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Employee Benefit Plans, Components of Net Periodic Benefit Cost (Details) - Defined Benefit Pension Plans Combined [Member] - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Components of Net Periodic Benefit Cost [Abstract]    
Service cost $ 938 $ 849
Interest cost 1,457 1,645
Expected return on plan assets (3,198) (2,733)
Amortization of loss 370 421
Amortization of prior service credit (13) (13)
Rate-regulated adjustment 2,746 2,131
Net periodic benefit cost 2,300 $ 2,300
Change in defined benefit plan regulatory asset from pension contribution greater (less) than net periodic benefit cost 2,746  
Amortization of Regulatory Assets to be Reclassified into Net Periodic Benefit Cost [Abstract]    
Net loss 582  
Net prior service credit (13)  
Total amortization of regulatory assets to be reclassified into net periodic benefit cost during the next fiscal year $ 569