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Pension Benefit and Retirement Health and Life Insurance Benefits (Estimated Future Payments) (Details)
$ in Thousands
Dec. 31, 2017
USD ($)
Pension Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2018 $ 9,261
2019 9,335
2020 9,423
2021 9,644
2022 9,956
2023-2027 52,676
Retirement Health and Life Insurance Benefits  
Defined Benefit Plan Disclosure [Line Items]  
2018 352
2019 316
2020 267
2021 174
2022 128
2023-2027 $ 795