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Employee Benefit Plans, Changes in Benefit Obligation and Plan Assets (Details) - Defined Benefit Pension Plans Combined [Member] - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Employee Benefit Plans [Abstract]    
Maximum period of eligible service 30 years  
Change in Benefit Obligation [Abstract]    
Pension benefit obligation, beginning of year $ 47,530 $ 41,511
Service cost 938 849
Interest cost 1,457 1,645
Actuarial loss 6,165 5,241
Benefit payments (1,984) (1,716)
Pension benefit obligation, end of year 54,106 47,530
Change in Plan Assets [Abstract]    
Fair value of plan assets, beginning of year 49,349 40,624
Actual return on plan assets 6,650 8,141
Employer contributions 2,300 2,300
Benefits paid (1,984) (1,716)
Fair value of plan assets, end of year 56,315 49,349
Funded status of plans at end of year $ 2,209 $ 1,819
Change in discount rate (0.80%) (1.00%)
Threshold for amortization of gains and losses 10.00%  
Changes in Plan Assets and Benefit Obligations Recognized in Regulatory Assets [Abstract]    
Net gain (loss) arising during the period $ 2,713 $ (167)
Recognized net actuarial loss (370) (421)
Recognized prior service credit 13 13
Total changes in regulatory asset during the year 2,356 (575)
Amounts Recognized in Regulatory Assets that Have Not Yet Been Recognized as Components of Net Periodic Benefit Cost [Abstract]    
Net loss 10,497 8,154
Prior service credit (63) (76)
Regulatory asset $ 10,434 $ 8,078