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Employee Benefit Plans: AOCI Amounts (After-Tax) (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Net (gain) loss $ 5,511 $ 5,322
Prior service cost (gain) 0 0
Total amounts included in AOCI, after-tax not yet recognized as components of net periodic expense 5,511 5,322
Supplemental Employee Retirement Plan    
Defined Benefit Plan Disclosure [Line Items]    
Net (gain) loss 9,323 9,893
Prior service cost (gain) 0 2
Total amounts included in AOCI, after-tax not yet recognized as components of net periodic expense 9,323 9,895
Other Postretirement Benefits Plan    
Defined Benefit Plan Disclosure [Line Items]    
Net (gain) loss 100 90
Prior service cost (gain) (144) (230)
Total amounts included in AOCI, after-tax not yet recognized as components of net periodic expense $ (44) $ (140)