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Employee Benefit Plans: Employee Benefit Plan Obligations (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Pension Plan      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Projected benefit obligation at beginning of year $ 514,008 $ 485,376  
Service Cost 5,038 5,411 $ 5,383
Interest cost 9,313 13,426 17,374
Actuarial (gain) loss (14,037) 47,064  
Amendments (561) 0  
Benefits paid (35,499) (37,269)  
Plan participants’ contributions 0 0  
Projected benefit obligation at end of year 478,262 514,008 485,376
Supplemental Employee Retirement Plan      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Projected benefit obligation at beginning of year 55,054 54,088  
Service Cost 3,149 1,579 4,995
Interest cost 706 1,099 1,295
Actuarial (gain) loss (1,073) 962  
Amendments 0 0  
Benefits paid (2,576) (2,674)  
Plan participants’ contributions 0 0  
Projected benefit obligation at end of year 55,260 55,054 54,088
Supplemental Employee Retirement Plan | Error Correction, Other      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Service Cost 1,400    
Other Postretirement Benefits Plan      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Projected benefit obligation at beginning of year 70,238 65,277  
Service Cost 2,237 2,056 1,815
Interest cost 1,058 1,649 2,247
Actuarial (gain) loss (5,165) 5,804  
Amendments 0 0  
Benefits paid (6,432) (6,058)  
Plan participants’ contributions 1,548 1,510  
Projected benefit obligation at end of year $ 63,484 $ 70,238 $ 65,277