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Inventories - Schedule of Inventory (Details) - Successor - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Inventories [Line Items]    
Materials and supplies $ 103.1 $ 101.5
Raw coal 56.6 78.1
Saleable coal 105.8 111.7
Total 265.5 291.3
Reserve for materials and supplies    
Inventories [Line Items]    
Valuation Allowances and Reserves, Balance $ 0.5 $ 0.6