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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

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