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Investments (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Jan. 05, 2011
Available-for-sale Securities [Abstract]        
Cost $ 27.4 $ 27.3 $ 27.4  
Gross Unrealized Gain 0.5 0.1 0.2  
Gross Unrealized (Loss) (1.1) (2.7) (2.4)  
Fair Value 26.8 24.7 25.2  
Net Proceeds 1.5 7.8 0.6  
Gross Realized Gain 0 0 0  
Gross Realized (Loss) 0 0 0  
Net Unrealized Gain (Loss) in Other Comprehensive Income 1.2 (0.3) 0.8  
Available for sale securities, Issued by States of the United States (ARS)       6.7
Investments [Abstract]        
ALLETE Properties 91.1 91.3    
Available-for-sale Securities 26.8 24.7    
Other 25.6 16.3    
Total Investments 143.5 132.3    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]        
Land Inventory Beginning Balance 86.0 86.0    
Deeds to Collateralized Property 0.5 1.8    
Land Impairment 0 (1.7)    
Cost of Sales (0.2) (0.3)    
Capitalized Improvements and Other 0.2 0.2    
Land Inventory Ending Balance 86.5 86.0 86.0  
Long-Term Finance Receivables (net of allowances of $0.6 and $0.6) 1.4 2.0    
Other 3.2 3.3    
Total Real Estate Assets 91.1 91.3    
Long-Term Finance Receivables, Allowance $ 0.6 $ 0.6