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Note 4 - Other Long-term Assets (Tables) | 3 Months Ended
--- | ---
Mar. 31, 2022
Notes Tables |
Schedule of Other Assets, Noncurrent [Table Text Block] |
|  | March 31, |  |  | December 31, |
|  | 2022 |  |  | 2021 |
Advanced coal royalties |  | $ | 6,668 |  |  | $ | 6,678 |
Other |  |  | 1,665 |  |  |  | 1,694 |
Total other assets |  | $ | 8,333 |  |  | $ | 8,372 |

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- Definition Tabular disclosure of noncurrent assets. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(17)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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