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Equity Investments - Narrative (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2022 USD ($) | Dec. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) MW | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Equity Investments [Line Items] |  |  |  |  |
Cash Investments |  |  | $ 8.2 | $ 5.9 | $ 17.6
Wind Turbine Generators | Tenaska | Nobles 2 PPA | Minnesota Power |  |  |  |  |
Equity Investments [Line Items] |  |  |  |  |
Generating capacity counterparty owned (MW) | MW |  |  | 250 |  |
Contract term |  |  | 20 years |  |
ATC |  |  |  |  |
Equity Investments [Line Items] |  |  |  |  |
Approximate refund for reversal of FERC ROE order | $ 31.0 |  |  |  |
Approximate refund for reversal of FERC ROE Order, ALLETE's proportionate share | $ 2.4 |  |  |  |
Basis point incentive adder |  |  | 50.00% |  |
Approximate reduction in pretax income for FERC proposal to limit incentive adder, ALLETE's proportionate share |  |  | $ 1.0 |  |
ATC |  |  |  |  |
Equity Investments [Line Items] |  |  |  |  |
Ownership percentage |  |  | 8.00% |  |
Cash Investments |  |  | $ 8.2 | $ 5.9 |
Authorized return on equity |  |  | 10.02% |  |
Authorized return on equity, including incentive adder |  |  | 10.52% |  |
ATC | Forecast |  |  |  |  |
Equity Investments [Line Items] |  |  |  |  |
Expected additional investment |  | $ 5.8 |  |  |
Nobles 2 |  |  |  |  |
Equity Investments [Line Items] |  |  |  |  |
Ownership percentage |  |  | 49.00% |  |

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