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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