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Components of Retirement Plan Costs (Detail) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Pension Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost $ 868 $ 831
Interest Cost 1,251 1,444
Expected Return on Plan Assets (2,423) (2,255)
Prior Service Cost Amortization 75 80
Actuarial Loss Amortization 2,022 1,618
Sub-total 1,793 1,718
Amounts Capitalized and Deferred (713) (630)
Net Periodic Benefit Cost Recognized 1,080 1,088
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost 758 675
Interest Cost 685 780
Expected Return on Plan Assets (627) (516)
Prior Service Cost Amortization 302 303
Actuarial Loss Amortization 262 186
Sub-total 1,380 1,428
Amounts Capitalized and Deferred (662) (532)
Net Periodic Benefit Cost Recognized 718 896
Supplemental Employee Retirement Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost 89 71
Interest Cost 114 137
Prior Service Cost Amortization 14 14
Actuarial Loss Amortization 372 259
Sub-total 589 481
Amounts Capitalized and Deferred (178) (138)
Net Periodic Benefit Cost Recognized $ 411 $ 343