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Investments (Details) (USD $)
In Millions, unless otherwise specified
 6 Months Ended 12 Months Ended
Jun. 30, 2013
Dec. 31, 2012
Investments [Abstract]    
ALLETE Properties $ 91.1 $ 91.1
Available-for-sale Securities 20.4 [1] 26.8 [1]
Cash Equivalents 24.4 20.7
Other 4.3 4.9
Total Other Investments 140.2 143.5
Available-for-sale Corporate Debt Securities, Maturities, One Year or Less 1.2  
Available-for-sale Corporate Debt Securities, Maturities, More than One Year to Less than Three Years 5.1  
Available-for-sale Corporate Debt Securities, Maturities, Three Years to Less than Five Years 1.3  
Available-for-sale Corporate Debt Securities, Maturities, Five Years or More 0.9  
ALLETE Properties [Rollforward]    
Land Inventory Beginning Balance 86.5 86.0
Deeds to Collateralized Property 0 0.5
Cost of Sales 0 (0.2)
Capitalized Improvements and Other 0.1 0.2
Land Inventory Ending Balance 86.6 86.5
Long-Term Finance Reveivables (net of allowances of $0.6 and $0.6) 1.4 1.4
Other 3.1 3.2
Total Real Estate Assets 91.1 91.1
Impairment of Real Estate 0 0
Long-Term Finance Receivables, Allowance $ 0.6 $ 0.6
[1] As of June 30, 2013, the aggregate amount of available-for-sale corporate debt securities maturing in one year or less was $1.2 million, in one year to less than three years was $5.1 million, in three years to less than five years was $1.3 million, and in five or more years was $0.9 million.