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Note 7 - Revenue 1 (Details Textual) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Contract with Customer, Liability, Current | $ 0.3 |
Coal [Member] | INDIANA | Revenue from Contract with Customer Benchmark [Member] | Geographic Concentration Risk [Member] |  |
Concentration Risk, Percentage | 74.00% | 74.00%

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