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Inventories (Summary of Inventory) (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023
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Inventory Disclosure [Abstract] |  |
Coal | $ 19,576 | $ 23,784
Mining supplies | 56,179 | 53,216
Total inventories | $ 75,755 | $ 77,000

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