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Investment in ATC (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Balance Sheet Data [Abstract]        
Current Assets   $ 80.7 $ 63.1  
Non-Current Assets   3,509.5 3,274.7  
Total Assets   3,590.2 3,337.8  
Current Liabilities   381.4 251.5  
Long-Term Debt   1,550.0 1,550.0  
Other Non-Current Liabilities   126.2 95.8  
Members' Equity   1,532.6 1,440.5  
Total Liabilities and Members' Equity   3,590.2 3,337.8  
Income Statement Data [Abstract]        
Revenue   626.3 603.2 567.2
Operating Expense   295.1 281.0 261.6
Other Expense   83.6 84.8 81.7
Net Income   247.6 237.4 223.9
ALLETE's Equity in Net Income   20.3 19.4 18.4
Investment in ATC [Abstract]        
Equity Method Investment, Ownership Percentage   8.00%    
Equity Method Investment, Approved Return on Common Equity   12.20%    
Equity Method Investment, Total Expected Investment for Current Year   5.8    
ALLETE's Investment in ATC [Roll Forward]        
Equity Investment Beginning Balance 114.6 107.3 98.9  
Cash Investments 1.2 3.1 4.7  
Equity Earnings in ATC   20.3 19.4 18.4
Distributed ATC Earnings   (16.1) (15.7)  
Equity Investment Ending Balance   $ 114.6 $ 107.3 $ 98.9