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Property, Plant and Equipment (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment – Net $ 3,904.4 $ 3,822.4  
Asset Retirement Obligation [Roll Forward]      
Beginning Obligation 122.7 136.6  
Accretion 7.0 7.6  
Liabilities Settled (5.3) (5.9)  
Revisions in Estimated Cash Flows 14.2 (15.6) $ 3.7
Ending Obligation 138.6 122.7 $ 136.6
Regulated Operations [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment in Service 4,490.6 4,523.7  
Construction Work in Progress 251.1 121.6  
Accumulated Depreciation (1,549.6) (1,520.5)  
Property, Plant and Equipment – Net $ 3,192.1 3,124.8  
Regulated Operations [Member] | Generation [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 5 years    
Regulated Operations [Member] | Generation [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 50 years    
Regulated Operations [Member] | Transmission [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 52 years    
Regulated Operations [Member] | Transmission [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 71 years    
Regulated Operations [Member] | Distribution [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 19 years    
Regulated Operations [Member] | Distribution [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 68 years    
ALLETE Clean Energy [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment in Service $ 488.4 482.5  
Construction Work in Progress 164.5 144.9  
Accumulated Depreciation (73.0) (60.8)  
Property, Plant and Equipment – Net $ 579.9 566.6  
ALLETE Clean Energy [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 5 years    
ALLETE Clean Energy [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 35 years    
U.S. Water Services [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment in Service $ 30.1 24.8  
Accumulated Depreciation (14.0) (10.4)  
Property, Plant and Equipment – Net $ 16.1 14.4  
U.S. Water Services [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 5 years    
U.S. Water Services [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 39 years    
Corporate and Other [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment in Service [1] $ 214.3 204.7  
Construction Work in Progress [1] 6.6 5.0  
Accumulated Depreciation [1] (104.6) (93.1)  
Property, Plant and Equipment – Net $ 116.3 $ 116.6  
Corporate and Other [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 3 years    
Corporate and Other [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 50 years    
[1] Primarily includes BNI Energy and a small amount of non-rate base generation.