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Retirement Benefit Obligations - Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Pension Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | $ 436 | $ 498 | $ 1,308 | $ 1,492
Interest Cost | 2,016 | 1,887 | 6,048 | 5,659
Expected Return on Plan Assets | (2,624) | (2,649) | (7,873) | (7,947)
Prior Service Cost Amortization | 63 | 80 | 191 | 242
Actuarial Loss Amortization | 320 | 361 | 958 | 1,085
Sub-total | 211 | 177 | 632 | 531
Amounts Capitalized and Deferred | 6 | 30 | 141 | 165
Net Periodic Benefit Cost Recognized | 217 | 207 | 773 | 696
Other Postretirement Benefits Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 502 | 497 | 1,505 | 1,491
Interest Cost | 992 | 817 | 2,976 | 2,451
Expected Return on Plan Assets | (1,086) | (970) | (3,257) | (2,908)
Prior Service Cost Amortization | 0 | 0 | 0 | 0
Actuarial Loss Amortization | (187) | (218) | (561) | (655)
Sub-total | 221 | 126 | 663 | 379
Amounts Capitalized and Deferred | (64) | (8) | (98) | 170
Net Periodic Benefit Cost Recognized | 157 | 118 | 565 | 549
Supplemental Employee Retirement Plan [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost | 39 | 53 | 118 | 162
Interest Cost | 172 | 180 | 516 | 542
Expected Return on Plan Assets | 0 | 0 | 0 | 0
Prior Service Cost Amortization | 3 | 4 | 7 | 8
Actuarial Loss Amortization | (54) | 0 | (161) | 0
Sub-total | 160 | 237 | 480 | 712
Amounts Capitalized and Deferred | (51) | (73) | (152) | (221)
Net Periodic Benefit Cost Recognized | $ 109 | $ 164 | $ 328 | $ 491

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