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Retirement Benefit Obligations - Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Pension Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost $ 497 $ 523 $ 994 $ 1,046
Interest Cost 1,886 1,870 3,772 3,740
Expected Return on Plan Assets (2,649) (2,672) (5,298) (5,344)
Prior Service Cost Amortization 81 88 162 177
Actuarial (Gain) Loss Amortization $ 362 0 $ 724 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) $ 354 (381)
Amounts Capitalized and Deferred 27 366 135 864
Net Periodic Benefit Cost Recognized 204 175 489 483
Other Postretirement Benefits Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 497 373 994 746
Interest Cost 817 725 1,634 1,450
Expected Return on Plan Assets (969) (852) (1,938) (1,704)
Prior Service Cost Amortization 0 199 0 397
Actuarial (Gain) Loss Amortization $ (218) (366) $ (437) (732)
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 $ 127 79 $ 253 157
Amounts Capitalized and Deferred 54 112 178 333
Net Periodic Benefit Cost Recognized 181 191 431 490
Supplemental Employee Retirement Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 55 63 109 125
Interest Cost 181 188 362 377
Expected Return on Plan Assets 0 0    
Prior Service Cost Amortization 2 14 4 28
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 $ 238 265 $ 475 530
Amounts Capitalized and Deferred (74) (82) (148) (164)
Net Periodic Benefit Cost Recognized $ 164 $ 183 $ 327 $ 366