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Employee Benefit Plans: Employee Benefit Plan Obligations (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plan      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Projected benefit obligation at beginning of year $ 485,376 $ 445,381  
Service Cost 5,411 5,383 $ 6,834
Interest cost 13,426 17,374 15,470
Actuarial (gain) loss 47,064 56,384  
Benefits paid (37,269) (39,146)  
Plan participants’ contributions 0 0  
Projected benefit obligation at end of year 514,008 485,376 445,381
Supplemental Employee Retirement Plan      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Projected benefit obligation at beginning of year 54,088 43,010  
Service Cost 1,579 4,995  
Interest cost 1,099 1,295  
Actuarial (gain) loss 962 7,132  
Benefits paid (2,674) (2,344)  
Plan participants’ contributions 0 0  
Projected benefit obligation at end of year 55,054 54,088 43,010
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 65,277 60,817  
Service Cost 2,056 1,815 2,291
Interest cost 1,649 2,247 2,085
Actuarial (gain) loss 5,804 5,976  
Benefits paid (6,058) (7,033)  
Plan participants’ contributions 1,510 1,455  
Projected benefit obligation at end of year $ 70,238 $ 65,277 $ 60,817