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Commitments, Contingencies and Legal Proceedings - Narrative (Detail) $ in Thousands | 6 Months Ended | 9 Months Ended |
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Jun. 30, 2021 Operable_Unit | Sep. 30, 2021 USD ($) | Dec. 31, 2020 USD ($)
Commitments Contingencies And Litigation [Line Items] |  |  |
Number of operable units | Operable_Unit | 5 |  |
Term of agreement with third party for monitoring and maintenance in OU1-OU4a |  | 30 years |
Difference recorded in escrow account |  | $ 1,600 |
Total amount available in escrow account |  | 8,800 |
Reserve for environmental liabilities, current portion |  | 2,529 | $ 3,700
Other Long-Term Liabilities |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |
Reserve for environmental liabilities, remaining portion |  | $ 14,300 |
Minimum |  |  |
Commitments Contingencies And Litigation [Line Items] |  |  |
Period over which cash expenditure to be incurred for monitoring activities |  | 30 years |

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- Definition The specific areas of the site into which the contaminated area has been separated by the US EPA. + References No definition available. + Details
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- Definition Carrying value of the obligation (known or estimated) arising from requirements to perform activities to remediate one or more sites, payable in twelve months or in the next operating cycle if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.20) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 10 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121557415&loc=d3e14326-108349 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=124098289&loc=d3e6911-107765 + Details
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