CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' (DEFICIT) EQUITY (Unaudited) - USD ($) $ in Thousands	Total	IPO	Redeemable Convertible Preferred Stock	Common Stock	Common Stock IPO	Additional Paid-in Capital	Additional Paid-in Capital IPO	Accumulated Deficit
Temporary Equity, Balance at Dec. 31, 2018			$ 519,100
Temporary Equity, Balance, shares at Dec. 31, 2018			66,825,300
Balance at Dec. 31, 2018	$ (296,866)			$ 8				$ (296,874)
Balance (in shares) at Dec. 31, 2018				8,571,386
Stock-based compensation	869					$ 869
Exercise of stock options	347					347
Exercise of stock options (in shares)				101,950
Repurchase of common stock	(542)					(1,216)		674
Repurchase of common stock (in shares)				(93,186)
Accretion of redeemable convertible preferred stock	(17,964)							(17,964)
Temporary equity, Accretion of redeemable convertible preferred stock			$ 17,964
Net loss	(27,139)							(27,139)
Temporary Equity, Balance at Mar. 31, 2019			$ 537,064
Temporary Equity, Balance (in shares) at Mar. 31, 2019			66,825,300
Balance at Mar. 31, 2019	(341,295)			$ 8				(341,303)
Balance (in shares) at Mar. 31, 2019				8,580,150
Temporary Equity, Balance at Dec. 31, 2018			$ 519,100
Temporary Equity, Balance, shares at Dec. 31, 2018			66,825,300
Balance at Dec. 31, 2018	(296,866)			$ 8				(296,874)
Balance (in shares) at Dec. 31, 2018				8,571,386
Net loss	(100,243)
Temporary Equity, Balance at Sep. 30, 2019			$ 628,629
Temporary Equity, Balance (in shares) at Sep. 30, 2019			66,825,300
Balance at Sep. 30, 2019	(504,660)			$ 8				(504,668)
Balance (in shares) at Sep. 30, 2019				8,620,922
Temporary Equity, Balance at Dec. 31, 2018			$ 519,100
Temporary Equity, Balance, shares at Dec. 31, 2018			66,825,300
Balance at Dec. 31, 2018	(296,866)			$ 8				(296,874)
Balance (in shares) at Dec. 31, 2018				8,571,386
Temporary equity, Accretion of redeemable convertible preferred stock	132,800
Temporary Equity, Balance at Dec. 31, 2019	$ 874,332		$ 874,332
Temporary Equity, Balance (in shares) at Dec. 31, 2019	83,568,628		83,568,628
Balance at Dec. 31, 2019	$ (573,852)			$ 8				(573,860)
Balance (in shares) at Dec. 31, 2019				8,650,922
Temporary Equity, Balance at Mar. 31, 2019			$ 537,064
Temporary Equity, Balance, shares at Mar. 31, 2019			66,825,300
Balance at Mar. 31, 2019	(341,295)			$ 8				(341,303)
Balance (in shares) at Mar. 31, 2019				8,580,150
Stock-based compensation	667					667
Repurchase of common stock						(667)		667
Accretion of redeemable convertible preferred stock	(25,879)							(25,879)
Temporary equity, Accretion of redeemable convertible preferred stock			$ 25,879
Net loss	(33,340)							(33,340)
Temporary Equity, Balance at Jun. 30, 2019			$ 562,943
Temporary Equity, Balance (in shares) at Jun. 30, 2019			66,825,300
Balance at Jun. 30, 2019	(399,847)			$ 8				(399,855)
Balance (in shares) at Jun. 30, 2019				8,580,150
Stock-based compensation	619					619
Exercise of stock options	18					18
Exercise of stock options (in shares)				4,000
Vesting of restricted stock units (in shares)				36,772
Repurchase of common stock						(637)		637
Accretion of redeemable convertible preferred stock	(65,686)							(65,686)
Temporary equity, Accretion of redeemable convertible preferred stock			$ 65,686
Net loss	(39,764)							(39,764)
Temporary Equity, Balance at Sep. 30, 2019			$ 628,629
Temporary Equity, Balance (in shares) at Sep. 30, 2019			66,825,300
Balance at Sep. 30, 2019	(504,660)			$ 8				(504,668)
Balance (in shares) at Sep. 30, 2019				8,620,922
Temporary Equity, Balance at Dec. 31, 2019	$ 874,332		$ 874,332
