Operations and Significant Accounting Policies - Other Non-Current Assets (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Contract Assets	$ 19.1	$ 21.0
Operating Lease Right-of-use Assets	11.6	12.7
Finance Lease Right-of-use Assets	2.1	0.0
ALLETE Properties	19.4	19.1
Restricted Cash	2.4	2.3	$ 2.3	$ 2.3
Other Postretirement Benefit Plans	60.2	58.8
Other	101.5	90.4
Total Other Non-Current Assets	$ 216.3	$ 204.3