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Short-Term and Long-Term Debt (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2012
Lines of Credit [Member]
Dec. 31, 2011
Lines of Credit [Member]
Dec. 31, 2012
ALLETE LOC January 2014 [Member]
Jun. 30, 2012
ALLETE LOC January 2014 [Member]
Feb. 01, 2012
ALLETE LOC January 2014 [Member]
Dec. 31, 2012
ALLETE LOC June 2015 [Member]
Dec. 31, 2012
ALLETE Bonds 4.86% Due 2013 [Member]
Dec. 31, 2012
ALLETE Bonds 3.20% Due July 2026 [Member]
Dec. 31, 2012
ALLETE Bonds 4.08% Due July2042 [Member]
Dec. 31, 2012
Industrial Development Revenue Bonds 6.5% Due 2025 [Member]
Financial Covenants [Abstract]                        
Required Indebtedness to Total Capital Ratio 0.65                      
Actual Indebtedness to Total Capital Ratio 0.46                      
Maturities of Long-term Debt [Abstract]                        
Long-Term Debt $ 933.6 $ 857.9                    
Long-Term Debt Due Within One Year 84.5 5.4             60.0      
Long-Term Debt, 2014 Maturities 94.8                      
Long-Term Debt, 2015 Maturities 17.4                      
Long-Term Debt, 2016 Maturities 21.7                      
Long-Term Debt, 2017 Maturities 51.2                      
Long-Term Debt, Thereafter Maturities 748.5                      
Debt Instrument [Line Items]                        
Short-Term Debt 84.5 6.5                    
Line of Credit Facility, Maximum Borrowing Capacity     406.4 256.4 150.0   150.0 250.0        
Letter of Credit Maximum Issuances 50.0           10.0          
Lines of Credit, Current 0 1.1                    
Proceeds from Issuance of First Mortgage Bond 160.0                 75.0 85.0  
First Mortgage Bonds, Interest Rate                   3.20% 4.08% 6.50%
Redemption of Bond                       6.0
Line of Credit Facility, Amount Outstanding           $ 14.0