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Employee Benefit Plans: Plan Contributions (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year $ 13,000  
Pension Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Payment for Pension and Other Postretirement Benefits 12,700 $ 27,700
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Payment for Pension and Other Postretirement Benefits 5,298 4,332
Supplemental Employee Retirement Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Payment for Pension and Other Postretirement Benefits 2,073 3,217
Defined Contribution Plan, Company Retirement    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Employer Contributions 8,766 10,223
Defined Contribution Plan, 401K    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Employer Contributions $ 13,559 $ 9,811