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Property, Plant and Equipment - Property, Plant and Equipment and Estimated Useful Lives of Property, Plant and Equipment (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment – Net $ 5,181.5 $ 5,013.4
Regulated Operations    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment in Service 5,387.8 5,167.2
Construction Work in Progress 197.5 146.7
Accumulated Depreciation (2,077.8) (1,969.4)
Property, Plant and Equipment – Net 3,507.5 3,344.5
ALLETE Clean Energy    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment in Service 1,605.3 1,612.8
Construction Work in Progress 51.9 48.9
Accumulated Depreciation (272.9) (229.1)
Property, Plant and Equipment – Net 1,384.3 1,432.6
Corporate and Other    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment in Service 377.4 355.8
Construction Work in Progress 77.5 27.5
Accumulated Depreciation (165.2) (147.0)
Property, Plant and Equipment – Net $ 289.7 $ 236.3
Minimum | ALLETE Clean Energy    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 5 years  
Minimum | Corporate and Other    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 3 years  
Maximum | ALLETE Clean Energy    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 35 years  
Maximum | Corporate and Other    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 50 years  
Generation | Minimum | Regulated Operations    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 3 years  
Generation | Maximum | Regulated Operations    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 50 years  
Transmission | Minimum | Regulated Operations    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 50 years  
Transmission | Maximum | Regulated Operations    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 75 years  
Distribution | Minimum | Regulated Operations    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 18 years  
Distribution | Maximum | Regulated Operations    
Property, Plant and Equipment [Line Items]    
Estimated Useful Lives 70 years