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Inventories (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Inventory [Line Items] |  |
Materials and supplies | $ 102.6 | $ 116.3
Raw coal | 70.5 | 85.1
Saleable coal | 88.5 | 130.1
Inventories | 261.6 | 331.5
Material And Supplies |  |
Inventory [Line Items] |  |
Inventory reserves | 10.4 | 7.9
Coal |  |
Inventory [Line Items] |  |
Inventory reserves | $ 7.6 | $ 8.3

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