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Business Segments (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2021
USD ($)
a
Segments
Mar. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
Business Segments [Line Items]      
Number of Reportable Segments | Segments 2    
Operating Revenue $ 339.2 $ 311.6  
Net Income (Loss) Attributable to ALLETE 51.8 66.3  
Assets $ 6,274.5   $ 6,084.6
Regulated Operations [Member]      
Business Segments [Line Items]      
Number of Operating Segments | Segments 3    
Operating Revenue $ 293.0 265.3  
Net Income (Loss) Attributable to ALLETE 45.0 57.5  
Assets 4,184.1   4,196.8
Regulated Operations [Member] | Residential [Member]      
Business Segments [Line Items]      
Operating Revenue 45.6 40.9  
Regulated Operations [Member] | Commercial [Member]      
Business Segments [Line Items]      
Operating Revenue 39.7 37.3  
Regulated Operations [Member] | Municipal [Member]      
Business Segments [Line Items]      
Operating Revenue 12.6 10.3  
Regulated Operations [Member] | Industrial [Member]      
Business Segments [Line Items]      
Operating Revenue 129.0 118.8  
Regulated Operations [Member] | Other Power Suppliers [Member]      
Business Segments [Line Items]      
Operating Revenue 38.4 38.3  
Regulated Operations [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue 27.7 19.7  
ALLETE Clean Energy [Member]      
Business Segments [Line Items]      
Operating Revenue 20.9 20.1  
Net Income (Loss) Attributable to ALLETE 7.4 11.7  
Assets 1,596.6   1,483.3
ALLETE Clean Energy [Member] | Long-term PSA [Member]      
Business Segments [Line Items]      
Operating Revenue 18.1 17.3  
ALLETE Clean Energy [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue, Other Than Customer Revenue $ 2.8 2.8  
Corporate and Other [Member]      
Business Segments [Line Items]      
Number of Operating Segments | Segments 2    
Land in Minnesota (Acres) | a 4,000    
Operating Revenue $ 25.3 26.2  
Net Income (Loss) Attributable to ALLETE (0.6) (2.9)  
Assets 493.8   $ 404.5
Corporate and Other [Member] | Long-term Contract [Member]      
Business Segments [Line Items]      
Operating Revenue 21.7 22.4  
Corporate and Other [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue $ 3.6 $ 3.8