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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ in Thousands, T in Millions | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2024 USD ($) contract T | Sep. 30, 2024 USD ($) | Jun. 30, 2024 USD ($) | Mar. 31, 2024 USD ($) | Dec. 31, 2024 USD ($) customer contract segment T $ / MWh | Dec. 31, 2023 USD ($) customer | Nov. 30, 2023 USD ($) | May 31, 2023 USD ($) | Dec. 31, 2022 USD ($)
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Number of reportable segments | segment |  |  |  |  | 2 |  |  |  |
Accounts Receivable, Allowance for Credit Loss, Ending Balance | $ 0 |  |  |  | $ 0 | $ 0 |  |  |
Asset impairment | 215,100 |  |  |  | 215,136 | 0 |  |  |
Contract with Customer, Liability | 146,719 |  |  |  | 146,719 | 113,741 |  |  | $ 207,695
Contract value | 52,100 |  |  |  |  |  |  |  |
Other operating and maintenance costs |  |  |  |  | 118,364 | 199,855 |  |  |
Regulatory Asset | $ 30,800 |  |  |  | 30,800 |  |  |  |
Undiscounted Asset Retirement Obligations |  |  |  |  | $ 26,100 | $ 26,600 |  |  |
Number of coal contracts | contract | 0 |  |  |  | 0 |  |  |  |
Coal Supply Commitment (US Ton) | T | 8.4 |  |  |  | 8.4 |  |  |  |
Year Supply Commitments End |  |  |  |  | 2028 |  |  |  |
Sunrise Energy, LLC |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Percentage of equity method investment | 50.00% |  |  |  | 50.00% |  |  |  |
Minimum |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  | 2 years |  |  |  |
Minimum | Measurement Input, Discount Rate |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
ARO measurement input | 7 |  |  |  | 7 |  |  |  |
Maximum |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Coal Supply Commitment (US Ton) | T | 9.2 |  |  |  | 9.2 |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year) |  |  |  |  | 4 years |  |  |  |
Maximum | Measurement Input, Discount Rate |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
ARO measurement input | 10 |  |  |  | 10 |  |  |  |
Revision of Prior Period, Reclassification, Adjustment [Member] |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Other operating and maintenance costs |  | $ (1,700) | $ (2,600) | $ (2,100) |  |  |  |  |
Surface And Underground Mining Equipment | Minimum |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Property, Plant and Equipment, Useful Life (Year) | 3 years |  |  |  | 3 years |  |  |  |
Surface And Underground Mining Equipment | Maximum |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Property, Plant and Equipment, Useful Life (Year) | 15 years |  |  |  | 15 years |  |  |  |
Power Plant Equipment | Minimum |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Property, Plant and Equipment, Useful Life (Year) | 7 years |  |  |  | 7 years |  |  |  |
Power Plant Equipment | Maximum |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Property, Plant and Equipment, Useful Life (Year) | 9 years |  |  |  | 9 years |  |  |  |
Revenue, Product and Service Benchmark | Customer Concentration Risk | Five Customers | Coal |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  |  | 93.00% |  |  |
Number Of Major Customers | customer |  |  |  |  |  | 5 |  |  |
Revenue, Product and Service Benchmark | Customer Concentration Risk | Three Customers | Coal |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  | 94.00% |  |  |  |
Number Of Major Customers | customer |  |  |  |  | 3 |  |  |  |
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Three Customers | Delivered energy |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  | 89.00% |  |  |  |
Number Of Major Customers | customer |  |  |  |  | 3 |  |  |  |
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Three Customers | Energy Capacity |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  |  | 91.00% |  |  |
Number Of Major Customers | customer |  |  |  |  |  | 3 |  |  |
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Four Customers | Energy Capacity |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  | 88.00% |  |  |  |
Number Of Major Customers | customer |  |  |  |  | 4 |  |  |  |
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Hoosier | Delivered energy |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  |  | 100.00% |  |  |
Accounts Receivable | Customer Concentration Risk | Two Customer |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  |  | 100.00% |  |  |
Number Of Major Customers | customer |  |  |  |  |  | 2 |  |  |
Accounts Receivable | Customer Concentration Risk | Three Customers |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  | 100.00% |  |  |  |
Number Of Major Customers | customer |  |  |  |  | 3 |  |  |  |
Accounts Receivable | Customer Concentration Risk | Four Customers | Coal |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Concentration Risk, Percentage |  |  |  |  | 98.00% | 85.00% |  |  |
Number Of Major Customers | customer |  |  |  |  | 4 | 4 |  |  |
Coal Purchase Agreement |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Contract with Customer, Asset, Amortized Expense |  |  |  |  |  | $ 19,600 |  |  |
Coal Purchase Agreement | Operating Expense |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Contract with Customer, Asset, Amortized Expense |  |  |  |  |  | 30,700 |  |  |
Power Purchase Agreement | Hoosier |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Additional Obligations | $ 186,600 |  |  |  | $ 186,600 |  |  |  |
Long-term Purchase Commitment, Minimum Quantity Required, Price Per Megawatt Hour | $ / MWh |  |  |  |  | 56 |  |  |  |
Capacity payment reductions |  |  |  |  |  |  | $ 7,500 | $ 7,500 |
Contract with Customer, Liability | 43,500 |  |  |  | $ 43,500 |  |  |  |
Contract With Customer Liability Amortization Expense |  |  |  |  | 47,100 | $ 70,500 |  |  |
Capacity Payment Reduction Agreement |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Contract with Customer, Liability | 42,000 |  |  |  | 42,000 |  |  |  |
Physically Delivered Prepaid Power Contract |  |  |  |  |  |  |  |  |
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES |  |  |  |  |  |  |  |  |
Contract with Customer, Liability | 61,200 |  |  |  | 61,200 |  |  |  |
Contract value |  |  |  |  | 60,000 |  |  |  |
Contract with Customer, Liability, Implied Interest | $ 1,200 |  |  |  | $ 1,200 |  |  |  |

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