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Business Segments (Details)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
USD ($)
a
Sep. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
a
Segments
Sep. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Business Segments [Line Items]          
Operating Revenue $ 349.6 $ 462.5 $ 998.2 $ 1,105.8  
Net Income (Loss) Attributable to ALLETE 40.3 60.4 111.0 122.8  
Assets [1] 4,874.8   $ 4,874.8   $ 4,894.5
Regulated Operations [Member]          
Business Segments [Line Items]          
Number of Operating Segments | Segments     3    
Operating Revenue 253.3 250.2 $ 740.5 743.0  
Net Income (Loss) Attributable to ALLETE 45.0 43.8 110.0 108.1  
Assets [1] 3,838.8   3,838.8   3,853.1
ALLETE Clean Energy [Member]          
Business Segments [Line Items]          
Operating Revenue 14.7 151.1 57.1 197.5  
Net Income (Loss) Attributable to ALLETE 1.0 13.2 9.7 18.7  
Assets [1] 483.5   483.5   501.5
U.S. Water Services [Member]          
Business Segments [Line Items]          
Operating Revenue 37.8 36.1 104.5 86.0  
Net Income (Loss) Attributable to ALLETE 1.5 1.0 2.0 1.5  
Assets $ 259.4   $ 259.4   258.3
Corporate and Other [Member]          
Business Segments [Line Items]          
Number of Operating Segments | Segments     2    
Land in Minnesota (Acres) | a 5,000   5,000    
Operating Revenue $ 43.8 25.1 $ 96.1 79.3  
Net Income (Loss) Attributable to ALLETE (7.2) $ 2.4 (10.7) $ (5.5)  
Assets $ 293.1   $ 293.1   $ 281.6
[1] As a result of revised accounting guidance adopted in the first quarter of 2016, we reclassified unamortized debt issuance costs from Other Non-Current Assets to Long-Term Debt on the Consolidated Balance Sheet. Prior period segment assets have been revised to conform to the current presentation. (See Note 1. Operations and Significant Accounting Policies.)