Nature of Business and Basis of Presentation - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands					3 Months Ended						9 Months Ended		12 Months Ended
	Apr. 26, 2019	Apr. 12, 2019	Apr. 01, 2019	Apr. 02, 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	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Net proceeds from initial public offering after underwriting discounts and commissions and offering expenses												$ 150,843
Conversion of convertible preferred stock into common stock upon initial public offering, Shares				24,168,656
Common stock, shares issued					44,028,647						44,028,647		37,509,815
Net loss					$ 29,627	$ 26,270	$ 23,852	$ 14,148	$ 12,426	$ 10,217	$ 79,749	$ 36,791	$ 55,577
Accumulated deficit					$ 175,507						$ 175,507		$ 95,758
Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold									10,350,000
Conversion of convertible preferred stock into common stock upon initial public offering, Shares									24,168,656
Sales Agreement
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Common stock, shares issued					0						0
Initial Public Offering
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold				10,350,000
Common stock at a public offering price				$ 16.00
Net proceeds from initial public offering after underwriting discounts and commissions and offering expenses				$ 150,800
Common Stock, Preferred Stock, Debt Securities, Warrants and Units | Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds through future financings			$ 350,000
At-The-Market Sales Agreement | Maximum
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds through future financings			$ 100,000
Follow-on Public Offering | Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold		5,555,556
Shares issued price per share		$ 22.50
Net proceeds after deducting underwriting discounts and commissions and offering expenses		$ 116,900
Underwriters Option | Common Stock
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares issued and sold	833,333
Shares issued price per share	$ 22.50
Net proceeds after deducting underwriting discounts and commissions and offering expenses	$ 17,600