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Inventories - Schedule of Inventory (Details) - USD ($) $ in Millions | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Inventories [Line Items] |  |
Materials and supplies | $ 123.8 | $ 118.1
Raw coal | 74.9 | 53.6
Saleable coal | 123.4 | 108.5
Total | 322.1 | 280.2
Reserve for materials and supplies |  |
Inventories [Line Items] |  |
SEC Schedule, 12-09, Valuation Allowances and Reserves, Amount | $ 0.8 | $ 0.2

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