Operations and Significant Accounting Policies - Other Non-Current Assets (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Contract asset - noncurrent	$ 18.5	$ 21.0
ALLETE Properties	10.8	19.1
Restricted Cash	2.4	2.3	$ 2.3
Other Postretirement Benefit Plans	106.3	58.8
Other	124.9	103.1
Total Other Non-Current Assets	$ 262.9	$ 204.3