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Environmental Obligations - Additional Information (Detail) - USD ($) | 1 Months Ended | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | ---
Jul. 31, 2012 | Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Dec. 31, 2020
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 |  | $ 47,932,000 |  | $ 47,932,000 |  | $ 48,084,000
Accretion expense |  |  |  | 863,000 | $ 922,000 |
Amount of credits to environmental expenses |  |  |  | 1,039,000 | 1,228,000 |
Changes in environmental estimates |  |  |  | 92,000 | 73,000 |
Increase in carrying value of property |  |  |  | $ 1,211,000 | 1,441,000 |
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 |  |  |  | $ 1,971,000 | 1,995,000 |
Capitalized asset retirement costs |  | 39,370,000 |  | 39,370,000 |  | 39,610,000
Impairments |  | 756,000 | $ 507,000 | 1,532,000 | 1,538,000 |
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 |  |  |  | 1,500,000 | $ 1,446,000 |
Reasonably Estimable Environmental Remediation Obligation [Member] |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Environmental remediation obligations |  | 11,920,000 |  | 11,920,000 |  | 11,718,000
Future Environmental Liabilities for Preexisting Unknown Contamination [Member] |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Environmental remediation obligations |  | 36,012,000 |  | 36,012,000 |  | 36,366,000
Capitalized asset retirement costs |  | 15,786,000 |  | 15,786,000 |  | 16,037,000
Known Environmental Liabilities [Member] |  |  |  |  |  |
Other Commitments [Line Items] |  |  |  |  |  |
Capitalized asset retirement costs |  | $ 23,584,000 |  | $ 23,584,000 |  | $ 23,573,000
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% |  |

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- Definition The current period expense charged against earnings for the allocation of capitalized asset retirement costs over the remaining useful life of the underground storage tank or other period of time if circumstances warrant. + References No definition available. + Details
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