Nature of the Business and Basis of Presentation - Additional Information (Detail)		3 Months Ended						9 Months Ended		12 Months Ended
	Oct. 03, 2019 USD ($) shares	Sep. 30, 2019 USD ($) Tranche	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2019 USD ($) Tranche	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Number of tranches in loan agreement | Tranche		4						4
Proceeds from lines of credit								$ 29,500,000
Net losses		$ 21,149,000	$ 20,767,000	$ 19,426,000	$ 15,938,000	$ 11,941,000	$ 12,102,000	61,342,000	$ 39,981,000	$ 59,900,000
Accumulated deficit		$ 295,179,000						295,179,000		$ 233,837,000
Subsequent Event [Member] | Private Placement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Aggregate common stock issued | shares	7,647,057
Net proceeds received	$ 64,900,000
Loan Agreement [Member]
Organization Consolidation And Presentation Of Financial Statements Disclosure [Line Items]
Line of credit facility, maximum borrowing capacity tranche four								$ 40,000,000