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Inventories (Summary of Inventory) (Details) - USD ($) $ in Thousands | Mar. 31, 2023 | Dec. 31, 2022
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Inventory Disclosure [Abstract] |  |
Coal | $ 26,229 | $ 27,927
Mining supplies | 45,017 | 43,561
Total inventories | $ 71,246 | $ 71,488

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