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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, 2018
Dec. 31, 2016
Jun. 30, 2017
Successor          
Operating Leased Assets [Line Items]          
Operating Leases, Rent Expense   $ 144.2 $ 155.1    
Capital Leased Assets, Gross   125.3 125.0    
Capital Leases, Lessee Balance Sheet, Assets by Major Class, Accumulated Depreciation   20.6 47.9    
Royalty Expense   $ 364.6 $ 474.3    
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 $ 40.0   $ 79.9
Predecessor          
Operating Leased Assets [Line Items]          
Operating Leases, Rent Expense $ 57.0     $ 261.8  
Royalty Expense $ 115.2     $ 389.7