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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

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