Temporary Equity, Balance, shares at Dec. 31, 2019	83,568,628		83,568,628
Balance at Dec. 31, 2019	$ (573,852)			$ 8				(573,860)
Balance (in shares) at Dec. 31, 2019				8,650,922
Stock-based compensation	600					600
Exercise of stock options	6					6
Exercise of stock options (in shares)				2,774
Repurchase of common stock	(1,818)					(606)		(1,212)
Repurchase of common stock (in shares)				(200,000)
Issuance of Series H redeemable convertible preferred stock, net of issuance costs			$ 26,714
Issuance of Series H redeemable convertible preferred stock, net of issuance costs, shares			1,964,766
Net loss	(41,059)							(41,059)
Temporary Equity, Balance at Mar. 31, 2020			$ 901,046
Temporary Equity, Balance (in shares) at Mar. 31, 2020			85,533,394
Balance at Mar. 31, 2020	(616,123)			$ 8				(616,131)
Balance (in shares) at Mar. 31, 2020				8,453,696
Temporary Equity, Balance at Dec. 31, 2019	$ 874,332		$ 874,332
Temporary Equity, Balance, shares at Dec. 31, 2019	83,568,628		83,568,628
Balance at Dec. 31, 2019	$ (573,852)			$ 8				(573,860)
Balance (in shares) at Dec. 31, 2019				8,650,922
Exercise of stock options (in shares)	34,374
Net loss	$ (142,137)
Temporary Equity, Balance (in shares) at Sep. 30, 2020	0
Balance at Sep. 30, 2020	$ 1,277,850			$ 130		1,994,929		(717,209)
Balance (in shares) at Sep. 30, 2020				130,230,591
Temporary Equity, Balance at Mar. 31, 2020			$ 901,046
Temporary Equity, Balance, shares at Mar. 31, 2020			85,533,394
Balance at Mar. 31, 2020	(616,123)			$ 8				(616,131)
Balance (in shares) at Mar. 31, 2020				8,453,696
Issuance of common stock, net of offering costs	2,127	$ 496,510			$ 24	2,127	$ 496,486
Issuance of common stock, net of offering costs (in shares)				183,870	24,437,500
Conversion of redeemable convertible preferred stock to common stock	901,046			$ 86		900,960
Temporary equity, Conversion of redeemable convertible preferred stock to common stock (in shares)			(85,533,394)
Temporary equity, Conversion of redeemable convertible preferred stock to common stock			$ (901,046)
Conversion of redeemable convertible preferred stock to common stock (in shares)				85,533,394
Conversion of redeemable convertible preferred stock warrant to common stock warrant	21,873					21,873
Stock-based compensation	4,100					4,100
Exercise of stock options	7					7
Exercise of stock options (in shares)				500
Exercise of common stock warrants	1			$ 1
Exercise of common stock warrants (in shares)				636,112
Vesting of restricted stock units (in shares)				133,334
Common stock shares withheld to satisfy employee tax withholding obligations	(878)					(878)
Common stock shares withheld to satisfy employee tax withholding obligations (in shares)				(41,818)
Net loss	(63,228)							(63,228)
Balance at Jun. 30, 2020	745,435			$ 119		1,424,675		(679,359)
Balance (in shares) at Jun. 30, 2020				119,336,588
Issuance of common stock, net of offering costs		$ 567,952			$ 11		$ 567,941
Issuance of common stock, net of offering costs (in shares)					10,800,000
Stock-based compensation	4,230					4,230
Exercise of stock options	120					120
Exercise of stock options (in shares)				31,100
Vesting of restricted stock units (in shares)				104,000
Common stock shares withheld to satisfy employee tax withholding obligations	(2,037)					(2,037)
Common stock shares withheld to satisfy employee tax withholding obligations (in shares)				(41,097)
Net loss	$ (37,850)							(37,850)
Temporary Equity, Balance (in shares) at Sep. 30, 2020	0
Balance at Sep. 30, 2020	$ 1,277,850			$ 130		$ 1,994,929		$ (717,209)
Balance (in shares) at Sep. 30, 2020				130,230,591