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Note 7 - Revenue 1 (Details Textual) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Coal [Member] | INDIANA | Revenue from Contract with Customer Benchmark [Member] | Geographic Concentration Risk [Member] |  |  |  |
Concentration Risk, Percentage | 67.00% | 75.00% | 72.00% | 75.00%

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