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Retirement Benefits - Health Care Cost Trends and OPEB Future Ben Pmts (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]  
Effects of 1% increase on service and interest cost $ 2.3
Effects of 1% increase on APBO 79.4
Effects of 1% decrease on service and interest cost (1.9)
Effects of 1% decrease on APBO (67.0)
Retirement benefits other than pension  
Estimated Future Benefits Payments [Line Items]  
2018 41.5
2019 40.9
2020 40.8
2021 40.3
2022 39.2
2023 through 2027 179.2
UMWA Plans  
Estimated Future Benefits Payments [Line Items]  
2018 34.2
2019 34.0
2020 34.4
2021 34.3
2022 33.6
2023 through 2027 156.3
Black Lung and Other Plans  
Estimated Future Benefits Payments [Line Items]  
2018 7.3
2019 6.9
2020 6.4
2021 6.0
2022 5.6
2023 through 2027 $ 22.9