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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