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Business Segments (Details)
$ in Millions
 3 Months Ended
Mar. 31, 2019
USD ($)
a
Segments
Mar. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
Business Segments [Line Items]      
Number of Reportable Segments | Segments 3    
Operating Revenue $ 357.2 $ 358.2  
Net Income (Loss) 70.5 51.0  
Approximate Gain on Sale of U.S. Water Services, After-tax 10.0    
Assets $ 5,218.8   $ 5,165.0
Regulated Operations [Member]      
Business Segments [Line Items]      
Number of Operating Segments | Segments 3    
Operating Revenue $ 282.2 270.2  
Net Income (Loss) 51.5 43.9  
Assets 3,962.7   3,952.5
Regulated Operations [Member] | Residential [Member]      
Business Segments [Line Items]      
Operating Revenue 45.2 40.7  
Regulated Operations [Member] | Commercial [Member]      
Business Segments [Line Items]      
Operating Revenue 38.9 36.6  
Regulated Operations [Member] | Municipal [Member]      
Business Segments [Line Items]      
Operating Revenue 15.4 14.0  
Regulated Operations [Member] | Industrial [Member]      
Business Segments [Line Items]      
Operating Revenue 121.6 114.9  
Regulated Operations [Member] | Other Power Suppliers [Member]      
Business Segments [Line Items]      
Operating Revenue 39.4 43.7  
Regulated Operations [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue 21.7 20.3  
ALLETE Clean Energy [Member]      
Business Segments [Line Items]      
Operating Revenue 17.5 24.6  
Net Income (Loss) 5.8 8.1  
Assets 643.8   606.6
ALLETE Clean Energy [Member] | Long-term PSA [Member]      
Business Segments [Line Items]      
Operating Revenue 14.6 18.6  
ALLETE Clean Energy [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue, Other Than Customer Revenue 2.9 6.0  
U.S. Water Services [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 33.4 38.2  
Net Income (Loss) (1.1) (1.4)  
Assets [2] 0.0   295.8
U.S. Water Services [Member] | Point-in-Time [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 19.0 22.3  
U.S. Water Services [Member] | Contract [Member]      
Business Segments [Line Items]      
Operating Revenue [1] 9.2 9.5  
U.S. Water Services [Member] | Capital Project [Member]      
Business Segments [Line Items]      
Operating Revenue [1] $ 5.2 6.4  
Corporate and Other [Member]      
Business Segments [Line Items]      
Number of Operating Segments | Segments 2    
Land in Minnesota (Acres) | a 4,000    
Operating Revenue $ 24.1 25.2  
Net Income (Loss) 14.3 0.4  
Assets 612.3   $ 310.1
Corporate and Other [Member] | Long-term Contract [Member]      
Business Segments [Line Items]      
Operating Revenue 20.2 20.0  
Corporate and Other [Member] | Other [Member]      
Business Segments [Line Items]      
Operating Revenue $ 3.9 $ 5.2  
[1] On March 26, 2019, ALLETE completed the sale of U.S. Water Services. The Company recognized a gain on the sale of approximately $10 million after-tax reflected in Corporate and Other in 2019. (See Note 1. Operations and Significant Accounting Policies.)
[2] On March 26, 2019, ALLETE completed the sale of U.S. Water Services. (See Note 1. Operations and Significant Accounting Policies.)