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Retirement Benefit Obligations - Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Pension Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost $ 498 $ 522 $ 1,492 $ 1,568
Interest Cost 1,887 1,870 5,659 5,610
Expected Return on Plan Assets (2,649) (2,673) (7,947) (8,017)
Prior Service Cost Amortization 80 90 242 267
Actuarial (Gain) Loss Amortization $ 361 0 $ 1,085 0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense)   Other Nonoperating Income (Expense)  
Sub-total $ 177 (191) $ 531 (572)
Amounts Capitalized and Deferred 30 333 165 1,197
Net Periodic Benefit Cost Recognized 207 142 696 625
Other Postretirement Benefits Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 497 374 1,491 1,120
Interest Cost 817 724 2,451 2,174
Expected Return on Plan Assets (970) (852) (2,908) (2,556)
Prior Service Cost Amortization 0 198 0 595
Actuarial (Gain) Loss Amortization $ (218) (366) $ (655) (1,098)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense)   Other Nonoperating Income (Expense)  
Sub-total $ 126 78 $ 379 235
Amounts Capitalized and Deferred (8) 82 170 415
Net Periodic Benefit Cost Recognized 118 160 549 650
Supplemental Employee Retirement Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 53 63 162 188
Interest Cost 180 188 542 565
Expected Return on Plan Assets 0 0 0 0
Prior Service Cost Amortization 4 14 8 42
Actuarial (Gain) Loss Amortization $ 0 0 $ 0 0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense)   Other Nonoperating Income (Expense)  
Sub-total $ 237 265 $ 712 795
Amounts Capitalized and Deferred (73) (82) (221) (246)
Net Periodic Benefit Cost Recognized $ 164 $ 183 $ 491 $ 549