Property Acquisitions - Additional Information (Detail)				9 Months Ended	12 Months Ended
	Jun. 17, 2019 USD ($) Property Option	Aug. 01, 2018 USD ($) Property Option	Apr. 17, 2018 USD ($) Property Option	Sep. 30, 2019 USD ($) Property	Dec. 31, 2018 USD ($) Property
Business Acquisition [Line Items]
Number of gasoline stations and convenience stores acquired during the period | Property	6			14	41
Aggregate purchase price of the gasoline stations and convenience stores acquired during the period	$ 24,724,000			$ 43,324,000	$ 77,972,000
Unitary lease description				The unitary lease requires our tenant to pay a fixed annual rent plus all amounts pertaining to the properties, including environmental expenses, real estate taxes, assessments, license and permit fees, charges for public utilities and all other governmental charges. Rent is scheduled to increase annually during the initial and renewal terms of the lease.
Unitary lease initial term	15 years
Number of unitary lease renewal options | Option	2
Unitary lease renewal term	10 years
Purchase price allocation, land acquired	$ 18,086,000			$ 8,034,000
Purchase price allocation, buildings and improvements acquired	4,789,000			9,590,000
Purchase price allocated to in-place leases	$ 1,849,000			$ 976,000
Gasoline Stations and Convenience Store Properties [Member]
Business Acquisition [Line Items]
Number of gasoline stations and convenience stores acquired during the period | Property				8
Aggregate purchase price of the gasoline stations and convenience stores acquired during the period				$ 18,600,000
GPM Investments, LLC [Member]
Business Acquisition [Line Items]
Number of gasoline stations and convenience stores acquired during the period | Property			30		5
Aggregate purchase price of the gasoline stations and convenience stores acquired during the period			$ 52,592,000		$ 7,968,000
Unitary lease description				The unitary lease requires GPM to pay a fixed annual rent plus all amounts pertaining to the properties, including environmental expenses, real estate taxes, assessments, license and permit fees, charges for public utilities and all other governmental charges. Rent is scheduled to increase annually during the initial and renewal terms of the lease.
Unitary lease initial term			15 years
Number of unitary lease renewal options | Option			4
Unitary lease renewal term			5 years
Purchase price allocation, land acquired			$ 31,633,000		4,929,000
Purchase price allocation, buildings and improvements acquired			17,489,000		2,753,000
Purchase price allocated to in-place leases			4,047,000		$ 286,000
Purchase price allocated to below market leases			$ 577,000
Applegreen PLC [Member]
Business Acquisition [Line Items]
Number of gasoline stations and convenience stores acquired during the period | Property		6
Aggregate purchase price of the gasoline stations and convenience stores acquired during the period		$ 17,412,000
Unitary lease description				The unitary lease requires Applegreen to pay a fixed annual rent plus all amounts pertaining to the properties, including environmental expenses, real estate taxes, assessments, license and permit fees, charges for public utilities and all other governmental charges. Rent is scheduled to increase annually during the initial and renewal terms of the lease.
Unitary lease initial term		15 years
Number of unitary lease renewal options | Option		4
Unitary lease renewal term		5 years
Purchase price allocation, land acquired		$ 8,930,000
Purchase price allocation, buildings and improvements acquired		6,773,000
Purchase price allocated to in-place leases		1,371,000
Purchase price allocated to below market leases		435,000
Purchase price allocated to above market leases		$ 773,000