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Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost $ 831 $ 776 $ 1,662 $ 1,552
Interest Cost 1,443 1,621 2,887 3,242
Expected Return on Plan Assets (2,255) (2,118) (4,510) (4,237)
Prior Service Cost Amortization 80 80 160 160
Actuarial Loss Amortization 1,618 1,081 3,236 2,162
Sub-total 1,717 1,440 3,435 2,879
Amounts Capitalized and Deferred (807) (597) (1,437) (1,009)
Net Periodic Benefit Cost Recognized 910 843 1,998 1,870
Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 675 576 1,350 1,152
Interest Cost 779 856 1,559 1,712
Expected Return on Plan Assets (516) (411) (1,032) (822)
Prior Service Cost Amortization 303 303 606 606
Actuarial Loss Amortization 186 57 372 114
Sub-total 1,427 1,381 2,855 2,762
Amounts Capitalized and Deferred (695) (606) (1,227) (1,080)
Net Periodic Benefit Cost Recognized 732 775 1,628 1,682
Supplemental Employee Retirement Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 71 63 142 123
Interest Cost 138 145 275 284
Prior Service Cost Amortization 14 25 28 28
Actuarial Loss Amortization 259 156 518 314
Sub-total 482 389 963 749
Amounts Capitalized and Deferred (152) (112) (290) (215)
Net Periodic Benefit Cost Recognized $ 330 $ 277 $ 673 $ 534