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Business Segments (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2023
USD ($)
Segments
Mar. 31, 2022
USD ($)
Dec. 31, 2022
USD ($)
Business Segments [Line Items]      
Number of Reportable Segments | Segments 2    
Operating Revenue $ 564.9 $ 383.5  
Net Income (Loss) Attributable to ALLETE 58.2 66.3  
Assets $ 6,704.5   $ 6,845.6
Regulated Operations [Member]      
Business Segments [Line Items]      
Number of Operating Segments | Segments 3    
Operating Revenue $ 312.6 329.0  
Net Income (Loss) Attributable to ALLETE 40.6 51.5  
Assets 4,268.8   4,291.4
Regulated Operations [Member] | Residential [Member]      
Business Segments [Line Items]      
Operating Revenue 49.4 56.0  
Regulated Operations [Member] | Commercial [Member]      
Business Segments [Line Items]      
Operating Revenue 47.7 48.3  
Regulated Operations [Member] | Municipal [Member]      
Business Segments [Line Items]      
Operating Revenue 8.9 12.1  
Regulated Operations [Member] | Industrial [Member]      
Business Segments [Line Items]      
Operating Revenue 144.9 147.8  
Regulated Operations [Member] | Other Power Suppliers [Member]      
Business Segments [Line Items]      
Operating Revenue 35.9 41.0  
Regulated Operations [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue 25.8 23.8  
ALLETE Clean Energy [Member]      
Business Segments [Line Items]      
Operating Revenue 201.5 28.2  
Net Income (Loss) Attributable to ALLETE 8.5 16.5  
Assets 1,697.3   1,873.3
ALLETE Clean Energy [Member] | Long-term PSA [Member]      
Business Segments [Line Items]      
Operating Revenue 18.4 25.4  
ALLETE Clean Energy [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue, Other Than Customer Revenue 1.3 2.8  
ALLETE Clean Energy [Member] | Sale of Wind Energy Facility      
Business Segments [Line Items]      
Operating Revenue 181.8 0.0  
Corporate and Other [Member]      
Business Segments [Line Items]      
Operating Revenue 50.8 26.3  
Net Income (Loss) Attributable to ALLETE 9.1 (1.7)  
Assets 738.4   $ 680.9
Corporate and Other [Member] | Long-term Contract [Member]      
Business Segments [Line Items]      
Operating Revenue 25.5 22.6  
Corporate and Other [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue 5.5 3.7  
Corporate and Other [Member] | Sale of Renewable Development Projects      
Business Segments [Line Items]      
Operating Revenue $ 19.8 $ 0.0