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Investment in ATC (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Jan. 31, 2012
ALLETE's Investment in ATC [Roll Forward]        
Equity Investment Beginning Balance $ 93.3 $ 88.4    
Cash Investments 2.0 1.6    
Equity Earnings in ATC 18.4 17.9 17.5  
Distributed ATC Earnings (14.8) (14.6)    
Equity Investment Ending Balance 98.9 93.3 88.4  
Balance Sheet Data [Abstract]        
Current Assets 58.7 59.9    
Non-Current Assets 3,053.7 2,888.4    
Total Assets 3,112.4 2,948.3    
Current Liabilities 298.5 428.4    
Long-Term Debt 1,400.0 1,175.0    
Other Non-Current Liabilities 82.6 84.9    
Members' Equity 1,331.3 1,260.0    
Total Liabilities and Members' Equity 3,112.4 2,948.3    
Income Statement Data [Abstract]        
Revenue 567.2 556.7 521.5  
Operating Expense 261.6 251.1 230.3  
Other Expense 81.7 85.9 77.8  
Net Income 223.9 219.7 213.4  
ALLETE's Equity in Net Income 18.4 17.9 17.5  
Investment in ATC [Abstract]        
Equity Method Investment, Expected Future Investment 3      
Equity Method Investment - Additional Investment Amount       $ 0.8
Equity Method Investment, Approved Return on Common Equity 12.20%      
Equity Method Investment, Ownership Percentage 8.00%