CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Thousands	Common stock	Additional paid-in capital	Accumulated other comprehensive loss	Accumulated deficit	Convertible preferred stock	Series B convertible preferred stock	Series C convertible preferred stock	Total
Balances at Beginning of period (in shares) at Dec. 31, 2016	7,886,292
Balances at Beginning of period at Dec. 31, 2016	$ 8			$ (17,132)				$ (17,124)
Issuance of common stock upon exercise of stock options (in shares)	250,572
Issuance of common stock upon exercise of stock options		$ 37						37
Issuance of common stock for one‑time bonus payment (in shares)	213,439
Issuance of restricted common stock upon early exercise of stock options (in shares)	1,400,000
Issuance of restricted common stock upon early exercise of stock options	$ 1	(1)
Issuance of restricted common stock (in shares)	5,227,014
Issuance of restricted common stock	$ 6	(6)
Stock-based compensation expense		17,903						17,903
Accretion of Series A redeemable convertible preferred stock to redemption value		(656)						(656)
Net loss				(43,847)				(43,847)
Balances at End of period (in shares) at Dec. 31, 2017	14,977,317
Balances at End of period at Dec. 31, 2017	$ 15	17,277		(60,979)				(43,687)
Balances at Beginning of period (in shares) at Dec. 31, 2016					29,570,662
Balances at Beginning of period at Dec. 31, 2016					$ 19,067
Convertible preferred stock
Issuance of stock, net of issuance costs (in shares)						14,362,344
Issuance of stock, net of issuance costs						$ 120,067
Accretion of Series A redeemable convertible preferred stock to redemption value					$ 656
Balances at End of period (in shares) at Dec. 31, 2017					43,933,006
Balances at End of period at Dec. 31, 2017					$ 139,790
Shares issued	12,055,450
Issuance of common stock, initial public offering, net of issuance cost $3,548	$ 12	254,306						254,318
Cashless exercise of warrants	131,273
Issuance of common stock upon exercise of stock options (in shares)	225,375
Issuance of common stock upon exercise of stock options		64						64
Stock-based compensation expense		27,528						27,528
Vesting of restricted common stock		180						180
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering		3,053						3,053
Conversion of redeemable convertible preferred stock to common stock (in shares)	51,845,438
Conversion of redeemable convertible preferred stock to common stock	$ 52	240,724						240,776
Unrealized gains (losses) on investments			$ (29)					(29)
Net loss				(89,195)				(89,195)
Balances at End of period (in shares) at Dec. 31, 2018	79,234,853
Balances at End of period at Dec. 31, 2018	$ 79	543,040	(29)	(150,082)				$ 393,008
Convertible preferred stock
Issuance of stock, net of issuance costs (in shares)							7,912,432
Issuance of stock, net of issuance costs							$ 100,986
Conversion of redeemable convertible preferred stock to common stock (in shares)					(51,845,438)
Conversion of redeemable convertible preferred stock to common stock					$ (240,776)
Balances at End of period (in shares) at Dec. 31, 2018								0
Cumulative effect adjustment for adoption		(92)		92
Issuance of common stock upon exercise of stock options (in shares)	1,449,309
Issuance of common stock upon exercise of stock options	$ 2	2,443						$ 2,445
Stock-based compensation expense		41,271						41,271
Repurchase of unvested restricted common stock (in shares)	(667,917)
Repurchase of unvested restricted common stock	$ (1)							(1)
Vesting of restricted common stock		44						44
Unrealized gains (losses) on investments			104					104
Net loss				(163,458)				(163,458)
Balances at End of period (in shares) at Dec. 31, 2019	80,016,245
Balances at End of period at Dec. 31, 2019	$ 80	$ 586,798	$ 75	(312,740)				$ 274,213
Balances at End of period (in shares) at Dec. 31, 2019								0
Cumulative effect adjustment for adoption				$ 800				$ 800