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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, 2023
Dec. 31, 2022
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment – Net $ 5,013.4 $ 5,004.0
Regulated Operations    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment in Service 5,167.2 5,198.6
Construction Work in Progress 146.7 74.0
Accumulated Depreciation (1,969.4) (1,972.3)
Property, Plant and Equipment – Net 3,344.5 3,300.3
ALLETE Clean Energy    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment in Service 1,612.8 1,619.4
Construction Work in Progress 48.9 51.1
Accumulated Depreciation (229.1) (176.8)
Property, Plant and Equipment – Net 1,432.6 1,493.7
Corporate and Other    
Property, Plant and Equipment [Line Items]    
Property, Plant and Equipment in Service 355.8 295.2
Construction Work in Progress 27.5 50.9
Accumulated Depreciation (147.0) (136.1)
Property, Plant and Equipment – Net $ 236.3 $ 210.0
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 4 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