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Investment in ATC (Details) - USD ($)
$ in Millions
 12 Months Ended
Jan. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Sep. 27, 2016
ALLETE's Investment in ATC [Roll Forward]          
Equity Investment Beginning Balance   $ 135.6      
Cash Investments   7.8 $ 5.4 $ 1.6  
Equity Earnings in ATC   22.5 18.5 16.3  
Remeasurement of Deferred Income Taxes   (27.9) 0.0 0.0  
Equity Investment Ending Balance   118.7 135.6    
Income Statement Data [Abstract]          
ALLETE's Equity in Net Income   $ 22.5 18.5 16.3  
FERC [Member] | Return on Equity Complaint 2 [Member]          
Income Statement Data [Abstract]          
Proposed Return on Equity   9.70%      
Proposed Return on Equity, Including Incentive Adder   10.20%      
ATC [Member]          
Investment in ATC [Line Items]          
Ownership Percentage   8.00%      
Expected Additional Investment in 2018   $ 6.0      
ALLETE's Investment in ATC [Roll Forward]          
Equity Investment Beginning Balance   135.6 124.5    
Cash Investments   7.8 5.4    
Equity Earnings in ATC   22.5 18.5 16.3  
Distributed ATC Earnings   (19.3) (12.8)    
Equity Investment Ending Balance   118.7 135.6 124.5  
Balance Sheet Data [Abstract]          
Current Assets   87.7 75.8    
Non-Current Assets   4,598.9 4,312.9    
Total Assets   4,686.6 4,388.7    
Current Liabilities   767.2 495.1    
Long-Term Debt   1,790.6 1,865.3    
Other Non-Current Liabilities   240.3 271.5    
Members' Equity   1,888.5 1,756.8    
Total Liabilities and Members' Equity   4,686.6 4,388.7    
Income Statement Data [Abstract]          
Revenue   721.6 650.8 615.8  
Operating Expense   344.9 322.5 319.3  
Other Expense   104.1 95.5 96.1  
Net Income   272.6 232.8 200.4  
ALLETE's Equity in Net Income   $ 22.5 18.5 $ 16.3  
Authorized Return on Equity   10.32%     12.20%
Authorized Return on Equity, Including Incentive Adder   10.82%      
ATC [Member] | Subsequent Event [Member]          
ALLETE's Investment in ATC [Roll Forward]          
Cash Investments $ 1.6        
Investment in ATC [Member]          
ALLETE's Investment in ATC [Roll Forward]          
Remeasurement of Deferred Income Taxes   $ (27.9) $ 0.0