Condensed Consolidated Statements of Stockholder's Equity (Deficit) - USD ($) $ in Thousands	Total	Redeemable Convertible Preferred Stock	Accumulated Other Comprehensive Income (Loss)	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Beginning balance at Dec. 31, 2019	$ (111,962)			$ 1	$ 5,525	$ (117,488)
Beginning balance, Shares at Dec. 31, 2019				1,426,641
Redeemable convertible preferred stock, Beginning balance, Shares at Dec. 31, 2019		113,145,900
Redeemable convertible preferred stock, Beginning balance at Dec. 31, 2019		$ 110,995
Exercise of stock options	156				156
Exercise of stock options, Shares				49,031
Stock-based compensation	116				116
Repurchase of common stock	(202)				(202)
Vested stock option settlement	(728)				(728)
Net loss	(11,912)					(11,912)
Ending balance at Mar. 31, 2020	(124,532)			$ 1	4,867	(129,400)
Ending balance, Shares at Mar. 31, 2020				1,475,672
Redeemable convertible preferred stock, Ending balance, Shares at Mar. 31, 2020		113,145,900
Redeemable convertible preferred stock, Ending balance at Mar. 31, 2020		$ 110,995
Beginning balance at Dec. 31, 2019	(111,962)			$ 1	5,525	(117,488)
Beginning balance, Shares at Dec. 31, 2019				1,426,641
Redeemable convertible preferred stock, Beginning balance, Shares at Dec. 31, 2019		113,145,900
Redeemable convertible preferred stock, Beginning balance at Dec. 31, 2019		$ 110,995
Unrealized gain (loss) on marketable securities	8
Net loss	(44,540)
Ending balance at Sep. 30, 2020	(155,924)		$ 8	$ 1	6,095	(162,028)
Ending balance, Shares at Sep. 30, 2020				1,634,121
Redeemable convertible preferred stock, Ending balance, Shares at Sep. 30, 2020		256,003,042
Redeemable convertible preferred stock, Ending balance at Sep. 30, 2020		$ 256,520
Beginning balance at Mar. 31, 2020	(124,532)			$ 1	4,867	(129,400)
Beginning balance, Shares at Mar. 31, 2020				1,475,672
Redeemable convertible preferred stock, Beginning balance, Shares at Mar. 31, 2020		113,145,900
Redeemable convertible preferred stock, Beginning balance at Mar. 31, 2020		$ 110,995
Issuance of Series B redeemable convertible preferred stock		$ 141,026
Issuance of Series B redeemable convertible preferred stock, Shares		138,571,428
Exercise of stock options	109				109
Exercise of stock options, Shares				38,378
Stock-based compensation	161				161
Unrealized gain (loss) on marketable securities	(2)		(2)
Repurchase of common stock	78				78
Repurchase of common stock, Shares				(23,714)
Vested stock option settlement	(85)				(85)
Net loss	(10,793)					(10,793)
Ending balance at Jun. 30, 2020	(135,064)		(2)	$ 1	5,130	(140,193)
Ending balance, Shares at Jun. 30, 2020				1,490,336
Redeemable convertible preferred stock, Ending balance, Shares at Jun. 30, 2020		251,717,328
Redeemable convertible preferred stock, Ending balance at Jun. 30, 2020		$ 252,021
Issuance of Series B redeemable convertible preferred stock		$ 4,499
Issuance of Series B redeemable convertible preferred stock, Shares		4,285,714
Exercise of stock options	529				529
Exercise of stock options, Shares				164,057
Stock-based compensation	436				436
Unrealized gain (loss) on marketable securities	10		10
Repurchase of common stock, Shares				(20,272)
Net loss	(21,835)					(21,835)
Ending balance at Sep. 30, 2020	(155,924)		8	$ 1	6,095	(162,028)
Ending balance, Shares at Sep. 30, 2020				1,634,121
Redeemable convertible preferred stock, Ending balance, Shares at Sep. 30, 2020		256,003,042
Redeemable convertible preferred stock, Ending balance at Sep. 30, 2020		$ 256,520
Beginning balance at Dec. 31, 2020	280,791		13	$ 4	464,597	(183,823)
Beginning balance, Shares at Dec. 31, 2020				43,059,632
Exercise of stock options	166				166
Exercise of stock options, Shares				49,328
Stock-based compensation	3,845				3,845
Unrealized gain (loss) on marketable securities	(107)		(107)
Net loss	(20,971)					(20,971)
Other	(19)				(19)
Ending balance at Mar. 31, 2021	263,705		(94)	$ 4	468,589	(204,794)
Ending balance, Shares at Mar. 31, 2021				43,108,960
Beginning balance at Dec. 31, 2020	$ 280,791		13	$ 4	464,597	(183,823)
Beginning balance, Shares at Dec. 31, 2020				43,059,632
Exercise of stock options, Shares	380,688
Unrealized gain (loss) on marketable securities	$ (168)
Net loss	(68,234)
Ending balance at Sep. 30, 2021	398,780		(155)	$ 5	650,987	(252,057)
Ending balance, Shares at Sep. 30, 2021				48,593,398
Beginning balance at Mar. 31, 2021	263,705		(94)	$ 4	468,589	(204,794)
Beginning balance, Shares at Mar. 31, 2021				43,108,960
Issuance of common stock, net of estimated issuance costs of $11.3 million (Note 10)	169,483			$ 1	169,482
Issuance of common stock, net of estimated issuance costs of $11.3 million (Note 10), Shares				4,887,500
Shares issued upon warrant exercise - related party (Note 10), Shares				256,038
Exercise of stock options	539				539
Exercise of stock options, Shares				126,150
Stock-based compensation	5,175				5,175
Unrealized gain (loss) on marketable securities	25		25
Net loss	(22,580)					(22,580)
Other	80				80
Other, Shares				1,411
Ending balance at Jun. 30, 2021	416,427		(69)	$ 5	643,865	(227,374)
Ending balance, Shares at Jun. 30, 2021				48,380,059
Exercise of stock options	873				873
Exercise of stock options, Shares				205,210
Stock-based compensation	6,027				6,027
Unrealized gain (loss) on marketable securities	(86)		(86)
Net loss	(24,683)					(24,683)
Other	222				222
Other, Shares				8,129
Ending balance at Sep. 30, 2021	$ 398,780		$ (155)	$ 5	$ 650,987	$ (252,057)
Ending balance, Shares at Sep. 30, 2021				48,593,398