Nature of the Business and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended						9 Months Ended		12 Months Ended
	Apr. 01, 2019	Apr. 25, 2018	Apr. 03, 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											$ 66,793
Net loss				$ (11,923)	$ (10,516)	$ (11,691)	$ (10,168)	$ (9,023)	$ (6,735)	$ (34,130)	$ (25,926)	$ (34,500)
Accumulated deficit				$ (126,189)						$ (126,189)		$ (92,059)
Funding term of borrowings for operating expense and capital expenditure requirement										12 months
Cowen [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares of common stock issued and sold										0
Maximum [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Proceeds from issuance of equity and debt financing	$ 150,000
Maximum [Member] | Cowen [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Stock offering cost	$ 50,000
Common Stock [Member] | IPO [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares of common stock issued and sold		215,000	5,770,000					5,985,000
Public offering price, per share		$ 12.00	$ 12.00
Net proceeds from initial public offering		$ 2,400	$ 61,500
Common Stock [Member] | Private Placement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Shares of common stock issued and sold			416,666					416,666
Sale of stock, consideration received on transaction			$ 5,000