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Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost $ 829 $ 776 $ 2,491 $ 2,328
Interest Cost 1,445 1,621 4,332 4,863
Expected Return on Plan Assets (2,254) (2,119) (6,764) (6,356)
Prior Service Cost Amortization 80 80 240 240
Actuarial Loss Amortization 1,618 1,081 4,854 3,243
Sub-total 1,718 1,439 5,153 4,318
Amounts Capitalized and Deferred (861) (650) (2,298) (1,659)
Net Periodic Benefit Cost Recognized 857 789 2,855 2,659
Other Postretirement Benefit Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 673 576 2,023 1,728
Interest Cost 782 857 2,341 2,569
Expected Return on Plan Assets (515) (412) (1,547) (1,234)
Prior Service Cost Amortization 301 304 907 910
Actuarial Loss Amortization 187 56 559 170
Sub-total 1,428 1,381 4,283 4,143
Amounts Capitalized and Deferred (860) (650) (2,087) (1,730)
Net Periodic Benefit Cost Recognized 568 731 2,196 2,413
Supplemental Employee Retirement Plans, Defined Benefit        
Defined Benefit Plan Disclosure [Line Items]        
Service Cost 70 62 212 185
Interest Cost 137 142 412 426
Prior Service Cost Amortization 15 14 43 42
Actuarial Loss Amortization 259 157 777 471
Sub-total 481 375 1,444 1,124
Amounts Capitalized and Deferred (144) (107) (434) (322)
Net Periodic Benefit Cost Recognized $ 337 $ 268 $ 1,010 $ 802