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Business Segments (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2018
USD ($)
a
Segments
Mar. 31, 2017
USD ($)
Dec. 31, 2017
USD ($)
Business Segments [Line Items]      
Number of Reportable Segments | Segments 3    
Operating Revenue $ 358.2 $ 365.6  
Net Income (Loss) 51.0 49.0  
Assets $ 5,072.1   $ 5,080.0
Regulated Operations [Member]      
Business Segments [Line Items]      
Number of Operating Segments | Segments 3    
Operating Revenue $ 270.2 281.6  
Net Income (Loss) 43.9 43.5  
Assets 3,877.0   3,886.6
Regulated Operations [Member] | Residential [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 40.7 39.5  
Regulated Operations [Member] | Commercial [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 36.6 38.2  
Regulated Operations [Member] | Municipal [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 14.0 18.2  
Regulated Operations [Member] | Industrial [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 114.9 121.7  
Regulated Operations [Member] | Other Power Suppliers [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 43.7 41.2  
Regulated Operations [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 20.3 22.8  
ALLETE Clean Energy [Member]      
Business Segments [Line Items]      
Operating Revenue 24.6 23.7  
Net Income (Loss) 8.1 6.7  
Assets 620.4   600.5
ALLETE Clean Energy [Member] | Long-term PSA [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 18.6 17.8  
ALLETE Clean Energy [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue, Other Than Customer Revenue [1] 6.0 5.9  
U.S. Water Services [Member]      
Business Segments [Line Items]      
Operating Revenue 38.2 32.1  
Net Income (Loss) (1.4) (0.3)  
Assets 287.4   292.4
U.S. Water Services [Member] | Point-in-Time [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 22.3 21.8  
U.S. Water Services [Member] | Contract [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 9.5 8.9  
U.S. Water Services [Member] | Capital Project [Member]      
Business Segments [Line Items]      
Operating Revenue [1] $ 6.4 1.4  
Corporate and Other [Member]      
Business Segments [Line Items]      
Number of Operating Segments | Segments 2    
Land in Minnesota (Acres) | a 5,000    
Operating Revenue $ 25.2 28.2  
Net Income (Loss) 0.4 (0.9)  
Assets 287.3   $ 300.5
Corporate and Other [Member] | Long-term Contract [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 20.0 22.1  
Corporate and Other [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue [1] $ 5.2 $ 6.1  
[1] With the adoption of new revenue recognition guidance, the Company has enhanced the presentation of business segment Operating Revenue. (See Note 1. Operations and Significant Accounting Policies.)