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Employee Benefit Plans: Changes in Plan Assets (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Pension Plans, Defined Benefit    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]    
Beginning fair value of plan assets $ 416,343 $ 364,695
Investment income (loss) (17,939) 48,617
Employer contributions 12,700 27,700
Retiree contributions 0 0
Benefits paid (20,308) (24,669)
Ending fair value of plan assets 390,796 416,343
Supplemental Employee Retirement Plans, Defined Benefit    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]    
Beginning fair value of plan assets 0 0
Investment income (loss) 0 0
Employer contributions 2,073 3,217
Retiree contributions 0 0
Benefits paid (2,073) (3,217)
Ending fair value of plan assets 0 0
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]    
Beginning fair value of plan assets 8,621 8,470
Investment income (loss) (149) 120
Employer contributions 3,543 3,025
Retiree contributions 1,445 1,338
Benefits paid (5,298) (4,332)
Ending fair value of plan assets $ 8,162 $ 8,621