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v3.25.0.1

Short-Term and Long-Term Debt - Narrative (Details) |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Jul. 31, 2024 USD ($) | Apr. 23, 2024 USD ($) | Dec. 31, 2024 USD ($) | Sep. 05, 2024 USD ($) | Dec. 31, 2023 USD ($)
Debt Instrument [Line Items] |  |  |  |  |
Short-term debt - outstanding |  |  | $ 94,700,000 |  | $ 111,400,000
Short-term debt - unamortized debt issuance costs |  |  | 0 |  |
Long-term debt outstanding |  |  | 1,704,700,000 |  | 1,679,900,000
Long-term debt - unamortized debt issuance costs |  |  | 8,600,000 |  |
Long-term debt maturing in 2025 |  |  | 94,700,000 |  |
Long-term debt maturing in 2026 |  |  | 80,200,000 |  |
Long-term debt maturing in 2027 |  |  | 182,500,000 |  |
Long-term debt maturing in 2028 |  |  | 55,800,000 |  |
Long-term debt maturing in 2029 |  |  | 220,300,000 |  |
Long-term debt maturing thereafter |  |  | $ 1,174,500,000 |  |
Actual indebtedness to total capitalization ratio |  |  | 0.36 |  |
Dividend payable percentage limit |  |  | 5.00% |  |
Dividend payable period |  |  | 12 months |  |
Maximum |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Allowed indebtedness to total capitalization ratio |  |  | 0.65 |  |
Bank Lines of Credit |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Maximum borrowing capacity |  |  | $ 362,000,000 |  | 423,100,000
Standby letters of credit |  |  | 16,200,000 |  | 19,400,000
Draws outstanding |  |  | 20,000,000.0 |  | $ 34,100,000
Senior Notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Proceeds from issuance of long-term debt |  |  |  | $ 150,000,000 |
Camp Ripley Financing |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Annual financing payment |  |  | $ 1,400,000 |  |
Financing renewal term |  |  | 2 years |  |
Purchase option |  |  | $ 4,000,000 |  |
ALLETE Bonds 5.72% Due April 2039 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Proceeds from issuance of first mortgage bond |  | $ 100,000,000 |  |  |
Interest rate |  | 5.72% |  |  |
2.65% Senior Notes Due September 10, 2025 | Senior Notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Interest rate | 2.65% |  |  |  |
Repayments of senior debt | $ 150,000,000 |  |  |  |
Percentage of principal amount prepaid | 100.00% |  |  |  |
5.94% Senior Unsecured Notes Due September 2029 | Senior Notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Interest rate |  |  |  | 5.94% |
Proceeds from issuance of long-term debt |  |  |  | $ 100,000,000 |
6.18% Senior Unsecured Notes Due September 2034 | Senior Notes |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |
Interest rate |  |  |  | 6.18% |
Proceeds from issuance of long-term debt |  |  |  | $ 50,000,000 |

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