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Commitments, Guarantees and Contingencies - Other Matters (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2023 | Dec. 31, 2023 | Dec. 31, 2005 | Dec. 31, 2022
Guarantor Obligations [Line Items] |  |  |  |
Proceeds from legal settlements, gross | $ 68.3 |  |  |
Litigation settlement, amount awarded from other party, prejudgment interest | 5.1 |  |  |
Litigation settlement, amount awarded from other party, arbitration costs recovery | 3.6 |  |  |
Litigation settlement, amount awarded from other party, pre tax gain | 58.4 |  |  |
Proceeds from legal settlements | $ 60.0 |  |  |
ALLETE, Inc. | Letters of Credit |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Standby letters of credit |  | $ 149.8 |  |
Regulated Operations | Letters of Credit |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Standby letters of credit |  | 24.2 |  |
ALLETE Clean Energy | Letters of Credit |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Standby letters of credit |  | 91.6 |  |
BNI Energy Reclamation Liability |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Estimated obligation |  | 82.1 |  |
BNI Energy Reclamation Liability | Surety Bonds |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Collateral |  | 82.4 |  |
ALLETE South Wind | Letters of Credit |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Standby letters of credit |  | 10.1 |  |
South Shore Energy | Letters of Credit |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Standby letters of credit |  | 23.9 |  |
ALLETE Properties Development and Maintenance Obligations |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Estimated obligation |  | 1.0 |  |
ALLETE Properties Development and Maintenance Obligations | Surety Bonds and Letters of Credit |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Collateral |  | $ 2.0 |  |
Town Center District Capital Improvement Bonds |  |  |  |
Guarantor Obligations [Line Items] |  |  |  |
Bonds |  |  | $ 26.4 |
Bond interest rate |  |  | 6.00% |
Bond term |  |  | 31 years |
Ownership percentage of benefited property |  | 33.00% |  | 42.00%
Annual assessment |  | $ 0.7 |  |

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