Condensed Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Series A Redeemable Convertible Preferred Stock	Series B Redeemable Convertible Preferred Stock	Series C Redeemable Convertible Preferred Stock	Series D Redeemable Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Deficit
Beginning balance at Dec. 31, 2018	$ (57,728)					$ 6	$ 1,339	$ (59,073)
Beginning balance, shares at Dec. 31, 2018		6,225,719	6,786,896	3,688,740
Beginning balance at Dec. 31, 2018		$ 24,967	$ 55,151	$ 37,692
Beginning balance, shares at Dec. 31, 2018						3,757,403
Vesting of early exercised stock options	20						20
Stock-based compensation expense	266						266
Net loss	(13,671)							(13,671)
Ending balance at Mar. 31, 2019	(71,113)					$ 6	1,625	(72,744)
Ending balance, shares at Mar. 31, 2019		6,225,719	6,786,896	3,688,740
Ending balance at Mar. 31, 2019		$ 24,967	$ 55,151	$ 37,692
Ending balance, shares at Mar. 31, 2019						3,757,403
Beginning balance at Dec. 31, 2018	$ (57,728)					$ 6	1,339	(59,073)
Beginning balance, shares at Dec. 31, 2018		6,225,719	6,786,896	3,688,740
Beginning balance at Dec. 31, 2018		$ 24,967	$ 55,151	$ 37,692
Beginning balance, shares at Dec. 31, 2018						3,757,403
Exercise of stock options (in shares)	280,836
Net loss	$ (35,653)
Ending balance at Sep. 30, 2019	(92,160)					$ 7	2,559	(94,726)
Ending balance, shares at Sep. 30, 2019		6,225,719	6,786,896	3,688,740
Ending balance at Sep. 30, 2019		$ 24,967	$ 55,151	$ 37,692
Ending balance, shares at Sep. 30, 2019						4,038,239
Beginning balance at Mar. 31, 2019	(71,113)					$ 6	1,625	(72,744)
Beginning balance, shares at Mar. 31, 2019		6,225,719	6,786,896	3,688,740
Beginning balance at Mar. 31, 2019		$ 24,967	$ 55,151	$ 37,692
Beginning balance, shares at Mar. 31, 2019						3,757,403
Exercise of stock options	2						2
Exercise of stock options (in shares)						889
Vesting of early exercised stock options	20						20
Stock-based compensation expense	278						278
Net loss	(10,665)							(10,665)
Ending balance at Jun. 30, 2019	(81,478)					$ 6	1,925	(83,409)
Ending balance, shares at Jun. 30, 2019		6,225,719	6,786,896	3,688,740
Ending balance at Jun. 30, 2019		$ 24,967	$ 55,151	$ 37,692
Ending balance, shares at Jun. 30, 2019						3,758,292
Exercise of stock options	$ 314					$ 1	313
Exercise of stock options (in shares)	279,947					220,671
Issuance of common stock related to early exercised stock options						59,276
Vesting of early exercised stock options	$ 55						55
Stock-based compensation expense	266						266
Net loss	(11,317)							(11,317)
Ending balance at Sep. 30, 2019	(92,160)					$ 7	2,559	(94,726)
Ending balance, shares at Sep. 30, 2019		6,225,719	6,786,896	3,688,740
Ending balance at Sep. 30, 2019		$ 24,967	$ 55,151	$ 37,692
Ending balance, shares at Sep. 30, 2019						4,038,239
Beginning balance at Dec. 31, 2019	$ (106,373)					$ 7	2,967	(109,347)
Beginning balance, shares at Dec. 31, 2019	20,390,095	6,225,719	6,786,896	7,377,480
Beginning balance at Dec. 31, 2019	$ 160,310	$ 24,967	$ 55,151	$ 80,192
Beginning balance, shares at Dec. 31, 2019						4,059,909
Exercise of stock options	49						49
Exercise of stock options (in shares)						28,837
Issuance of common stock related to early exercised stock options						14,819
Vesting of early exercised stock options	128						128
Issuance of preferred stock					$ 109,875
Issuance of preferred stock, net of issuance cost (in shares)					8,220,242
Stock-based compensation expense	372						372
Net loss	(27,142)							(27,142)
Ending balance at Mar. 31, 2020	(132,966)					$ 7	3,516	(136,489)
Ending balance, shares at Mar. 31, 2020		6,225,719	6,786,896	7,377,480	8,220,242
Ending balance at Mar. 31, 2020		$ 24,967	$ 55,151	$ 80,192	$ 109,875
Ending balance, shares at Mar. 31, 2020						4,103,565
Beginning balance at Dec. 31, 2019	$ (106,373)					$ 7	2,967	(109,347)
Beginning balance, shares at Dec. 31, 2019	20,390,095	6,225,719	6,786,896	7,377,480
Beginning balance at Dec. 31, 2019	$ 160,310	$ 24,967	$ 55,151	$ 80,192
Beginning balance, shares at Dec. 31, 2019						4,059,909
Exercise of stock options (in shares)	263,626
Net loss	$ (68,480)
Ending balance at Sep. 30, 2020	364,075					$ 54	541,848	(177,827)
Ending balance, shares at Sep. 30, 2020		0	0	0
Ending balance, shares at Sep. 30, 2020						50,949,490
Beginning balance at Mar. 31, 2020	(132,966)					$ 7	3,516	(136,489)
Beginning balance, shares at Mar. 31, 2020		6,225,719	6,786,896	7,377,480	8,220,242
Beginning balance at Mar. 31, 2020		$ 24,967	$ 55,151	$ 80,192	$ 109,875
Beginning balance, shares at Mar. 31, 2020						4,103,565
Conversion of preferred stock	270,190	$ (24,967)	$ (55,151)	$ (80,192)	$ (109,879)	$ 29	270,161
Conversion of preferred stock (in shares)		(6,225,719)	(6,786,896)	(7,377,480)	(8,220,242)
Conversion of preferred stock (in shares)						28,610,337
Conversion of common stock warrant						30,278
Conversion of preferred stock warrant						16,591
Warrant liability write-off	629						629
Issuance of common stock upon initial public offering, net of issuance costs of $3,296	264,079					$ 18	264,061
Issuance of common stock upon initial public offering, net of issuance cost (in shares)						17,968,750
Exercise of stock options	297						297
Exercise of stock options (in shares)						152,177
Vesting of early exercised stock options	12						12
Issuance of preferred stock					$ 4
Issuance costs for initial public offering						$ (3,296)
Stock-based compensation expense	1,289						1,289
Net loss	(20,320)							(20,320)
Ending balance at Jun. 30, 2020	383,210					$ 54	539,965	(156,809)
Ending balance, shares at Jun. 30, 2020						50,881,698
Exercise of stock options	$ 108						108
Exercise of stock options (in shares)	67,792					67,792
Vesting of early exercised stock options	$ 9						9
Issuance costs for initial public offering	(72)						(72)
Stock-based compensation expense	1,838						1,838
Net loss	(21,018)							(21,018)
Ending balance at Sep. 30, 2020	$ 364,075					$ 54	$ 541,848	$ (177,827)
Ending balance, shares at Sep. 30, 2020		0	0	0
Ending balance, shares at Sep. 30, 2020						50,949,490