Environmental Obligations - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2012	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Other Commitments [Line Items]
Pollution legal liability insurance policy duration	10 years
Pollution legal liability insurance policy aggregate limit	$ 50,000,000
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 which existed prior to commencement of the lease and is discovered (other than as a result of a voluntary site investigation) during the first ten 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		$ 61,828,000		$ 61,828,000		$ 63,565,000
Accretion expense				1,308,000	$ 1,795,000
Amounts of credits to environmental expenses included in continuing and discontinued operations				608,000	5,719,000
Increase in carrying value of property				$ 2,116,000	1,654,000
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				$ 2,099,000	2,185,000
Capitalized asset retirement costs		44,973,000		44,973,000		45,380,000
Impairment charges		1,160,000	$ 914,000	3,977,000	4,651,000
Continuing and Discontinued Operations for Capitalized Asset Retirement Costs [Member]
Other Commitments [Line Items]
Impairment charges				1,817,000	$ 3,034,000
Reasonably Estimable Environmental Remediation Obligation [Member]
Other Commitments [Line Items]
Environmental remediation obligations		16,724,000		16,724,000		18,537,000
Future Environmental Liabilities for Preexisting Unknown Contamination [Member]
Other Commitments [Line Items]
Environmental remediation obligations		45,104,000		45,104,000		45,028,000
Capitalized asset retirement costs		25,992,000		25,992,000		26,688,000
Known Environmental Liabilities [Member]
Other Commitments [Line Items]
Capitalized asset retirement costs		$ 18,981,000		$ 18,981,000		$ 18,692,000
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%