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Leases Textuals (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Apr. 01, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 30, 2017
Operating Leased Assets [Line Items]          
Contingent lease expense   $ 0.0 $ 0.0    
Predecessor          
Operating Leased Assets [Line Items]          
Operating Leases, Rent Expense $ 57.6   264.7 $ 290.1  
Capital Leased Assets, Gross   125.3 77.9    
Capital Leases, Lessee Balance Sheet, Assets by Major Class, Accumulated Depreciation     48.6    
Royalty Expense $ 115.2   389.7 $ 444.5  
Successor          
Operating Leased Assets [Line Items]          
Operating Leases, Rent Expense   146.3      
Contingent lease expense   0.0      
Capital Leased Assets, Gross   125.3      
Capital Leases, Lessee Balance Sheet, Assets by Major Class, Accumulated Depreciation   20.6      
Royalty Expense   $ 364.6      
Federal Mineral Lease Term   ten years      
Minimum Annual Production Of Federal Coal Mining Leases On Original Amount   1.00%      
Monthly federal royalties payable from sale using surface mining methods   12.50%      
Monthly federal royalties payable of production using underground mining methods   8.00%      
Period to redetermine royalty rates on Indian tribe leased coal reserves   every ten years      
Company lease Obligations Subject to Surety Bonds   $ 95.4      
Capital lease and other obligations   $ 76.0 $ 20.1   $ 79.9