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Benefit Plans Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 5,790 $ 4,699 $ 4,447
Interest cost 6,879 6,993 6,830
Expected return on assets (9,255) (7,888) (7,288)
Amortization of prior service cost 51 94 376
Recognized actuarial loss 3,986 3,844 3,527
Net periodic pension cost 7,451 7,742 7,892
Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 616 529 527
Interest cost 627 634 655
Expected return on assets (450) (376) (292)
Amortization of prior service cost 197 198 197
Recognized actuarial loss 321 273 316
Net periodic pension cost 1,311 1,258 1,403
Other production expenses | Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic pension cost 1,534 1,231 1,165
Other production expenses | Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic pension cost 163 139 138
Administrative and general expense | Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic pension cost 3,249 2,632 2,490
Administrative and general expense | Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic pension cost 346 296 295
Maintenance expense | Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic pension cost 1,007 836 792
Maintenance expense | Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic pension cost 107 94 94
Pension non-service costs | Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic pension cost 1,661 3,043 3,445
Pension non-service costs | Other Postretirement Benefits Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Net periodic pension cost $ 695 $ 729 $ 876