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Inventories - Schedule of Inventory (Details) - Successor - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Inventories [Line Items]    
Materials and supplies $ 104.3 $ 101.5
Raw coal 57.5 78.1
Saleable coal 115.3 111.7
Total 277.1 291.3
Reserve for materials and supplies    
Inventories [Line Items]    
Valuation Allowances and Reserves, Balance $ 0.2 $ 0.6