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Discontinued Operations - Patriot Related Matters (Details) - Spinoff - Patriot - USD ($)
$ in Millions
6 Months Ended 12 Months Ended 24 Months Ended
Jan. 25, 2017
Jun. 30, 2016
Dec. 31, 2013
Dec. 31, 2015
Jun. 30, 2017
Dec. 31, 2007
Black Lung Occupational Disease Liability            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Potential exposure from Patriot bankruptcy         $ 124.6 $ 150.0
UMWA 1974 Pension Plan (UMWA Plan) Litigation            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Settlement of claim, amount awarded to other party $ 75.0          
Litigation settlement, liability $ 54.3       $ 52.6  
Funding of VEBA Pursuant to Settlement with UMWA and Patriot Coal Corporation            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Funding of the newly established VEBA     $ 310.0      
Number of years of payments to fund VEBA     4 years      
Payments made to fund VEBA       $ 165.0    
Settlement agreement, amount   $ 75.0        
Gain on VEBA settlement   $ 68.1