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Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost $ 3,322 $ 3,104 $ 3,393
Interest Cost 5,776 6,484 5,878
Expected Return on Plan Assets (9,019) (8,475) (7,785)
Prior Service Cost Amortization 320 320 324
Actuarial Loss Amortization 6,472 4,324 5,786
Sub-total 6,871 5,757 7,596
Amounts Capitalized or Deferred (3,083) (2,227) (3,465)
NPBC Recognized 3,788 3,530 4,131
Other Postretirement Benefit Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 2,698 2,304 2,933
Interest Cost 3,121 3,426 3,404
Expected Return on Plan Assets (2,063) (1,645) (1,635)
Prior Service Cost Amortization 1,210 1,213 1,309
Actuarial Loss Amortization 744 227 1,383
Sub-total 5,710 5,525 7,394
Amounts Capitalized or Deferred (2,865) (2,317) (3,416)
NPBC Recognized 2,845 3,208 3,978
Supplemental Employee Retirement Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 283 247 487
Interest Cost 549 567 404
Prior Service Cost Amortization 57 56 189
Actuarial Loss Amortization 1,036 628 486
Sub-total 1,925 1,498 1,566
Amounts Capitalized or Deferred (579) (430) (451)
NPBC Recognized $ 1,346 $ 1,068 $ 1,115