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Retirement Plans and Postretirement Medical Benefits (Nonpension Estimated Future Benefit Payments) (Details)
$ in Thousands
Dec. 31, 2018
USD ($)
Pension Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2019 $ 137,312
2020 131,672
2021 129,641
2022 128,972
2023 128,774
Thereafter 627,369
Total 1,283,740
Nonpension benefits  
Defined Benefit Plan Disclosure [Line Items]  
2019 17,040
2020 16,376
2021 15,783
2022 14,737
2023 13,706
Thereafter 59,518
Total $ 137,160