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Business Segments (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2022
USD ($)
Sep. 30, 2021
USD ($)
Sep. 30, 2022
USD ($)
Segments
Sep. 30, 2021
USD ($)
Dec. 31, 2021
USD ($)
Business Segments [Line Items]          
Number of Reportable Segments | Segments     2    
Operating Revenue $ 388.3 $ 345.4 $ 1,144.9 $ 1,020.2  
Net Income (Loss) Attributable to ALLETE 33.7 27.6 137.6 107.3  
Assets 6,858.0   6,858.0   $ 6,435.0
New Energy          
Business Segments [Line Items]          
After Tax Impact for Purchase Price Accounting     5.7    
Business Combination, Acquisition Related Costs     $ 2.6    
Regulated Operations [Member]          
Business Segments [Line Items]          
Number of Operating Segments | Segments     3    
Operating Revenue 322.6 304.8 $ 960.3 888.2  
Net Income (Loss) Attributable to ALLETE 38.3 32.9 119.4 99.4  
Assets 4,307.3   4,307.3   4,289.4
Regulated Operations [Member] | Residential [Member]          
Business Segments [Line Items]          
Operating Revenue 40.1 37.0 136.7 118.0  
Regulated Operations [Member] | Commercial [Member]          
Business Segments [Line Items]          
Operating Revenue 49.0 44.3 141.7 124.7  
Regulated Operations [Member] | Municipal [Member]          
Business Segments [Line Items]          
Operating Revenue 9.8 14.4 31.9 38.5  
Regulated Operations [Member] | Industrial [Member]          
Business Segments [Line Items]          
Operating Revenue 147.3 139.7 445.7 407.8  
Regulated Operations [Member] | Other Power Suppliers [Member]          
Business Segments [Line Items]          
Operating Revenue 46.7 41.2 124.6 116.9  
Regulated Operations [Member] | Other [Member]          
Business Segments [Line Items]          
Operating Revenue 29.7 28.2 79.7 82.3  
ALLETE Clean Energy [Member]          
Business Segments [Line Items]          
Operating Revenue 15.6 16.9 65.0 59.4  
Net Income (Loss) Attributable to ALLETE (7.3) (0.8) 15.0 11.7  
Assets 1,893.2   1,893.2   1,719.4
ALLETE Clean Energy [Member] | Long-term PSA [Member]          
Business Segments [Line Items]          
Operating Revenue 14.4 14.0 58.7 50.8  
ALLETE Clean Energy [Member] | Other [Member]          
Business Segments [Line Items]          
Operating Revenue, Other Than Customer Revenue 1.2 2.9 6.3 8.6  
Corporate and Other [Member]          
Business Segments [Line Items]          
Operating Revenue 50.1 23.7 119.6 72.6  
Net Income (Loss) Attributable to ALLETE [1] 2.7 (4.5) 3.2 (3.8)  
Assets 657.5   657.5   $ 426.2
Corporate and Other [Member] | Long-term Contract [Member]          
Business Segments [Line Items]          
Operating Revenue 23.5 20.3 67.5 63.4  
Corporate and Other [Member] | Other [Member]          
Business Segments [Line Items]          
Operating Revenue 4.5 3.4 15.5 9.2  
Corporate and Other [Member] | Sale of Renewable Development Projects          
Business Segments [Line Items]          
Operating Revenue $ 22.1 $ 0.0 $ 36.6 $ 0.0  
[1] Net Income in 2022 includes a $5.7 million after-tax expense as a result of purchase price accounting related to projects under development at the time of acquisition and $2.6 million after-tax of transaction costs related to the acquisition of New Energy.