balance at Sep. 30, 2019	740,398		1,013,582	(273,551)	73			$ 87			$ 207
Beginning balance (in shares) at Dec. 31, 2019						0
Beginning balance at Dec. 31, 2019						$ 0
Ending balance (in shares) at Sep. 30, 2020						0
Ending balance at Sep. 30, 2020						$ 0
Beginning balance (in shares) at Dec. 31, 2019							87,071,783	87,072,000		213,101,364	213,101,000
Beginning balance at Dec. 31, 2019	$ 725,828		1,027,179	(301,706)	61			$ 87			$ 207
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock in connection with acquisition (in shares)	948,000							107,000
Issuance of common stock in connection with acquisition	$ 1,821		1,821
Issuance of unvested restricted stock in connection with acquisition (in shares)								841,000
Issuance of common stock upon exercise of stock options (in shares)	3,587,000										3,509,000
Issuance of common stock upon exercise of stock options	$ 5,725		5,721								$ 4
Repurchases of unvested common stock (in shares)								(39,000)			0
Issuance of common stock related to early exercised stock options (in shares)											78,000
Vesting of shares issued upon early exercise of stock options	3,651		3,649								$ 2
Issuance of common stock related to settlement of RSUs (in shares)								279,000			2,094,000
Issuance of common stock related to settlement of RSUs	0		(2)								$ 2
Tax withholding on RSU settlement (in shares)								(11,000)			(410,000)
Tax withholding on RSU settlement	(7,606)		(7,606)
Conversion of Class B to Class A common stock (in shares)									(144,852,000)			(144,852,000)
Conversion of Class B to Class A common stock		0							$ 146			$ (146)
Equity component of convertible senior notes, net of issuance costs	200,812		200,812
Purchases of capped calls related to convertible senior notes	(67,333)		(67,333)
Common stock issued under employee stock purchase plan (in shares)								421,000
Common stock issued under employee stock purchase plan	5,447		5,447
Tax withholding on common stock issued under employee stock purchase plan (in shares)								(13,000)
Tax withholding on common stock issued under employee stock purchase plan	(376)		(376)
Stock-based compensation	42,762		42,762
Net loss	(85,349)			(85,349)				$ (45,059)			$ (40,290)
Other comprehensive income (loss)	509				509
Ending balance (in shares) at Sep. 30, 2020							233,509,363	233,509,000		73,520,225	73,520,000
Ending balance at Sep. 30, 2020	826,447		1,212,074	(386,499)	570			$ 233			$ 69
Beginning balance (in shares) at Jun. 30, 2020						0
Beginning balance at Jun. 30, 2020						$ 0
Ending balance (in shares) at Sep. 30, 2020						0
Ending balance at Sep. 30, 2020						$ 0
Beginning balance (in shares) at Jun. 30, 2020								184,900,000			120,524,000
Beginning balance at Jun. 30, 2020	835,618		1,194,125	(360,031)	1,224			$ 184			$ 116
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)											927,000
Issuance of common stock upon exercise of stock options	1,390		1,388								$ 2
Repurchases of unvested common stock (in shares)								(2,000)
Issuance of common stock related to early exercised stock options (in shares)											42,000
Vesting of shares issued upon early exercise of stock options	1,140		1,140								$ 0
Issuance of common stock related to settlement of RSUs (in shares)								197,000			449,000
Issuance of common stock related to settlement of RSUs	(1)		(1)								$ 0
Tax withholding on RSU settlement (in shares)											(8,000)
Tax withholding on RSU settlement	(299)		(299)
Conversion of Class B to Class A common stock (in shares)									(48,414,000)			(48,414,000)
Conversion of Class B to Class A common stock		$ 0							$ 49			$ (49)
Stock-based compensation	15,721		15,721
Net loss	(26,468)			(26,468)				$ (18,119)			$ (8,349)
Other comprehensive income (loss)	(654)				(654)
Ending balance (in shares) at Sep. 30, 2020							233,509,363	233,509,000		73,520,225	73,520,000
Ending balance at Sep. 30, 2020	$ 826,447		$ 1,212,074	$ (386,499)	$ 570			$ 233			$ 69