CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Series A Convertible Preferred Stock	Series B Convertible Preferred Stock	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Loss	Accumulated Deficit
Balance balance at Dec. 31, 2016		$ 17,392
Beginning balance at Dec. 31, 2016	$ (9,892)				$ 298		$ (10,190)
Beginning balance, Shares at Dec. 31, 2016		33,395,907
Beginning balance, Shares at Dec. 31, 2016				2,121,156
Issuance of convertible preferred stock, net of issuance costs		$ 20,479	$ 93,101
Issuance of convertible preferred stock, net of issuance costs, Shares		28,873,237	64,930,561
Reclassification of tranche liability upon issuance of convertible preferred stock		$ 5,123
Allocation of collaboration proceeds to carrying value of Series B convertible preferred stock			$ 1,667
Vesting of common stock from option exercise	225				225
Vesting of common stock from option exercise, Shares				483,200
Vesting of founders restricted common stock	$ 21				21
Vesting of founders restricted common stock, Shares				32,655
Issuance of common stock from option exercises, Shares	0
Stock-based compensation	$ 256				256
Other comprehensive loss	(73)					$ (73)
Net loss	(29,991)						(29,991)
Ending balance at Dec. 31, 2017	(39,454)				800	(73)	(40,181)
Ending balance, Shares at Dec. 31, 2017		62,269,144	64,930,561
Ending balance at Dec. 31, 2017	137,762	$ 42,994	$ 94,768
Ending balance, Shares at Dec. 31, 2017				2,637,011
Conversion of convertible preferred stock into common stock upon initial public offering		$ (42,994)	$ (94,768)
Conversion of convertible preferred stock into common stock upon initial public offering, Shares		(62,269,144)	(64,930,561)
Conversion of convertible preferred stock into common stock upon initial public offering	137,762			$ 2	137,760
Conversion of convertible preferred stock into common stock upon initial public offering, Shares				24,168,656
Issuance of common stock in initial public offering, net of discounts and issuance costs	150,843			$ 1	150,842
Issuance of common stock in initial public offering, net of discounts and issuance costs, Shares				10,350,000
Vesting of common stock from option exercise	75				75
Vesting of common stock from option exercise, Shares				129,959
Issuance of common stock from option exercises	$ 87				87
Issuance of common stock from option exercises, Shares	89,050			72,900
Stock-based compensation	$ 2,623				2,623
Other comprehensive loss	(4)					(4)
Net loss	(57,215)						(57,215)
Ending balance at Dec. 31, 2018	$ 194,717			$ 3	$ 292,187	$ (77)	$ (97,396)
Ending balance, Shares at Dec. 31, 2018				37,358,526