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OPERATIONS AND SIGNIFICANT ACCOUNTING POLICIES - Other Non-Current Assets (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Other Postretirement Benefit Plans	$ 105.4	$ 107.6
Contract Assets	15.3	15.9
ALLETE Properties	9.2	10.3
Restricted Cash	2.4	2.5	$ 2.4	$ 2.4
Other	136.7	134.2
Total Other Non-Current Assets	$ 269.0	$ 270.5

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