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BUSINESS SEGMENTS - Business Segments (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Dec. 31, 2024
Business Segments [Line Items]      
Other $ 1.5 $ 1.3  
Total Operating Revenue 400.2 403.3  
Net Income (Loss) Attributable to ALLETE 56.1 50.7  
Assets 6,866.3 6,604.6 $ 6,754.3
Operating Segments      
Business Segments [Line Items]      
Other 1.5 1.3  
Total Operating Revenue 370.7 374.4  
Net Income (Loss) Attributable to ALLETE 55.0 52.0  
Assets 6,375.8 6,221.0  
Corporate and Other      
Business Segments [Line Items]      
Other 0.0 0.0  
Total Operating Revenue 29.5 28.9  
Net Income (Loss) Attributable to ALLETE 1.1 (1.3)  
Assets 490.5 383.6  
Regulated Operations      
Business Segments [Line Items]      
Total Operating Revenue 332.8 338.3  
Net Income (Loss) Attributable to ALLETE 38.4 44.2  
Assets 4,560.2   4,489.4
Regulated Operations | Operating Segments      
Business Segments [Line Items]      
Other 0.0 0.0  
Total Operating Revenue 332.8 338.3  
Net Income (Loss) Attributable to ALLETE 38.4 44.2  
Assets 4,560.2 4,345.1  
Regulated Operations | Residential      
Business Segments [Line Items]      
Operating Revenue 56.4 51.5  
Regulated Operations | Commercial      
Business Segments [Line Items]      
Operating Revenue 51.7 49.7  
Regulated Operations | Municipal      
Business Segments [Line Items]      
Operating Revenue 9.6 9.0  
Regulated Operations | Industrial      
Business Segments [Line Items]      
Operating Revenue 139.9 159.5  
Regulated Operations | Other Power Suppliers      
Business Segments [Line Items]      
Operating Revenue 47.1 40.0  
Regulated Operations | Other      
Business Segments [Line Items]      
Operating Revenue 28.1 28.6  
ALLETE Clean Energy      
Business Segments [Line Items]      
Total Operating Revenue 16.5 19.1  
Net Income (Loss) Attributable to ALLETE 7.4 3.8  
Assets 1,464.2   1,477.4
ALLETE Clean Energy | Operating Segments      
Business Segments [Line Items]      
Other 1.5 1.3  
Total Operating Revenue 16.5 19.1  
Net Income (Loss) Attributable to ALLETE 7.4 3.8  
Assets 1,464.2 1,580.0  
ALLETE Clean Energy | Long-term PSA      
Business Segments [Line Items]      
Operating Revenue 15.0 17.8  
ALLETE Clean Energy | Other      
Business Segments [Line Items]      
Other 1.5 1.3  
New Energy      
Business Segments [Line Items]      
Total Operating Revenue 21.4 17.0  
Net Income (Loss) Attributable to ALLETE 9.2 4.0  
Assets 351.4   335.7
New Energy | Operating Segments      
Business Segments [Line Items]      
Other 0.0 0.0  
Total Operating Revenue 21.4 17.0  
Net Income (Loss) Attributable to ALLETE 9.2 4.0  
Assets 351.4 295.9  
New Energy | Sale of Renewable Development Projects      
Business Segments [Line Items]      
Operating Revenue 18.2 13.9  
New Energy | Other      
Business Segments [Line Items]      
Operating Revenue 3.2 3.1  
Corporate and Other      
Business Segments [Line Items]      
Total Operating Revenue 29.5 28.9  
Net Income (Loss) Attributable to ALLETE 1.1 (1.3)  
Assets 490.5   $ 451.8
Corporate and Other | Other      
Business Segments [Line Items]      
Operating Revenue 3.4 3.5  
Corporate and Other | Long-term Contract      
Business Segments [Line Items]      
Operating Revenue $ 26.1 $ 25.4