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Postretirement Health Care and Life Insurance Benefits (Details 1) - Postretirement Health Care and Life Insurance Benefits [Member]
$ in Millions
 12 Months Ended
Dec. 31, 2016
USD ($)
years
Dec. 31, 2015
USD ($)
years
Dec. 31, 2014
USD ($)
Defined Benefit Plan Disclosure [Line Items]      
Percentage of actuarial gains and losses amortized 0.00%    
Amortization period - future working lifetime of active employees | years 10.31 10.49  
Estimated net actuarial loss that will be amortized from accumulated other comprehensive (income) loss $ 22.0    
Prior service (cost) credit for the period 9.2    
Amounts recognized in accumulated other comprehensive loss      
Net actuarial loss (gain) arising during year 32.3 $ (35.1) $ 115.8
Prior service credit arising during year 0.0 0.0 (18.0)
Amortization:      
Actuarial loss (20.4) (24.9) (14.5)
Prior service credit (cost) 9.2 6.8 (1.3)
Settlement related to the Patriot bankruptcy      
Prior service cost [1] 7.2 (16.6) 0.0
Total recorded in other comprehensive (income) loss $ 28.3 $ (69.8) $ 82.0
[1] Refer to Note 27. "Matters Related to the Bankruptcy of Patriot Coal Corporation" herein for additional details related to this transaction.