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v3.24.0.1

Short-Term and Long-Term Debt - Narrative (Details) $ in Millions |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Apr. 27, 2023 USD ($) | Dec. 31, 2023 USD ($) | Oct. 17, 2023 USD ($) | Oct. 16, 2023 USD ($) | Dec. 31, 2022 USD ($)
Debt Instrument [Line Items] |  |  |  |  |
Short-term debt - outstanding |  | $ 111.4 |  |  | $ 272.6
Short-term debt - unamortized debt issuance costs |  | 0.1 |  |  |
Long-Term Debt |  | 1,679.9 |  |  | 1,648.2
Long-term debt - unamortized debt issuance costs |  | 8.0 |  |  |
Long-term debt maturing in 2024 |  | 111.4 |  |  |
Long-term debt maturing in 2025 |  | 244.7 |  |  |
Long-term debt maturing in 2026 |  | 80.2 |  |  |
Long-term debt maturing in 2027 |  | 162.5 |  |  |
Long-term debt maturing in 2028 |  | 55.8 |  |  |
Long-term debt maturing thereafter |  | $ 1,144.8 |  |  |
Actual indebtedness to total capitalization ratio |  | 0.36 |  |  |
Maximum |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Allowed indebtedness to total capitalization ratio |  | 0.65 |  |  |
Bank Lines of Credit |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Maximum borrowing capacity |  | $ 423.1 |  |  | 475.7
Standby letters of credit |  | 19.4 |  |  | 32.8
Draws outstanding |  | 34.1 |  |  | $ 31.3
Credit Agreement | Line of Credit |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Maximum borrowing capacity |  |  | $ 355.0 | $ 400.0 |
Extension period |  |  | 1 year |  |
Issuable letters of credit |  |  | $ 100.0 |  |
Camp Ripley Financing |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Annual financing payment |  | $ 1.4 |  |  |
Financing renewal term |  | 2 years |  |  |
Purchase option |  | $ 4.0 |  |  |
ALLETE Bonds 4.98% Due April 2033 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Proceeds from issuance of First Mortgage Bond | $ 125.0 |  |  |  |
Interest rate | 4.98% |  |  |  |

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