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Inventories (Details) - Successor - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Inventory [Line Items]    
Materials and supplies $ 118.1 $ 101.5
Raw coal 53.6 78.1
Saleable coal 108.5 111.7
Inventories 280.2 291.3
Reserve for materials and supplies    
Inventory [Line Items]    
Materials and supplies reserves $ 0.2 $ 0.6