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Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost $ 868 $ 831 $ 1,736 $ 1,662
Interest Cost 1,251 1,443 2,502 2,887
Expected Return on Plan Assets (2,423) (2,255) (4,846) (4,510)
Prior Service Cost Amortization 75 80 150 160
Actuarial Loss Amortization 2,022 1,618 4,044 3,236
Sub-total 1,793 1,717 3,586 3,435
Amounts Capitalized and Deferred (887) (807) (1,600) (1,437)
Net Periodic Benefit Cost Recognized 906 910 1,986 1,998
Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 758 675 1,516 1,350
Interest Cost 685 779 1,370 1,559
Expected Return on Plan Assets (627) (516) (1,254) (1,032)
Prior Service Cost Amortization 302 303 604 606
Actuarial Loss Amortization 262 186 524 372
Sub-total 1,380 1,427 2,760 2,855
Amounts Capitalized and Deferred (791) (695) (1,454) (1,227)
Net Periodic Benefit Cost Recognized 589 732 1,306 1,628
Supplemental Employee Retirement Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 88 71 177 142
Interest Cost 115 138 229 275
Prior Service Cost Amortization 14 14 28 28
Actuarial Loss Amortization 373 259 745 518
Sub-total 590 482 1,179 963
Amounts Capitalized and Deferred (178) (152) (356) (290)
Net Periodic Benefit Cost Recognized $ 412 $ 330 $ 823 $ 673