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Property, Plant and Equipment (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment – Net $ 3,822.4 $ 3,741.2  
Asset Retirement Obligation [Roll Forward]      
Beginning Obligation 136.6 131.4  
Accretion 7.6 8.0  
Liabilities Settled (5.9) (6.5)  
Revisions in Estimated Cash Flows (15.6) 3.7 $ 12.4
Ending Obligation 122.7 136.6 $ 131.4
Regulated Operations [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment in Service 4,523.7 4,437.0  
Construction Work in Progress 121.6 84.2  
Accumulated Depreciation (1,520.5) (1,426.1)  
Property, Plant and Equipment – Net $ 3,124.8 3,095.1  
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 44 years    
Regulated Operations [Member] | Transmission [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 67 years    
Regulated Operations [Member] | Distribution [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 18 years    
Regulated Operations [Member] | Distribution [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 65 years    
ALLETE Clean Energy [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment in Service $ 482.5 472.3  
Construction Work in Progress 144.9 101.0  
Accumulated Depreciation (60.8) (41.0)  
Property, Plant and Equipment – Net $ 566.6 532.3  
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 $ 24.8 19.5  
Accumulated Depreciation (10.4) (6.9)  
Property, Plant and Equipment – Net $ 14.4 12.6  
U.S. Water Services [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated Useful Lives 3 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] $ 204.7 179.8  
Construction Work in Progress [1] 5.0 2.8  
Accumulated Depreciation [1] (93.1) (81.4)  
Property, Plant and Equipment – Net $ 116.6 $ 101.2  
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.