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Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost $ 3,104 $ 3,393 $ 3,295
Interest Cost 6,484 5,878 6,057
Expected Return on Plan Assets (8,475) (7,785) (7,306)
Prior Service Cost Amortization 320 324 263
Actuarial Loss Amortization 4,324 5,786 4,662
Sub-total 5,757 7,596 6,971
Amounts Capitalized or Deferred (2,227) (3,465) (3,122)
NPBC Recognized 3,530 4,131 3,849
Other Postretirement Benefit Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 2,304 2,933 2,974
Interest Cost 3,426 3,404 3,913
Expected Return on Plan Assets (1,645) (1,635) (1,347)
Prior Service Cost Amortization 1,213 1,309 1,399
Actuarial Loss Amortization 227 1,383 2,098
Sub-total 5,525 7,394 9,037
Amounts Capitalized or Deferred (2,317) (3,416) (4,515)
NPBC Recognized 3,208 3,978 4,522
Supplemental Employee Retirement Plans, Defined Benefit      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 247 487 460
Interest Cost 567 404 392
Prior Service Cost Amortization 56 189 189
Actuarial Loss Amortization 628 486 295
Sub-total 1,498 1,566 1,336
Amounts Capitalized or Deferred (430) (451) (397)
NPBC Recognized $ 1,068 $ 1,115 $ 939