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Note 4 - Other Long-term Assets (Tables) | 9 Months Ended
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Sep. 30, 2022
Notes Tables |
Schedule of Other Assets, Noncurrent [Table Text Block] |
|  | September 30, |  |  | December 31, |
|  | 2022 |  |  | 2021 |
Advanced coal royalties |  | $ | 6,181 |  |  | $ | 6,678 |
Other |  |  | 1,647 |  |  |  | 1,694 |
Total other assets |  | $ | 7,828 |  |  | $ | 8,372 |

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