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Retirement Benefit Obligations - Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Pension Plan [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost $ 497 $ 523
Interest Cost 1,886 1,870
Expected Return on Plan Assets (2,649) (2,672)
Prior Service Cost Amortization 81 89
Actuarial Loss Amortization 362 (0)
Sub-total 177 (190)
Amounts Capitalized and Deferred 108 498
Net Periodic Benefit Cost Recognized $ 285 308
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 $ 497 373
Interest Cost 817 725
Expected Return on Plan Assets (969) (852)
Prior Service Cost Amortization 0 198
Actuarial Loss Amortization (219) (366)
Sub-total 126 78
Amounts Capitalized and Deferred 124 221
Net Periodic Benefit Cost Recognized $ 250 299
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 $ 54 63
Interest Cost 181 188
Prior Service Cost Amortization 2 14
Actuarial Loss Amortization (0) 0
Sub-total 237 265
Amounts Capitalized and Deferred (73) (82)
Net Periodic Benefit Cost Recognized $ 164 $ 183
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense)