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Investments (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Jan. 05, 2011
Available-for-sale Securities [Abstract]        
Cost $ 27.3 $ 27.4 $ 33.1  
Gross Unrealized Gain 0.1 0.2 0.1  
Gross Unrealized (Loss) (2.7) (2.4) (3.7)  
Fair Value 24.7 25.2 29.5  
Net Proceeds 5.5 (1.7) 6.7  
Gross Realized Gain 0 0 0  
Gross Realized (Loss) 0 0 0  
Net Unrealized Gain (Loss) in Other Comprehensive Income (0.4) 1.4 4.5  
Available for sale securities, Issued by States of the United States (ARS)       6.7
Investments [Abstract]        
ALLETE Properties 91.3 94.0    
Available-for-sale Securities 24.7 25.2    
Other 16.3 6.8    
Total Investments 132.3 126.0    
Notes, Loans and Financing Receivable, Net, Noncurrent [Abstract]        
Notes, Loans and Financing Receivable, Net, Noncurrent 2.0 3.7    
Financing Receivable, Allowance for Credit Losses, Provisions 0.6 0.8    
Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]        
Land Inventory Beginning Balance 86.0 74.9    
Deeds to Collateralized Property 1.8 [1] 9.9 [1]    
Land Impairment (1.7) [2] 0 [2]    
Cost of Real Estate Sold (0.3) 0    
Capitalized Improvements and Other 0.2 1.2    
Land Inventory Ending Balance 86.0 86.0 74.9  
Long-Term Finance Receivables (net of allowances of $0.6 and $0.8) 2.0 [1] 3.7 [1]    
Other 3.3 4.3    
Total Real Estate Assets 91.3 94.0    
Long-Term Finance Receivable Allowance $ 0.6 $ 0.8    
[1] (a)In 2010, the deeds to collateralized property resulted primarily from an entity which filed for Chapter 11 bankruptcy and were recorded at fair value net of estimated selling costs.
[2] (b)The land impairment charge was a result of an impairment analysis conducted in the fourth quarter of 2011 where the cost basis was reduced to the estimated fair value.