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Note 4 - Other Long-term Assets (Tables) | 6 Months Ended
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Jun. 30, 2020
Notes Tables |
Schedule of Other Assets, Noncurrent [Table Text Block] |
|  | June 30, |  |  | December 31, |
|  | 2020 |  |  | 2019 |
Advanced coal royalties |  | $ | 6,303 |  |  | $ | 6,105 |
Marketable equity securities available for sale, at fair value (restricted)* |  |  | — |  |  |  | 2,296 |
Other |  |  | 1,866 |  |  |  | 1,923 |
Total other assets |  | $ | 8,169 |  |  | $ | 10,324 |

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