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OPERATIONS AND SIGNIFICANT ACCOUNTING POLICIES - Other Non-Current Assets (Details) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023 | Sep. 30, 2023 | Dec. 31, 2022
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Organization, Consolidation and Presentation of Financial Statements [Abstract] |  |  |  |
Other Postretirement Benefit Plans | $ 101.1 | $ 106.3 |  |
Contract Assets | 16.6 | 18.5 |  |
Operating Lease Right-of-use Assets | 10.8 | 10.7 |  |
ALLETE Properties | 10.4 | 10.8 |  |
Restricted Cash | 2.5 | 2.4 | $ 2.4 | $ 2.3
Finance Lease Right-of-use Assets | 2.0 | 2.1 |  |
Other | 118.3 | 112.1 |  |
Total Other Non-Current Assets | $ 261.7 | $ 262.9 |  |

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