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Pension Plan and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Pension Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost–Benefit Earned During the Period | $ 1,866 | $ 1,655 | $ 5,597 | $ 4,966
Interest Cost on Projected Benefit Obligation | 2,915 | 3,264 | 8,745 | 9,790
Expected Return on Assets | (5,590) | (5,506) | (16,769) | (16,516)
Amortization of Net Actuarial Loss: |  |  |  |
From Regulatory Asset | 2,660 | 2,231 | 7,981 | 6,693
From Other Comprehensive Income | 68 | 55 | 204 | 165
Net Periodic Pension Cost | 1,919 | 1,699 | 5,758 | 5,098
Executive Survivor and Supplemental Retirement Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost–Benefit Earned During the Period | 47 | 45 | 140 | 134
Interest Cost on Projected Benefit Obligation | 307 | 362 | 921 | 1,086
Amortization of Net Actuarial Loss: |  |  |  |
From Regulatory Asset | 30 | 24 | 92 | 71
From Other Comprehensive Income | 124 | 85 | 373 | 256
Net Periodic Pension Cost | 508 | 516 | 1,526 | 1,547
Other Postretirement Benefits Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Service Cost–Benefit Earned During the Period | 430 | 461 | 1,291 | 1,385
Interest Cost on Projected Benefit Obligation | 472 | 598 | 1,418 | 1,795
Amortization of Prior Service Cost: |  |  |  |
From Regulatory Asset | (1,397) | (1,169) | (4,192) | (3,508)
From Other Comprehensive Income | (35) | (28) | (107) | (86)
Amortization of Net Actuarial Loss: |  |  |  |
From Regulatory Asset | 919 | 1,051 | 2,759 | 3,154
From Other Comprehensive Income | 24 | 26 | 71 | 78
Net Periodic Pension Cost | $ 413 | $ 939 | $ 1,240 | $ 2,818

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