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Business Segments (Details)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
a
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
a
Segments
Jun. 30, 2017
USD ($)
Dec. 31, 2017
USD ($)
Business Segments [Line Items]          
Number of Reportable Segments | Segments     3    
Operating Revenue $ 344.1 $ 353.3 $ 702.3 $ 718.9  
Net Income (Loss) 31.3 36.9 82.3 85.9  
Assets 5,114.7   $ 5,114.7   $ 5,080.0
Regulated Operations [Member]          
Business Segments [Line Items]          
Number of Operating Segments | Segments     3    
Operating Revenue 257.8 264.9 $ 528.0 546.5  
Net Income (Loss) 26.0 32.4 69.9 75.9  
Assets 3,898.2   3,898.2   3,886.6
Regulated Operations [Member] | Residential [Member]          
Business Segments [Line Items]          
Operating Revenue [1] 30.7 27.6 71.4 67.1  
Regulated Operations [Member] | Commercial [Member]          
Business Segments [Line Items]          
Operating Revenue [1] 36.2 33.7 72.8 71.9  
Regulated Operations [Member] | Municipal [Member]          
Business Segments [Line Items]          
Operating Revenue [1] 13.7 12.3 27.7 30.5  
Regulated Operations [Member] | Industrial [Member]          
Business Segments [Line Items]          
Operating Revenue [1] 115.3 121.6 230.2 243.3  
Regulated Operations [Member] | Other Power Suppliers [Member]          
Business Segments [Line Items]          
Operating Revenue [1] 42.7 41.7 86.4 82.9  
Regulated Operations [Member] | Conservation Improvement Programs [Member]          
Business Segments [Line Items]          
Operating Revenue [1],[2] 0.0 5.5 0.0 5.5  
Regulated Operations [Member] | Other [Member]          
Business Segments [Line Items]          
Operating Revenue [1] 19.2 22.5 39.5 45.3  
ALLETE Clean Energy [Member]          
Business Segments [Line Items]          
Operating Revenue 18.3 19.6 42.9 43.3  
Net Income (Loss) 6.8 3.8 14.9 10.5  
Assets 620.2   620.2   600.5
ALLETE Clean Energy [Member] | Long-term PSA [Member]          
Business Segments [Line Items]          
Operating Revenue [1] 12.4 13.7 31.0 31.5  
ALLETE Clean Energy [Member] | Other [Member]          
Business Segments [Line Items]          
Operating Revenue, Other Than Customer Revenue [1] 5.9 5.9 11.9 11.8  
U.S. Water Services [Member]          
Business Segments [Line Items]          
Operating Revenue 41.5 38.4 79.7 70.5  
Net Income (Loss) 0.2 0.6 (1.2) 0.3  
Assets 291.8   291.8   292.4
U.S. Water Services [Member] | Point-in-Time [Member]          
Business Segments [Line Items]          
Operating Revenue [1] 25.7 23.9 48.0 45.7  
U.S. Water Services [Member] | Contract [Member]          
Business Segments [Line Items]          
Operating Revenue [1] 9.5 8.8 19.0 17.7  
U.S. Water Services [Member] | Capital Project [Member]          
Business Segments [Line Items]          
Operating Revenue [1] $ 6.3 5.7 $ 12.7 7.1  
Corporate and Other [Member]          
Business Segments [Line Items]          
Number of Operating Segments | Segments     2    
Land in Minnesota (Acres) | a 4,000   4,000    
Operating Revenue $ 26.5 30.4 $ 51.7 58.6  
Net Income (Loss) (1.7) 0.1 (1.3) (0.8)  
Assets 304.5   304.5   $ 300.5
Corporate and Other [Member] | Long-term Contract [Member]          
Business Segments [Line Items]          
Operating Revenue [1] 22.7 23.2 42.7 45.3  
Corporate and Other [Member] | Other [Member]          
Business Segments [Line Items]          
Operating Revenue [1] $ 3.8 $ 7.2 $ 9.0 $ 13.3  
[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.)
[2] See Note 6. Regulatory Matters.