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Investment in ATC (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Balance Sheet Data [Abstract]        
Current Assets   $ 63.1 $ 58.7  
Non-Current Assets   3,274.7 3,053.7  
Total Assets   3,337.8 3,112.4  
Current Liabilities   251.5 298.5  
Long-Term Debt   1,550.0 1,400.0  
Other Non-Current Liabilities   95.8 82.6  
Members' Equity   1,440.5 1,331.3  
Total Liabilities and Members' Equity   3,337.8 3,112.4  
Income Statement Data [Abstract]        
Revenue   603.2 567.2 556.7
Operating Expense   281.0 261.6 251.1
Other Expense   84.8 81.7 85.9
Net Income   237.4 223.9 219.7
ALLETE's Equity in Net Income   19.4 18.4 17.9
Investment in ATC [Abstract]        
Equity Method Investment, Ownership Percentage   8.00%    
Equity Method Investment, Approved Return on Common Equity   12.20%    
Equity Method Investment, Expected Future Investment   2.0    
ALLETE's Investment in ATC [Roll Forward]        
Equity Investment Beginning Balance 107.3 98.9 93.3  
Cash Investments 0.4 4.7 2.0  
Equity Earnings in ATC   19.4 18.4 17.9
Distributed ATC Earnings   (15.7) (14.8)  
Equity Investment Ending Balance   $ 107.3 $ 98.9 $ 93.3