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SHORT-TERM AND LONG-TERM DEBT - Narrative (Details) | Jul. 31, 2024 USD ($) | Apr. 23, 2024 USD ($) | Sep. 30, 2024 USD ($) | Sep. 05, 2024 USD ($) | Dec. 31, 2023 USD ($)
--- | --- | --- | --- | --- | ---
Minimum |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Required ratio of indebtedness to total capitalization |  |  | 0.65 |  |
Actual ratio of indebtedness to total capitalization |  |  | 0.36 |  |
Maximum |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Required ratio of indebtedness to total capitalization |  |  | 1.00 |  |
Actual ratio of indebtedness to total capitalization |  |  | 1.00 |  |
ALLETE Bonds 5.72% Due April 2039 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Interest rate |  | 5.72% |  |  |
Proceeds from issuance of first mortgage bond |  | $ 100,000,000 |  |  |
Line of Credit | Credit Agreement |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Letters of credit outstanding |  |  | $ 23,400,000 |  | $ 19,400,000
Line of credit draw outstanding |  |  | $ 0 |  | $ 34,100,000
Senior Notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Proceeds from issuance of long-term debt |  |  |  | $ 150,000,000 |
Senior Notes | 5.94% Senior Unsecured Notes Due September 2029 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Proceeds from issuance of long-term debt |  |  |  | $ 100,000,000 |
Interest rate |  |  |  | 5.94% |
Senior Notes | 6.18% Senior Unsecured Notes Due September 2034 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Proceeds from issuance of long-term debt |  |  |  | $ 50,000,000 |
Interest rate |  |  |  | 6.18% |
Senior Notes | 2.65% Senior Notes Due September 10, 2025 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Interest rate | 2.65% |  |  |  |
Repayments of senior debt | $ 150,000,000 |  |  |  |
Percentage of principal amount prepaid | 100.00% |  |  |  |

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