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Asset Impairment and Mine Closure Costs Textuals (Details) - Predecessor - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Jun. 30, 2015
Dec. 31, 2016
Dec. 31, 2015
Nov. 30, 2016
Impaired Long-Lived Assets Held and Used [Line Items]            
Impairment of Long-Lived Assets to be Disposed of (after tax) $ 193.2          
Impairment of Long-Lived Assets Held-for-use       $ 247.9 $ 1,001.3  
Equity Method Investment, Other than Temporary Impairment         276.5  
Midwestern U.S. Mining [Member]            
Impaired Long-Lived Assets Held and Used [Line Items]            
Impairment of Long-Lived Assets Held-for-use   $ 9.7   54.7 40.2  
Equity Method Investment, Other than Temporary Impairment         0.0  
Australian Mining            
Impaired Long-Lived Assets Held and Used [Line Items]            
Impairment of Long-Lived Assets Held-for-use   144.5 $ 230.5 193.2 675.2  
Impairment of Long-Lived Assets to be Disposed of       17.2    
Equity Method Investment, Other than Temporary Impairment         0.0  
Remediation Property for Sale, Abandonment or Disposal [Member]            
Impaired Long-Lived Assets Held and Used [Line Items]            
Impairment of Long-Lived Assets Held-for-use   317.7        
Midwestern Inactive Surface Mine [Member]            
Impaired Long-Lived Assets Held and Used [Line Items]            
Impairment of Long-Lived Assets Held-for-use       30.5    
Corporate and Other            
Impaired Long-Lived Assets Held and Used [Line Items]            
Impairment of Long-Lived Assets Held-for-use   86.2   37.5 268.4  
Impairment of Long-Lived Assets to be Disposed of       182.2    
Middlemount Mine [Member]            
Impaired Long-Lived Assets Held and Used [Line Items]            
Equity Method Investment, Other than Temporary Impairment       46.6    
Subordinated Borrowing, Name [Domain]            
Impaired Long-Lived Assets Held and Used [Line Items]            
Equity Method Investment, Other than Temporary Impairment       229.9 229.9  
Middlemount Mine [Member] | Financing Receivable [Member]            
Impaired Long-Lived Assets Held and Used [Line Items]            
Investments and Other Noncurrent Assets $ 84.8 $ 65.2   $ 84.8 $ 65.2  
Metropolitan Collieries Pty Ltd [Member]            
Impaired Long-Lived Assets Held and Used [Line Items]            
Planned Divestiture, Agreed Upon Proceeds from Divestiture           $ 200.0