Environmental Obligations - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2012	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Other Commitments [Line Items]
Remediation agreement of lease				we have agreed to be responsible for environmental contamination at the premises that was known at the time the lease commenced, and for environmental contamination which existed prior to commencement of the lease and is discovered (other than as a result of a voluntary site investigation) during the first 10 years of the lease term (or a shorter period for a minority of such leases).
The maximum number of years during lease term during which contamination is discovered that the Company may be responsible for				10 years
Estimated maximum remaining useful life of underground storage tanks that the company may be responsible for sold properties				5 years
Environmental remediation obligations		$ 49,273,000		$ 49,273,000		$ 50,723,000
Accretion expense				922,000	$ 1,032,000
Amount of credits to environmental expenses				1,228,000	559,000
Changes in environmental estimates				73,000	175,000
Increase in carrying value of property				$ 1,441,000	2,334,000
Estimated maximum remaining useful life of underground storage tanks used to calculate depreciation of capitalized asset retirement costs				10 years
Depreciation and amortization expense for capitalized asset retirement costs				$ 1,995,000	2,078,000
Capitalized asset retirement costs		39,611,000		39,611,000		39,684,000
Impairments		507,000	$ 701,000	1,538,000	1,472,000
Pollution legal liability insurance policy duration	10 years
Pollution legal liability insurance policy aggregate limit	$ 50,000,000
Capitalized Asset Retirement Costs [Member]
Other Commitments [Line Items]
Impairments				1,446,000	$ 1,429,000
Reasonably Estimable Environmental Remediation Obligation [Member]
Other Commitments [Line Items]
Environmental remediation obligations		11,487,000		11,487,000		12,470,000
Future Environmental Liabilities for Preexisting Unknown Contamination [Member]
Other Commitments [Line Items]
Environmental remediation obligations		37,786,000		37,786,000		38,253,000
Capitalized asset retirement costs		16,924,000		16,924,000		17,534,000
Known Environmental Liabilities [Member]
Other Commitments [Line Items]
Capitalized asset retirement costs		$ 22,687,000		$ 22,687,000		$ 22,150,000
Maximum [Member]
Other Commitments [Line Items]
The maximum number of years during lease term during which contamination is discovered that the Company may be responsible for				10 years
Environmental remediation liabilities discount rate		7.00%		7.00%
Environmental remediation liability inflation rate adjustment				2.75%
Minimum [Member]
Other Commitments [Line Items]
Environmental remediation liabilities discount rate		4.00%		4.00%
Environmental remediation liability inflation rate adjustment				2.00%