Nature of the Business and Basis of Presentation (Details) - USD ($) $ in Thousands		1 Months Ended	3 Months Ended						9 Months Ended
	Jul. 20, 2018	Jul. 31, 2018	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
Sale of Equity
Net proceeds after deducting underwriting discounts and commissions and other offering costs										$ 257,866
Net loss			$ (47,015)	$ (39,390)	$ (32,581)	$ (26,362)	$ (21,239)	$ (14,411)	$ (118,986)	$ (62,012)
Accumulated deficit			$ (268,268)						$ (268,268)		$ (149,282)	$ (150,082)	$ (60,979)
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
IPO | Common stock
Sale of Equity
Shares issued (in shares)	12,055,450
Overallotment
Sale of Equity
Shares issued (in shares)	1,572,450