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Equity Investments - Investments (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2024
Sep. 30, 2024
Jun. 30, 2024
Mar. 31, 2024
Dec. 31, 2023
Sep. 30, 2023
Jun. 30, 2023
Mar. 31, 2023
Dec. 31, 2022
Sep. 30, 2022
Jun. 30, 2022
Mar. 31, 2022
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Equity Method Investments [Roll Forward]                              
Equity Investment Beginning Balance       $ 331.2       $ 322.7         $ 331.2 $ 322.7  
Cash Investments                         5.8 8.2 $ 5.9
Equity in ATC Earnings                         25.0 21.7 18.7
Equity Investment Ending Balance $ 340.1       $ 331.2       $ 322.7       340.1 331.2 322.7
Balance Sheet Data                              
Current Assets 435.2       468.1               435.2 468.1  
Total Assets 6,754.3       6,656.4       6,845.6       6,754.3 6,656.4 6,845.6
Current Liabilities 404.2       377.6               404.2 377.6  
Members’ Equity 3,390.1       3,406.6       3,348.3       3,390.1 3,406.6 3,348.3
Total Liabilities and Members’ Equity 6,754.3       6,656.4               6,754.3 6,656.4  
Income Statement Data                              
Revenue 364.8 $ 407.2 $ 354.5 403.3 402.7 $ 378.8 $ 533.4 564.9 425.8 $ 388.3 $ 373.1 $ 383.5 1,529.8 1,879.8 1,570.7
Equity Earnings                         25.0 21.7 18.7
ALLETE’s Equity in Net Income                         121.7 178.9 131.3
Net loss attributable to non-controlling interest                         (57.6) (68.2) (58.0)
ATC                              
Balance Sheet Data                              
Current Assets 126.6       115.2               126.6 115.2  
Non-Current Assets 6,792.6       6,337.0               6,792.6 6,337.0  
Total Assets 6,919.2       6,452.2               6,919.2 6,452.2  
Current Liabilities 482.4       495.9               482.4 495.9  
Long-Term Debt 3,083.4       2,736.0               3,083.4 2,736.0  
Other Non-Current Liabilities 545.0       585.2               545.0 585.2  
Members’ Equity 2,808.4       2,635.1               2,808.4 2,635.1  
Total Liabilities and Members’ Equity 6,919.2       6,452.2               6,919.2 6,452.2  
Income Statement Data                              
Revenue                         911.3 818.9 751.2
Operating Expense                         442.3 407.6 381.5
Other Expense                         137.8 131.7 122.9
ALLETE’s Equity in Net Income                         331.2 279.6 246.8
Nobles 2                              
Income Statement Data                              
Net loss attributable to non-controlling interest                         11.7    
ATC                              
Equity Method Investments [Roll Forward]                              
Equity Investment Beginning Balance       179.7       $ 165.4         179.7 165.4  
Cash Investments                         5.8 8.2  
Equity in ATC Earnings                         26.5 23.1 19.3
Distributed ATC Earnings                         (18.9) (18.3)  
Amortization of the Remeasurement of Deferred Income Taxes                         1.3 1.3  
Equity Investment Ending Balance 194.4       179.7       $ 165.4       194.4 179.7 165.4
Income Statement Data                              
Equity Earnings                         26.5 23.1 $ 19.3
Nobles 2                              
Equity Method Investments [Roll Forward]                              
Equity Investment Beginning Balance       $ 151.5                 151.5    
Equity in ATC Earnings                         (1.5)    
Distributed ATC Earnings                         (4.3)    
Equity Investment Ending Balance $ 145.7       $ 151.5               145.7 $ 151.5  
Income Statement Data                              
Equity Earnings                         $ (1.5)