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Inventories - Schedule of Inventory (Details) - USD ($) $ in Millions | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Inventories [Line Items] |  |
Materials and supplies | $ 111.2 | $ 116.3
Raw coal | 49.7 | 85.1
Saleable coal | 140.7 | 130.1
Total | 301.6 | 331.5
Reserve for materials and supplies |  |
Inventories [Line Items] |  |
Materials and Supplies Reserve | $ 9.3 | $ 7.9

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