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Employee Benefit Plans: Components of Net Periodic Expense (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost $ 5,411 $ 5,383 $ 6,834
Interest cost 13,426 17,374 15,470
Expected return on assets (22,591) (24,401) (24,741)
Net amortization of prior service cost 0 26 58
Recognized net actuarial loss (gain) 8,372 3,763 8,632
Net periodic benefit expense 4,618 2,145 6,253
Other Pension, Postretirement and Supplemental Plans      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 1,579 4,995 1,764
Interest cost 1,099 1,295 1,170
Expected return on assets 0 0 0
Net amortization of prior service cost 2 2 2
Recognized net actuarial loss (gain) 1,702 535 1,000
Net periodic benefit expense 4,382 6,827 3,936
Other Pension, Postretirement and Supplemental Plans | Error Correction, Other      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 1,400    
Other Postretirement Benefits Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service Cost 2,056 1,815 2,291
Interest cost 1,649 2,247 2,085
Expected return on assets (182) (230) (315)
Net amortization of prior service cost (546) (398) (398)
Recognized net actuarial loss (gain) 20 0 216
Net periodic benefit expense $ 2,997 $ 3,434 $ 3,879