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Retirement Benefit Obligations - Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Pension Plan [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost $ 436 $ 497
Interest Cost 2,016 1,886
Expected Return on Plan Assets (2,624) (2,649)
Prior Service Cost Amortization 64 81
Actuarial Loss Amortization 319 362
Sub-total 211 177
Amounts Capitalized and Deferred 109 108
Net Periodic Benefit Cost Recognized $ 320 285
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense)  
Other Postretirement Benefits Plan [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost $ 502 497
Interest Cost 992 817
Expected Return on Plan Assets (1,086) (969)
Prior Service Cost Amortization 0 0
Actuarial Loss Amortization (187) (219)
Sub-total 221 126
Amounts Capitalized and Deferred 14 124
Net Periodic Benefit Cost Recognized $ 235 250
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense)  
Supplemental Employee Retirement Plan [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost $ 39 54
Interest Cost 172 181
Expected Return on Plan Assets 0 0
Prior Service Cost Amortization 2 2
Actuarial Loss Amortization (53) 0
Sub-total 160 237
Amounts Capitalized and Deferred (51) (73)
Net Periodic Benefit Cost Recognized $ 109 $ 164
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense)