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 Jun. 30, 2019	0
Balance at Jun. 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)		20,257
Release of restricted stock units	0
Shares repurchased for tax withholdings on release of restricted stock units	(302,675)		(302,675)
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)		277
Issuance of common stock for cash upon exercise of stock options, net	702		702
Share-based compensation	1,135,293		1,135,293
Other comprehensive income (loss)	1,944			1,944
Net loss	(1,200,921)				(1,200,921)
Ending balance (in shares) at Jun. 30, 2019		542,704
Ending balance at Jun. 30, 2019	$ 2,073,240	$ 5	4,118,988	523	(2,046,276)
Balance (in shares) at Mar. 31, 2019	308,373
Balance at Mar. 31, 2019	$ 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 Jun. 30, 2019	0
Balance at Jun. 30, 2019	$ 0
Beginning balance (in shares) at Mar. 31, 2019		127,371
Beginning balance at Mar. 31, 2019	(633,989)	$ 1	253,016	(231)	(886,775)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Release of restricted stock units (in shares)		20,257
Release of restricted stock units	0
Shares repurchased for tax withholdings on release of restricted stock units	(302,675)		(302,675)
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)		204
Issuance of common stock for cash upon exercise of stock options, net	592		592
Share-based compensation	1,134,599		1,134,599
Other comprehensive income (loss)	754			754
Net loss	(1,159,501)				(1,159,501)
Ending balance (in shares) at Jun. 30, 2019		542,704
Ending balance at Jun. 30, 2019	$ 2,073,240	$ 5	4,118,988	523	(2,046,276)
Balance (in shares) at Dec. 31, 2019	0
Balance at Dec. 31, 2019	$ 0
Balance (in shares) at Jun. 30, 2020	0
Balance at Jun. 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)		8,143
Release of restricted stock units	0
Shares repurchased for tax withholdings on release of restricted stock units	$ (56,887)		(56,887)
Issuance of common stock for cash upon exercise of stock options, net (in shares)	15,518	15,518
Issuance of common stock for cash upon exercise of stock options, net	$ 32,749		32,749
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	143,169		143,169
Other comprehensive income (loss)	3,090			3,090
Net loss	(241,944)				(241,944)
Ending balance (in shares) at Jun. 30, 2020		596,264
Ending balance at Jun. 30, 2020	$ 1,906,630	$ 6	4,351,557	3,737	(2,448,670)
Balance (in shares) at Mar. 31, 2020	0
Balance at Mar. 31, 2020	$ 0
Balance (in shares) at Jun. 30, 2020	0
Balance at Jun. 30, 2020	$ 0
Beginning balance (in shares) at Mar. 31, 2020		581,840
Beginning balance at Mar. 31, 2020	1,938,480	$ 6	4,288,603	(2,207)	(2,347,922)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Release of restricted stock units (in shares)		5,230
Release of restricted stock units	0
Shares repurchased for tax withholdings on release of restricted stock units	(12,797)		(12,797)
Issuance of common stock for cash upon exercise of stock options, net (in shares)		6,595
Issuance of common stock for cash upon exercise of stock options, net	12,402		12,402
Issuance of common stock related to charitable contributions (in shares)		50
Issuance of common stock related to charitable contributions	1,204		1,204
Issuance of restricted stock awards (in shares)		2,549
Issuance of restricted stock awards	0
Share-based compensation	62,145		62,145
Other comprehensive income (loss)	5,944			5,944
Net loss	(100,748)				(100,748)
Ending balance (in shares) at Jun. 30, 2020		596,264
Ending balance at Jun. 30, 2020	$ 1,906,630	$ 6	$ 4,351,557	$ 3,737	$ (2,448,670)