CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY/(DEFICIT) - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Accumulated Deficit	Total
Balances at beginning of period at Dec. 31, 2018					$ 27,647
Convertible preferred stock, Shares outstanding, beginning of period at Dec. 31, 2018					103,015,190
Balances at end of period at Mar. 31, 2019					$ 27,647
Convertible preferred stock, Shares outstanding, end of period at Mar. 31, 2019					103,015,190
Balances at beginning of period at Dec. 31, 2018		$ 154		$ (7,383)	$ (7,229)
Balances at beginning of period (in shares) at Dec. 31, 2018	935,887
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)	197
Vesting of restricted common stock from early-exercised stock options and sales of restricted common stock		7			7
Stock-based compensation expense		18			18
Net loss				(5,909)	(5,909)
Balances at end of period at Mar. 31, 2019		179		(13,292)	(13,113)
Balances at end of period (in shares) at Mar. 31, 2019	936,084
Balances at beginning of period at Dec. 31, 2018					$ 27,647
Convertible preferred stock, Shares outstanding, beginning of period at Dec. 31, 2018					103,015,190
Balances at end of period at Sep. 30, 2019					$ 58,690
Convertible preferred stock, Shares outstanding, end of period at Sep. 30, 2019					178,349,005
Balances at beginning of period at Dec. 31, 2018		154		(7,383)	$ (7,229)
Balances at beginning of period (in shares) at Dec. 31, 2018	935,887
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss					(16,123)
Balances at end of period at Sep. 30, 2019		229		(23,506)	(23,277)
Balances at end of period (in shares) at Sep. 30, 2019	980,237
Balances at beginning of period at Mar. 31, 2019					$ 27,647
Convertible preferred stock, Shares outstanding, beginning of period at Mar. 31, 2019					103,015,190
Balances at end of period at Jun. 30, 2019					$ 27,647
Convertible preferred stock, Shares outstanding, end of period at Jun. 30, 2019					103,015,190
Balances at beginning of period at Mar. 31, 2019		179		(13,292)	$ (13,113)
Balances at beginning of period (in shares) at Mar. 31, 2019	936,084
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options		13			13
Issuance of common stock upon exercise of stock options (in shares)	16,608
Vesting of restricted common stock from early-exercised stock options and sales of restricted common stock		(9)			(9)
Stock-based compensation expense		18			18
Net loss				(2,004)	(2,004)
Balances at end of period at Jun. 30, 2019		201		(15,296)	(15,095)
Balances at end of period (in shares) at Jun. 30, 2019	952,692
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of convertible preferred stock, net					$ 25,016
Issuance of convertible preferred stock, net (in shares)					75,333,815
Reclassification of preferred stock tranche liability upon settlement					$ 6,027
Balances at end of period at Sep. 30, 2019					$ 58,690
Convertible preferred stock, Shares outstanding, end of period at Sep. 30, 2019					178,349,005
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options		43			$ 43
Issuance of common stock upon exercise of stock options (in shares)	27,545
Vesting of restricted common stock from early-exercised stock options and sales of restricted common stock		(36)			(36)
Stock-based compensation expense		21			21
Net loss				(8,210)	(8,210)
Balances at end of period at Sep. 30, 2019		229		(23,506)	(23,277)
Balances at end of period (in shares) at Sep. 30, 2019	980,237
Balances at beginning of period at Dec. 31, 2019					$ 58,690
Convertible preferred stock, Shares outstanding, beginning of period at Dec. 31, 2019					178,349,005
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of convertible preferred stock, net					$ 104,837
Issuance of convertible preferred stock, net (in shares)					221,399,223
Balances at end of period at Mar. 31, 2020					$ 163,527
Convertible preferred stock, Shares outstanding, end of period at Mar. 31, 2020					399,748,228
Balances at beginning of period at Dec. 31, 2019		323		(33,124)	$ (32,801)
Balances at beginning of period (in shares) at Dec. 31, 2019	997,165
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options		7			7
Issuance of common stock upon exercise of stock options (in shares)	13,630
Vesting of restricted common stock from early-exercised stock options and sales of restricted common stock		12			12
Stock-based compensation expense		111			111
Net loss				(10,440)	(10,440)
Balances at end of period at Mar. 31, 2020		453		(43,564)	(43,111)
Balances at end of period (in shares) at Mar. 31, 2020	1,010,795
Balances at beginning of period at Dec. 31, 2019					$ 58,690
Convertible preferred stock, Shares outstanding, beginning of period at Dec. 31, 2019					178,349,005
Convertible preferred stock, Shares outstanding, end of period at Sep. 30, 2020					0
Balances at beginning of period at Dec. 31, 2019		323		(33,124)	$ (32,801)
Balances at beginning of period (in shares) at Dec. 31, 2019	997,165
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss					(36,052)
Unrealized gain (loss) on marketable securities					8
Balances at end of period at Sep. 30, 2020	$ 3	390,473	$ 8	(69,176)	321,308
Balances at end of period (in shares) at Sep. 30, 2020	34,374,348
Balances at beginning of period at Mar. 31, 2020					$ 163,527
Convertible preferred stock, Shares outstanding, beginning of period at Mar. 31, 2020					399,748,228
Increase (Decrease) in Temporary Equity [Roll Forward]
Conversion of convertible preferred stock to common stock					$ (163,527)
Conversion of convertible preferred stock, shares					(399,748,228)
Balances at beginning of period at Mar. 31, 2020		453		(43,564)	$ (43,111)
Balances at beginning of period (in shares) at Mar. 31, 2020	1,010,795
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options		31			31
Issuance of common stock upon exercise of stock options (in shares)	15,181
Vesting of restricted common stock from early-exercised stock options and sales of restricted common stock		7			7
Stock-based compensation expense		490			490
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs	$ 1	223,849			223,850
Issuance of common stock upon completion of initial public offering, net of commissions, underwriting discounts and offering costs, shares	14,375,000
Conversion of Stock, Amount Issued	$ 2	163,510			163,512
Conversion of Stock, Shares Issued	18,969,672
Net loss				(12,523)	(12,523)
Balances at end of period at Jun. 30, 2020	$ 3	388,340		(56,087)	$ 332,256
Balances at end of period (in shares) at Jun. 30, 2020	34,370,648
Convertible preferred stock, Shares outstanding, end of period at Sep. 30, 2020					0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options		3			$ 3
Issuance of common stock upon exercise of stock options (in shares)	3,700
Vesting of restricted common stock from early-exercised stock options and sales of restricted common stock		54			54
Stock-based compensation expense		2,076			2,076
Net loss				(13,089)	(13,089)
Unrealized gain (loss) on marketable securities			8		8
Balances at end of period at Sep. 30, 2020	$ 3	$ 390,473	$ 8	$ (69,176)	$ 321,308
Balances at end of period (in shares) at Sep. 30, 2020	34,374,348