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OPERATIONS AND SIGNIFICANT ACCOUNTING POLICIES - Other Non-Current Assets (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Dec. 31, 2024 | Sep. 30, 2024 | Dec. 31, 2023
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Organization, Consolidation and Presentation of Financial Statements [Abstract] |  |  |  |
Other Postretirement Benefit Plans | $ 100.7 | $ 107.6 |  |
Contract Assets | 14.2 | 15.9 |  |
ALLETE Properties | 9.1 | 10.3 |  |
Restricted Cash | 2.5 | 2.5 | $ 2.5 | $ 2.4
Other | 139.3 | 134.2 |  |
Total Other Non-Current Assets | $ 265.8 | $ 270.5 |  |

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