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Postretirement Health Care and Life Insurance Benefits Estimated Future Benefit Payments (Details) - Postretirement Health Care and Life Insurance Benefits [Member]
$ in Millions
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure [Line Items]  
Postretirement Benefits, 2018 $ 52.2
Postretirement Benefits, 2019 53.5
Postretirement Benefits, 2020 55.9
Postretirement Benefits, 2021 57.6
Postretirement Benefits, 2022 56.9
Postretirement Benefits, Years 2023-2027 $ 266.4