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Retirement Benefits - Health Care Cost Trends and OPEB Future Ben Pmts (Details)
$ in Millions
12 Months Ended
Dec. 31, 2015
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 51.3
Effects of 1% decrease on service and interest cost (1.9)
Effects of 1% decrease on APBO (43.5)
UMWA Plans  
Estimated Future Benefits Payments [Line Items]  
2016 30.9
2017 31.2
2018 31.1
2019 30.8
2020 31.7
2021 through 2025 141.8
Black Lung and Other Plans  
Estimated Future Benefits Payments [Line Items]  
2016 6.3
2017 5.9
2018 5.6
2019 5.2
2020 4.9
2021 through 2025 19.6
Retirement benefits other than pension  
Estimated Future Benefits Payments [Line Items]  
2016 37.2
2017 37.1
2018 36.7
2019 36.0
2020 36.6
2021 through 2025 $ 161.4