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Inventories (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Apr. 01, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Successor          
Inventory [Line Items]          
Materials and supplies $ 101.5        
Raw coal 78.1        
Saleable coal 111.7        
Total 291.3        
Predecessor          
Inventory [Line Items]          
Materials and supplies     $ 104.5    
Raw coal     29.6    
Saleable coal     69.6    
Total     203.7    
Reserve for materials and supplies [Member] | Successor          
Inventory [Line Items]          
Valuation Allowances and Reserves, Balance $ 0.6 $ 0.0      
Reserve for materials and supplies [Member] | Predecessor          
Inventory [Line Items]          
Valuation Allowances and Reserves, Balance   $ 0.0 $ 5.6 $ 4.7 $ 4.6