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Note 1 - Summary of Significant Accounting Policies (Details Textual) $ in Thousands, T in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 USD ($) T | Dec. 31, 2019 USD ($)
Accounts Receivable, Allowance for Credit Loss, Ending Balance | $ | $ 0 | $ 0
Surety bonds | $ | $ 27,000 |
Coal Supply Commitment (US Ton) | T | 21.6 |
Year Supply Commitments End | 2027 |
Priced Coal Supply Commitment (US Ton) | T | 13.7 |
Revenue, Product and Service Benchmark [Member] | Customer Concentration Risk [Member] | Coal [Member] |  |
Number of Major Customers | 4 | 4
Revenue, Product and Service Benchmark [Member] | Customer Concentration Risk [Member] | Four Customers [Member] | Coal [Member] |  |
Concentration Risk, Percentage | 79.00% | 70.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Coal [Member] |  |
Number of Major Customers | 4 | 3
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Four Customers [Member] | Coal [Member] |  |
Concentration Risk, Percentage | 87.00% |
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Three Customers [Member] | Coal [Member] |  |
Concentration Risk, Percentage |  | 68.00%
Minimum [Member] |  |
Property, Plant and Equipment, Useful Life (Year) | 3 years |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) | 2 years |
Minimum [Member] | Measurement Input, Discount Rate [Member] |  |
ARO measurement input | 0.050 |
Maximum [Member] |  |
Property, Plant and Equipment, Useful Life (Year) | 25 years |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) | 4 years |
Maximum [Member] | Measurement Input, Discount Rate [Member] |  |
ARO measurement input | 0.10 |
Southwestern Indiana [Member] |  |
Number of Underground Mines Included in the Significant Operating Segment | 2 |

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