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Retirement Benefit Obligations - Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2025 | Jun. 30, 2024 | Jun. 30, 2025 | Jun. 30, 2024
Pension Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | $ 436 | $ 497 | $ 872 | $ 994
Interest Cost | 2,016 | 1,886 | 4,032 | 3,772
Expected Return on Plan Assets | (2,625) | (2,649) | (5,249) | (5,298)
Prior Service Cost Amortization | 64 | 81 | 128 | 162
Actuarial Loss Amortization | 319 | 362 | 638 | 724
Sub-total | 210 | 177 | 421 | 354
Amounts Capitalized and Deferred | 26 | 27 | 135 | 135
Net Periodic Benefit Cost Recognized | 236 | 204 | 556 | 489
Other Postretirement Benefits Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 501 | 497 | 1,003 | 994
Interest Cost | 992 | 817 | 1,984 | 1,634
Expected Return on Plan Assets | (1,085) | (969) | (2,171) | (1,938)
Prior Service Cost Amortization | 0 | 0 | 0 | 0
Actuarial Loss Amortization | (187) | (218) | (374) | (437)
Sub-total | 221 | 127 | 442 | 253
Amounts Capitalized and Deferred | (48) | 54 | (34) | 178
Net Periodic Benefit Cost Recognized | 173 | 181 | 408 | 431
Supplemental Employee Retirement Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 40 | 55 | 79 | 109
Interest Cost | 172 | 181 | 344 | 362
Expected Return on Plan Assets | 0 | 0 | 0 | 0
Prior Service Cost Amortization | 2 | 2 | 4 | 4
Actuarial Loss Amortization | (54) | 0 | (107) | 0
Sub-total | 160 | 238 | 320 | 475
Amounts Capitalized and Deferred | (50) | (74) | (101) | (148)
Net Periodic Benefit Cost Recognized | $ 110 | $ 164 | $ 219 | $ 327

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- Definition Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480535/715-20-45-3A Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (h)(5) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(5) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-6 + Details
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- Definition Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 3A -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480535/715-20-45-3A Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 18 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-18 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (h)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-6 + Details
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