Condensed Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Convertible Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Beginning Balance at Dec. 31, 2018	$ (97,398)		$ 1	$ 17,078	$ (42)	$ (114,435)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2018		45,374,836
Convertible preferred stock, beginning balance at Dec. 31, 2018		$ 210,520
Beginning balance (in shares) at Dec. 31, 2018			13,764,829
Issuance of common stock upon initial public offering, net of issuance costs	168,223		$ 1	168,222
Issuance of common stock upon initial public offering, net of issuance costs (in shares)			9,739,541
Conversion of convertible preferred stock into common stock	210,521	$ (210,520)	$ 5	210,516
Conversion of convertible preferred stock into common stock (in shares)		(45,374,836)	45,374,836
Exercise of stock options	5			5
Exercise of stock options (in shares)			625
Forfeiture of restricted common stock (in shares)			(8,039)
Stock-based compensation	3,245			3,245
Unrealized gain (loss) on marketable securities	150				150
Net loss	(18,560)					(18,560)
Ending Balance at Mar. 31, 2019	266,186		$ 7	399,066	108	(132,995)
Ending balance (in shares) at Mar. 31, 2019			68,871,792
Beginning Balance at Dec. 31, 2018	(97,398)		$ 1	17,078	(42)	(114,435)
Convertible preferred stock, beginning balance (in shares) at Dec. 31, 2018		45,374,836
Convertible preferred stock, beginning balance at Dec. 31, 2018		$ 210,520
Beginning balance (in shares) at Dec. 31, 2018			13,764,829
Net loss	(74,858)
Ending Balance at Sep. 30, 2019	218,587		$ 7	407,647	226	(189,293)
Ending balance (in shares) at Sep. 30, 2019			68,922,249
Beginning Balance at Mar. 31, 2019	266,186		$ 7	399,066	108	(132,995)
Beginning balance (in shares) at Mar. 31, 2019			68,871,792
Exercise of stock options	63			63
Exercise of stock options (in shares)			8,999
Forfeiture of restricted common stock (in shares)			(48,934)
Stock-based compensation	3,694			3,694
Unrealized gain (loss) on marketable securities	333				333
Net loss	(24,560)					(24,560)
Ending Balance at Jun. 30, 2019	245,716		$ 7	402,823	441	(157,555)
Ending balance (in shares) at Jun. 30, 2019			68,831,857
Exercise of stock options	720			720
Exercise of stock options (in shares)			90,392
Stock-based compensation	4,104			4,104
Unrealized gain (loss) on marketable securities	(215)				(215)
Net loss	(31,738)					(31,738)
Ending Balance at Sep. 30, 2019	218,587		$ 7	407,647	226	(189,293)
Ending balance (in shares) at Sep. 30, 2019			68,922,249
Beginning Balance at Dec. 31, 2019	194,743		$ 7	414,414	142	(219,820)
Beginning balance (in shares) at Dec. 31, 2019			69,052,873
Issuance of common stock upon initial public offering, net of issuance costs	224,511		$ 1	224,510
Issuance of common stock upon initial public offering, net of issuance costs (in shares)			9,602,500
Exercise of stock options	3,217			3,217
Exercise of stock options (in shares)			361,096
Stock-based compensation	6,642			6,642
Unrealized gain (loss) on marketable securities	2,640				2,640
Net loss	(40,019)					(40,019)
Ending Balance at Mar. 31, 2020	391,734		$ 8	648,783	2,782	(259,839)
Ending balance (in shares) at Mar. 31, 2020			79,016,469
Beginning Balance at Dec. 31, 2019	194,743		$ 7	414,414	142	(219,820)
Beginning balance (in shares) at Dec. 31, 2019			69,052,873
Net loss	(138,049)
Ending Balance at Sep. 30, 2020	310,191		$ 8	666,958	1,094	(357,869)
Ending balance (in shares) at Sep. 30, 2020			79,284,181
Beginning Balance at Mar. 31, 2020	391,734		$ 8	648,783	2,782	(259,839)
Beginning balance (in shares) at Mar. 31, 2020			79,016,469
Exercise of stock options	2,196			2,196
Exercise of stock options (in shares)			190,709
Purchase of common stock under employee stock option	715			715
Purchase of common stock under employee stock option (in shares)			45,217
Stock-based compensation	6,948			6,948
Unrealized gain (loss) on marketable securities	(936)				(936)
Net loss	(45,326)					(45,326)
Ending Balance at Jun. 30, 2020	355,331		$ 8	658,642	1,846	(305,165)
Ending balance (in shares) at Jun. 30, 2020			79,252,395
Exercise of stock options	286			286
Exercise of stock options (in shares)			31,786
Stock-based compensation	8,030			8,030
Unrealized gain (loss) on marketable securities	(752)				(752)
Net loss	(52,704)					(52,704)
Ending Balance at Sep. 30, 2020	$ 310,191		$ 8	$ 666,958	$ 1,094	$ (357,869)
Ending balance (in shares) at Sep. 30, 2020			79,284,181