OPERATIONS AND SIGNIFICANT ACCOUNTING POLICIES - Other Non-Current Assets (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Other Postretirement Benefit Plans	$ 102.9	$ 106.3
Contract Assets	17.2	18.5
Operating Lease Right-of-use Assets	11.6	10.7
ALLETE Properties	10.5	10.8
Restricted Cash	2.4	2.4	$ 2.4	$ 2.3
Finance Lease Right-of-use Assets	2.0	2.1
Other	116.9	112.1
Total Other Non-Current Assets	$ 263.5	$ 262.9