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Property, Plant and Equipment (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment – Net $ 4,377.0 $ 3,904.4  
Costs Incurred, Capitalized Asset Retirement Obligation Costs 20.7 14.2 $ (15.6)
Asset Retirement Obligation [Roll Forward]      
Beginning Obligation 138.6 122.7  
Accretion 7.2 7.0  
Liabilities Recognized 1.4    
Liabilities Settled (4.6) (5.3)  
Revisions in Estimated Cash Flows 17.7    
Ending Obligation 160.3 138.6 $ 122.7
Regulated Operations [Member]      
Property, Plant and Equipment [Line Items]      
Property, Plant and Equipment in Service 4,555.8 4,490.6  
Construction Work in Progress 383.6 251.1  
Accumulated Depreciation (1,635.3) (1,549.6)  
Property, Plant and Equipment – Net $ 3,304.1 3,192.1  
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 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 $ 686.0 488.4  
Construction Work in Progress 351.3 164.5  
Accumulated Depreciation (86.8) (73.0)  
Property, Plant and Equipment – Net $ 950.5 579.9  
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 [1] $ 0.0 30.1  
Accumulated Depreciation [1] 0.0 (14.0)  
Property, Plant and Equipment – Net [1] $ 0.0 16.1  
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 [2] $ 231.9 214.3  
Construction Work in Progress [2] 3.8 6.6  
Accumulated Depreciation [2] (113.3) (104.6)  
Property, Plant and Equipment – Net [2] $ 122.4 $ 116.3  
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]
On March 26, 2019, ALLETE completed the sale of U.S. Water Services. (See Note 1. Operations and Significant Accounting Policies.)
[2]
Primarily includes BNI Energy and a small amount of non-rate base generation.