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Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost $ 868 $ 829 $ 2,604 $ 2,491
Interest Cost 1,250 1,445 3,752 4,332
Expected Return on Plan Assets (2,422) (2,254) (7,268) (6,764)
Prior Service Cost Amortization 76 80 226 240
Actuarial Loss Amortization 2,021 1,618 6,065 4,854
Sub-total 1,793 1,718 5,379 5,153
Amounts Capitalized and Deferred (944) (861) (2,544) (2,298)
Net Periodic Benefit Cost Recognized 849 857 2,835 2,855
Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 759 673 2,275 2,023
Interest Cost 685 782 2,055 2,341
Expected Return on Plan Assets (627) (515) (1,881) (1,547)
Prior Service Cost Amortization 302 301 906 907
Actuarial Loss Amortization 260 187 784 559
Sub-total 1,379 1,428 4,139 4,283
Amounts Capitalized and Deferred (904) (860) (2,358) (2,087)
Net Periodic Benefit Cost Recognized 475 568 1,781 2,196
Supplemental Employee Retirement Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 89 70 266 212
Interest Cost 114 137 343 412
Prior Service Cost Amortization 14 15 42 43
Actuarial Loss Amortization 372 259 1,117 777
Sub-total 589 481 1,768 1,444
Amounts Capitalized and Deferred (178) (144) (534) (434)
Net Periodic Benefit Cost Recognized $ 411 $ 337 $ 1,234 $ 1,010