Nature of the Business and Basis of Presentation (Details) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
	Jul. 20, 2018	Jul. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Sale of Equity
Net proceeds after deducting underwriting discounts and commissions and other offering costs				$ 257,866
Net loss			$ (163,458)	(89,195)	$ (43,847)
Accumulated deficit			$ (312,740)	$ (150,082)		$ (60,887)
Common stock
Sale of Equity
Shares issued (in shares)				12,055,450
Conversion of redeemable convertible preferred stock to common stock (in shares)	51,845,438	51,845,438		51,845,438
IPO
Sale of Equity
Shares issued (in shares)	12,055,450
Net proceeds after deducting underwriting discounts and commissions and other offering costs	$ 254,300
Conversion of redeemable convertible preferred stock to common stock (in shares)		131,273
Overallotment
Sale of Equity
Shares issued (in shares)	1,572,450