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Note 1 - Summary of Significant Accounting Policies (Details Textual) $ in Thousands, T in Millions | 12 Months Ended | 24 Months Ended | 31 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2023 USD ($) T | Dec. 31, 2022 USD ($) | Dec. 31, 2028 | Dec. 31, 2026 | Dec. 31, 2025 | May 31, 2023 USD ($)
Number of Reportable Segments | 2 |  |  |  |  |
Accounts Receivable, Allowance for Credit Loss, Ending Balance | $ 0 | $ 0 |  |  |  |
Regulatory Asset | 37,500 |  |  |  |  |
Undiscounted Asset Retirement Obligations | $ 26,600 | $ 27,000 |  |  |  |
Capacity Payment Reductions |  |  |  |  |  | $ 7,500
Coal Supply Commitment (US Ton) | T | 9.2 |  |  |  |  |
Priced Coal Supply Commitment (US Ton) | T | 6.2 |  |  |  |  |
Year Supply Commitments End | 2028 |  |  |  |  |
Revenue, Product and Service Benchmark [Member] | Customer Concentration Risk [Member] | Coal [Member] |  |  |  |  |  |
Number of Major Customers | 5 | 5 |  |  |  |
Revenue, Product and Service Benchmark [Member] | Customer Concentration Risk [Member] | Coal [Member] | Five Customers [Member] |  |  |  |  |  |
Concentration Risk, Percentage | 93.00% | 90.00% |  |  |  |
Revenue, Product and Service Benchmark [Member] | Customer Concentration Risk [Member] | Electric Distribution [Member] |  |  |  |  |  |
Number of Major Customers | 2 | 1 |  |  |  |
Revenue, Product and Service Benchmark [Member] | Customer Concentration Risk [Member] | Electric Distribution [Member] | One Customer [Member] |  |  |  |  |  |
Concentration Risk, Percentage | 100.00% | 100.00% |  |  |  |
Revenue, Product and Service Benchmark [Member] | Customer Concentration Risk [Member] | Electric Distribution [Member] | Three Customers [Member] |  |  |  |  |  |
Concentration Risk, Percentage | 91.00% |  |  |  |  |
Number of Major Customers | 3 |  |  |  |  |
Revenue, Product and Service Benchmark [Member] | Customer Concentration Risk [Member] | Energy Output [Member] | Forecast [Member] |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  | 22.00% |
Revenue, Product and Service Benchmark [Member] | Customer Concentration Risk [Member] | Energy Capacity [Member] | Forecast [Member] |  |  |  |  |  |
Concentration Risk, Percentage |  |  | 28.00% |  | 32.00% |
Revenue, Product and Service Benchmark [Member] | Product Concentration Risk [Member] | Electricity, Generation [Member] |  |  |  |  |  |
Concentration Risk, Percentage | 100.00% |  |  |  |  |
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Coal [Member] |  |  |  |  |  |
Number of Major Customers | 4 | 4 |  |  |  |
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Coal [Member] | Five Customers [Member] |  |  |  |  |  |
Concentration Risk, Percentage | 85.00% | 86.00% |  |  |  |
Minimum [Member] |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) | 2 years |  |  |  |  |
Minimum [Member] | Revenue, Product and Service Benchmark [Member] | Customer Concentration Risk [Member] | Energy Capacity [Member] | Forecast [Member] |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  | 47.00% |  |
Minimum [Member] | Measurement Input, Discount Rate [Member] |  |  |  |  |  |
ARO measurement input | 7 |  |  |  |  |
Minimum [Member] | Surface and Underground Mining Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment, Useful Life (Year) | 3 years |  |  |  |  |
Minimum [Member] | Power Plant Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment, Useful Life (Year) | 4 years |  |  |  |  |
Maximum [Member] |  |  |  |  |  |
Coal Supply Commitment (US Ton) | T | 10.7 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) | 4 years |  |  |  |  |
Maximum [Member] | Revenue, Product and Service Benchmark [Member] | Customer Concentration Risk [Member] | Energy Capacity [Member] | Forecast [Member] |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  | 55.00% |  |
Maximum [Member] | Measurement Input, Discount Rate [Member] |  |  |  |  |  |
ARO measurement input |  | 0.10 |  |  |  |
Maximum [Member] | Surface and Underground Mining Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment, Useful Life (Year) | 25 years |  |  |  |  |
Maximum [Member] | Power Plant Equipment [Member] |  |  |  |  |  |
Property, Plant and Equipment, Useful Life (Year) | 9 years |  |  |  |  |
Coal Purchase Agreement [Member] |  |  |  |  |  |
Contract with Customer, Asset, Amortized Expense | $ 19,600 | $ 14,700 |  |  |  |
Coal Purchase Agreement [Member] | Operating Expense [Member] |  |  |  |  |  |
Contract with Customer, Asset, Amortized Expense | 30,700 | 3,600 |  |  |  |
Power Purchase Agreement [Member] |  |  |  |  |  |
Contract with Customer, Liability, Amortization Expense | $ 70,500 | $ 23,300 |  |  |  |
Sunrise Energy, LLC [Member] |  |  |  |  |  |
Equity Method Investment, Ownership Percentage | 50.00% |  |  |  |  |

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