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Retirement Benefit Plans - Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost $ 1,990 $ 2,091 $ 3,165
Interest Cost 7,546 7,480 5,486
Expected Return on Plan Assets (10,597) (10,689) (10,883)
Prior Service Cost Amortization 323 356 356
Actuarial Loss Amortization $ 1,446   5,507
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense)    
Sub-total $ 708 (762) 3,631
Amounts Capitalized or Deferred 227 1,598 (1,085)
NPBC Recognized 935 836 2,546
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 1,988 1,493 2,890
Interest Cost 3,269 2,899 3,194
Expected Return on Plan Assets (3,877) (3,408) (3,415)
Prior Service Cost Amortization 0 794 1,092
Actuarial Loss Amortization $ (874) (1,464) 1,020
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense)    
Sub-total $ 506 314 4,781
Amounts Capitalized or Deferred 191 555 (2,388)
NPBC Recognized 697 869 2,393
Supplemental Employee Retirement Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 216 250 273
Interest Cost 723 754 472
Prior Service Cost Amortization $ 10 55 55
Actuarial Loss Amortization     794
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration] Other Nonoperating Income (Expense)    
Sub-total $ 949 1,059 1,594
Amounts Capitalized or Deferred (295) (327) (472)
NPBC Recognized $ 654 $ 732 $ 1,122