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Note 7 - Revenue 1 (Details Textual) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2022 | Mar. 31, 2021 | Jan. 01, 2021
Provision for Loss on Contract, Recognized During Period | $ 0.2 |  |
Coal [Member] |  |  |
Accounts Receivable, after Allowance for Credit Loss, Total |  |  | $ 14.4
Coal [Member] | Revenue from Contract with Customer Benchmark [Member] | Geographic Concentration Risk [Member] | INDIANA |  |  |
Concentration Risk, Percentage | 87.00% | 77.00% |

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