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Employee Benefit Plans: Fair Value Employee Benefit Plan Assets (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Beginning fair value of plan assets $ 434,284 $ 390,796
Investment income (loss) 64,006 69,934
Employer contributions 12,700 12,700
Retiree contributions 0 0
Defined Benefit Plan, Plan Assets, Benefits Paid (37,269) (39,146)
Ending fair value of plan assets 473,721 434,284
Supplemental Employee Retirement Plan    
Defined Benefit Plan Disclosure [Line Items]    
Beginning fair value of plan assets 0 0
Investment income (loss) 0 0
Employer contributions 2,674 2,344
Retiree contributions 0 0
Defined Benefit Plan, Plan Assets, Benefits Paid (2,674) (2,344)
Ending fair value of plan assets 0 0
Other Postretirement Benefits Plan    
Defined Benefit Plan Disclosure [Line Items]    
Beginning fair value of plan assets 8,305 8,162
Investment income (loss) 33 260
Employer contributions 4,374 5,461
Retiree contributions 1,511 1,455
Defined Benefit Plan, Plan Assets, Benefits Paid (6,058) (7,033)
Ending fair value of plan assets $ 8,165 $ 8,305