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Note 7 - Revenue 1 (Details Textual) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Contract with Customer, Liability, Current | $ 0.5 |  | $ 0.5 |
Coal [Member] | INDIANA | Revenue from Contract with Customer Benchmark [Member] | Geographic Concentration Risk [Member] |  |  |  |
Concentration Risk, Percentage | 73.00% | 67.00% | 75.00% | 70.00%

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