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Employee Benefits (Future Contributions) (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Retirement Benefits [Abstract]  
Expected employer contribution in 2015 $ 25.4
Expected benefit payments  
2018 136.9
2019 87.5
2020 84.9
2021 82.2
2022 81.0
2023-2027 $ 374.3