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Pension Plan and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost–Benefit Earned During the Period $ 1,866 $ 1,655
Interest Cost on Projected Benefit Obligation 2,915 3,263
Expected Return on Assets (5,590) (5,505)
Amortization of Net Actuarial Loss:    
From Regulatory Asset 2,660 2,231
From Other Comprehensive Income 68 55
Net Periodic Pension Cost 1,919 1,699
Executive Survivor and Supplemental Retirement Plan    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost–Benefit Earned During the Period 47 45
Interest Cost on Projected Benefit Obligation 307 362
Amortization of Net Actuarial Loss:    
From Regulatory Asset 31 23
From Other Comprehensive Income 124 86
Net Periodic Pension Cost 509 516
Other Postretirement Benefits Plan    
Defined Benefit Plan Disclosure [Line Items]    
Service Cost–Benefit Earned During the Period 430 462
Interest Cost on Projected Benefit Obligation 473 598
Amortization of Prior Service Cost:    
From Regulatory Asset (1,397) (1,169)
From Other Comprehensive Income (36) (29)
Amortization of Net Actuarial Loss:    
From Regulatory Asset 920 1,051
From Other Comprehensive Income 23 26
Net Periodic Pension Cost $ 413 $ 939