Environmental Obligations - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended
	Jul. 31, 2012	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Environmental Matters And Other Commitments And Contingencies [Line Items]
Payment for pollution legal liability insurance policy	$ 3,062,000
Pollution legal liability insurance policy duration		10 years
Pollution legal liability insurance policy aggregate limit		50,000,000
Asset retirement obligations removed from balance sheet		11,955,000
Deferred rental revenue		1,782,000
Environmental remediation obligations		45,728,000		46,150,000
Accretion expense		627,000	774,000
The amount of credits to environmental expenses		352,000	512,000
Increase in carrying value of property		1,840,000	1,459,000
Estimated remaining useful life of underground storage tank for capitalized asset retirement costs		10 years
Depreciation and amortization expense for capitalized asset retirement costs		778,000	1,809,000
Capitalized asset retirement costs		20,981,000		23,549,000
USTs [Member]
Environmental Matters And Other Commitments And Contingencies [Line Items]
Net asset cost related to USTs removed from the balance sheet		$ 10,173,000		$ 10,173,000