Condensed Consolidated Statement of Redeemable Convertible Preferred Stock and Stockholder's Deficit - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Series Seed Redeemable Convertible Preferred Stock	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock
Beginning balance at Dec. 31, 2018	$ (79,750)	$ 1	$ 3,638		$ (83,389)
Redeemable convertible preferred stock, Beginning balance, Shares at Dec. 31, 2018						4,000,000	109,145,900
Redeemable convertible preferred stock, Beginning balance at Dec. 31, 2018						$ 1,000	$ 109,995
Beginning balance, Shares at Dec. 31, 2018		1,338,956
Exercise of stock options	260		260
Exercise of stock options, Shares		87,928
Stock-based compensation	913		913
Repurchase of common stock	(20)		(20)
Repurchase of common stock, Shares		(6,112)
Net loss	(24,076)				(24,076)
Ending balance at Sep. 30, 2019	(102,673)	$ 1	4,791		(107,465)
Redeemable convertible preferred stock, Ending balance, Shares at Sep. 30, 2019						4,000,000	109,145,900
Redeemable convertible preferred stock, Ending balance at Sep. 30, 2019						$ 1,000	$ 109,995
Ending balance, Shares at Sep. 30, 2019		1,420,772
Beginning balance at Jun. 30, 2019	(93,409)	$ 1	3,957	$ 4	(97,371)
Redeemable convertible preferred stock, Beginning balance, Shares at Jun. 30, 2019						4,000,000	109,145,900
Redeemable convertible preferred stock, Beginning balance at Jun. 30, 2019						$ 1,000	$ 109,995
Beginning balance, Shares at Jun. 30, 2019		1,354,373
Exercise of stock options	213		213
Exercise of stock options, Shares		72,511
Stock-based compensation	627		627
Repurchase of common stock	(6)		(6)
Repurchase of common stock, Shares		(6,112)
Net unrealized gain on available- for-sale securities	(4)			(4)
Net loss	(10,094)				(10,094)
Ending balance at Sep. 30, 2019	(102,673)	$ 1	4,791		(107,465)
Redeemable convertible preferred stock, Ending balance, Shares at Sep. 30, 2019						4,000,000	109,145,900
Redeemable convertible preferred stock, Ending balance at Sep. 30, 2019						$ 1,000	$ 109,995
Ending balance, Shares at Sep. 30, 2019		1,420,772
Beginning balance at Dec. 31, 2019	$ (111,963)	$ 1	5,524		(117,488)
Redeemable convertible preferred stock, Beginning balance, Shares at Dec. 31, 2019	113,145,900					4,000,000	109,145,900	0
Redeemable convertible preferred stock, Beginning balance at Dec. 31, 2019	$ 110,995					$ 1,000	$ 109,995
Beginning balance, Shares at Dec. 31, 2019		1,426,641
Issuance of Series B convertible preferred stock								$ 145,525
Issuance of Series B convertible preferred stock, Shares								142,857,142
Exercise of stock options	795		795
Exercise of stock options, Shares		251,466
Stock-based compensation	713		713
Repurchase of common stock	(210)		(210)
Repurchase of common stock, Shares		(43,986)
Vested stock option settlement	(727)		(727)
Unrealized loss on investments	8			8
Net loss	(44,540)				(44,540)
Ending balance at Sep. 30, 2020	$ (155,924)	$ 1	6,095	8	(162,028)
Redeemable convertible preferred stock, Ending balance, Shares at Sep. 30, 2020	256,003,042					4,000,000	109,145,900	142,857,142
Redeemable convertible preferred stock, Ending balance at Sep. 30, 2020	$ 256,520					$ 1,000	$ 109,995	$ 145,525
Ending balance, Shares at Sep. 30, 2020		1,634,121
Beginning balance at Jun. 30, 2020	(135,065)	$ 1	5,129	(2)	(140,193)
Redeemable convertible preferred stock, Beginning balance, Shares at Jun. 30, 2020						4,000,000	109,145,900	138,571,428
Redeemable convertible preferred stock, Beginning balance at Jun. 30, 2020						$ 1,000	$ 109,995	$ 141,026
Beginning balance, Shares at Jun. 30, 2020		1,490,336
Issuance of Series B convertible preferred stock								$ 4,499
Issuance of Series B convertible preferred stock, Shares								4,285,714
Exercise of stock options	530		530
Exercise of stock options, Shares		164,057
Stock-based compensation	436		436
Repurchase of common stock, Shares		(20,272)
Unrealized loss on investments	10			10
Net loss	(21,835)				(21,835)
Ending balance at Sep. 30, 2020	$ (155,924)	$ 1	$ 6,095	$ 8	$ (162,028)
Redeemable convertible preferred stock, Ending balance, Shares at Sep. 30, 2020	256,003,042					4,000,000	109,145,900	142,857,142
Redeemable convertible preferred stock, Ending balance at Sep. 30, 2020	$ 256,520					$ 1,000	$ 109,995	$ 145,525
Ending balance, Shares at Sep. 30, 2020		1,634,121