CONDENSED CONSOLIDATED STATEMENTS OF REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Accumulated Deficit
Balance (in shares) at Dec. 31, 2018	308,373
Balance at Dec. 31, 2018	$ 1,465,399
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of redeemable convertible preferred stock and redeemable convertible preferred stock warrants to common stock in connection with initial public offering (in shares)	(308,373)
Conversion of redeemable convertible preferred stock and redeemable convertible preferred stock warrants to common stock in connection with initial public offering	$ (1,465,399)
Balance (in shares) at Sep. 30, 2019	0
Balance at Sep. 30, 2019	$ 0
Beginning balance (in shares) at Dec. 31, 2018		127,298
Beginning balance at Dec. 31, 2018	(594,563)	$ 1	$ 252,212	$ (1,421)	$ (845,355)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Release of restricted stock units (in shares)		25,108
Release of restricted stock units	0
Shares repurchased for tax withholdings on release of restricted stock units	(424,965)		(424,965)
Conversion of redeemable convertible preferred stock and redeemable convertible preferred stock warrants to common stock in connection with initial public offering (in shares)		308,622
Conversion of redeemable convertible preferred stock and redeemable convertible preferred stock warrants to common stock in connection with initial public offering	1,470,077	$ 3	1,470,074
Issuance of common stock in connection with initial public offering net of underwriters' discounts and commissions and offering costs (in shares)		86,250
Issuance of common stock in connection with initial public offering net of underwriters' discounts and commissions and offering costs	1,563,383	$ 1	1,563,382
Issuance of common stock for cash upon exercise of stock options, net (in shares)		287
Issuance of common stock for cash upon exercise of stock options, net	744		744
Share-based compensation	1,265,581		1,265,581
Other comprehensive income (loss)	1,797			1,797
Net loss	(1,325,653)				(1,325,653)
Ending balance (in shares) at Sep. 30, 2019		547,565
Ending balance at Sep. 30, 2019	$ 1,956,401	$ 5	4,127,028	376	(2,171,008)
Balance (in shares) at Jun. 30, 2019	0
Balance at Jun. 30, 2019	$ 0
Balance (in shares) at Sep. 30, 2019	0
Balance at Sep. 30, 2019	$ 0
Beginning balance (in shares) at Jun. 30, 2019		542,704
Beginning balance at Jun. 30, 2019	2,073,240	$ 5	4,118,988	523	(2,046,276)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Release of restricted stock units (in shares)		4,851
Release of restricted stock units	0
Shares repurchased for tax withholdings on release of restricted stock units	(122,290)		(122,290)
Issuance of common stock for cash upon exercise of stock options, net (in shares)		10
Issuance of common stock for cash upon exercise of stock options, net	42		42
Share-based compensation	130,288		130,288
Other comprehensive income (loss)	(147)			(147)
Net loss	(124,732)				(124,732)
Ending balance (in shares) at Sep. 30, 2019		547,565
Ending balance at Sep. 30, 2019	$ 1,956,401	$ 5	4,127,028	376	(2,171,008)
Balance (in shares) at Dec. 31, 2019	0
Balance at Dec. 31, 2019	$ 0
Balance (in shares) at Sep. 30, 2020	0
Balance at Sep. 30, 2020	$ 0
Beginning balance (in shares) at Dec. 31, 2019		569,904
Beginning balance at Dec. 31, 2019	2,023,705	$ 6	4,229,778	647	(2,206,726)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Release of restricted stock units (in shares)		13,785
Release of restricted stock units	0
Shares repurchased for tax withholdings on release of restricted stock units	$ (56,894)		(56,894)
Issuance of common stock for cash upon exercise of stock options, net (in shares)	28,855	28,855
Issuance of common stock for cash upon exercise of stock options, net	$ 64,992		64,992
Issuance of common stock related to charitable contributions (in shares)		150
Issuance of common stock related to charitable contributions	2,748		2,748
Issuance of restricted stock awards (in shares)		2,549
Issuance of restricted stock awards	0
Share-based compensation	234,801		234,801
Other comprehensive income (loss)	1,416			1,416
Net loss	(336,164)				(336,164)
Ending balance (in shares) at Sep. 30, 2020		615,243
Ending balance at Sep. 30, 2020	$ 1,934,604	$ 6	4,475,425	2,063	(2,542,890)
Balance (in shares) at Jun. 30, 2020	0
Balance at Jun. 30, 2020	$ 0
Balance (in shares) at Sep. 30, 2020	0
Balance at Sep. 30, 2020	$ 0
Beginning balance (in shares) at Jun. 30, 2020		596,264
Beginning balance at Jun. 30, 2020	1,906,630	$ 6	4,351,557	3,737	(2,448,670)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Release of restricted stock units (in shares)		5,642
Release of restricted stock units	0
Shares repurchased for tax withholdings on release of restricted stock units	(7)		(7)
Issuance of common stock for cash upon exercise of stock options, net (in shares)		13,337
Issuance of common stock for cash upon exercise of stock options, net	32,243		32,243
Share-based compensation	91,632		91,632
Other comprehensive income (loss)	(1,674)			(1,674)
Net loss	(94,220)				(94,220)
Ending balance (in shares) at Sep. 30, 2020		615,243
Ending balance at Sep. 30, 2020	$ 1,934,604	$ 6	$ 4,475,425	$ 2,063	$ (2,542,890)