CONDENSED CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	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 B common stock	Class B common stock Common Stock
Beginning balance (in shares) at Dec. 31, 2017						152,022,000
Beginning balance at Dec. 31, 2017						$ 181,546
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series D redeemable convertible preferred stock, net of issuance costs of $25 (in shares)						13,636,000
Issuance of Series D redeemable convertible preferred stock, net of issuance costs of $25						$ 149,975
Ending balance (in shares) at Sep. 30, 2018						165,658,000
Ending balance at Sep. 30, 2018						$ 331,521
Beginning balance (in shares) at Dec. 31, 2017		79,116,000
Beginning balance at Dec. 31, 2017	$ (59,834)	$ 79	$ 48,907	$ (108,714)	$ (106)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		5,425,000
Issuance of common stock upon exercise of stock options	4,317	$ 5	4,312
Repurchases of unvested common stock (in shares)		(26,000)
Issuance of common stock related to early exercised stock options (in shares)		6,906,000
Vesting of shares issued upon early exercise of stock options	813		813
Vesting of restricted stock (in shares)		75,000
Vesting of restricted stock	3		3
Stock-based compensation	26,467		26,467
Net loss	(70,477)	$ (70,477)		(70,477)
Other comprehensive income (loss)	40				40
Ending balance (in shares) at Sep. 30, 2018		91,496,000
Ending balance at Sep. 30, 2018	(98,671)	$ 84	80,502	(179,191)	(66)
Beginning balance (in shares) at Jun. 30, 2018						152,022,000
Beginning balance at Jun. 30, 2018						$ 181,546
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of Series D redeemable convertible preferred stock, net of issuance costs of $25 (in shares)						13,636,000
Issuance of Series D redeemable convertible preferred stock, net of issuance costs of $25						$ 149,975
Ending balance (in shares) at Sep. 30, 2018						165,658,000
Ending balance at Sep. 30, 2018						$ 331,521
Beginning balance (in shares) at Jun. 30, 2018		80,879,000
Beginning balance at Jun. 30, 2018	(89,371)	$ 79	51,816	(141,202)	(64)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		4,870,000
Issuance of common stock upon exercise of stock options	3,721	$ 5	3,716
Repurchases of unvested common stock (in shares)		(2,000)
Issuance of common stock related to early exercised stock options (in shares)		5,674,000
Vesting of shares issued upon early exercise of stock options	447		447
Vesting of restricted stock (in shares)		75,000
Vesting of restricted stock	3		3
Stock-based compensation	24,520		24,520
Net loss	(37,989)	$ (37,989)		(37,989)
Other comprehensive income (loss)	(2)				(2)
Ending balance (in shares) at Sep. 30, 2018		91,496,000
Ending balance at Sep. 30, 2018	$ (98,671)	$ 84	80,502	(179,191)	(66)
Beginning balance (in shares) at Dec. 31, 2018	165,658,000					165,658,000
Beginning balance at Dec. 31, 2018	$ 331,521					$ 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 public offering						$ (331,521)
Ending balance (in shares) at Sep. 30, 2019	0					0
Ending balance at Sep. 30, 2019	$ 0					$ 0
Beginning balance (in shares) at Dec. 31, 2018							0	0	91,542,243	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 public 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)									174,347	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)	2,589,000							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							86,838,606	86,839,000	213,308,441	213,308,000
Ending balance at Sep. 30, 2019	$ 740,398		1,013,582	(273,551)	73			$ 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 public offering						$ (331,521)
Ending balance (in shares) at Sep. 30, 2019	0					0
Ending balance at Sep. 30, 2019	$ 0					$ 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 public 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)									174,347	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					$ 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							86,838,606	86,839,000	213,308,441	213,308,000
Ending balance at Sep. 30, 2019	$ 740,398		$ 1,013,582	$ (273,551)	$ 73			$ 87		$ 207