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REGULATORY MATTERS - Electric Rates (Details) $ in Thousands |  |  |  |  | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Apr. 01, 2025 USD ($) | May 03, 2024 USD ($) | Dec. 19, 2023 USD ($) | Nov. 01, 2023 USD ($) | Dec. 31, 2025 USD ($) | Dec. 31, 2024 USD ($) | Sep. 30, 2025 USD ($) | Sep. 30, 2024 USD ($) | Aug. 04, 2025 MW | Mar. 03, 2025 MW | Dec. 31, 2021 MW
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  | $ 371,700 | $ 360,400 |  |  |  |
Regulatory liability |  |  |  |  |  | 570,500 | 593,500 |  |  |  |
Depreciation expense |  |  |  |  |  |  | 215,300 | $ 201,700 |  |  |
MPUC | Minnesota Power | New Wind Energy Resources |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Wind project, generating production (MW) | MW |  |  |  |  |  |  |  |  | 200 |  |
ECO Annual Filing | MPUC | Minnesota Power |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from cost recovery riders and other regulatory programs | $ 2,600 |  |  |  |  | $ 2,200 |  |  |  |  |
2025 IRP | Minnesota Power |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Solar and wind projects, generating production (MW) | MW |  |  |  |  |  |  |  |  |  | 700 |
2025 IRP | Minnesota Power | New Wind Energy Resources |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Wind project, generating production (MW) | MW |  |  |  |  |  |  |  |  |  | 400 |
2025 IRP | Minnesota Power | Expanding Energy Storage Resources |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Energy storage project, generating production (MW) | MW |  |  |  |  |  |  |  |  |  | 100 |
2025 IRP | Minnesota Power | Natural Gas Capacity |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Natural gas project, generating production (MW) | MW |  |  |  |  |  |  |  |  |  | 1,000 |
2021 IRP | Minnesota Power |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Solar and wind projects, generating production (MW) | MW |  |  |  |  |  |  |  |  |  |  | 700
Electric Rates | Fuel Adjustment Clause 2024 | MPUC | Minnesota Power |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Regulatory assets |  |  |  |  |  |  | 4,500 |  |  |  |
Electric Rates | Fuel Adjustment Clause 2025 | MPUC | Minnesota Power |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Regulatory liability |  |  |  |  |  |  | 7,400 |  |  |  |
Retail Customers | Deferred Accounting Petition | MPUC |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Depreciation expense |  |  |  |  |  |  | 4,800 | 4,400 |  |  |
Retail Customers | Deferred Accounting Petition | MPUC | Forecast |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Depreciation expense |  |  |  |  | $ 9,200 |  |  |  |  |  |
Retail Customers | Electric Rates | Current Cost Recovery Rider | MPUC | Minnesota Power |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenue from cost recovery riders and other regulatory programs |  |  |  |  |  |  | $ 35,200 | $ 15,100 |  |  |
Retail Customers | Electric Rates | 2024 Minnesota General Rate Case | MPUC | Minnesota Power |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |
Requested rate increase percent |  |  |  | 12.00% |  |  |  |  |  |  |
Requested return on equity |  | 9.78% |  | 10.30% |  |  |  |  |  |  |
Requested equity capital structure |  |  |  | 53.00% |  |  |  |  |  |  |
Annual additional revenue generated from requested final rate increase |  | $ 33,970 |  | $ 89,000 |  |  |  |  |  |  |
Approved rate increase |  |  | $ 64,000 |  |  |  |  |  |  |  |

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