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Retirement Benefits - Health Care Cost Trends and OPEB Future Ben Pmts (Details)
$ in Millions
12 Months Ended
Dec. 31, 2016
USD ($)
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract]  
Effects of 1% increase on service and interest cost $ 2.2
Effects of 1% increase on APBO 53.5
Effects of 1% decrease on service and interest cost (1.9)
Effects of 1% decrease on APBO (45.6)
UMWA Plans  
Estimated Future Benefits Payments [Line Items]  
2017 31.9
2018 31.9
2019 31.7
2020 32.7
2021 31.9
2022 through 2026 141.9
Black Lung and Other Plans  
Estimated Future Benefits Payments [Line Items]  
2017 6.6
2018 6.2
2019 5.8
2020 5.4
2021 5.0
2022 through 2026 20.4
Retirement benefits other than pension  
Estimated Future Benefits Payments [Line Items]  
2017 38.5
2018 38.1
2019 37.5
2020 38.1
2021 36.9
2022 through 2026 $ 162.3