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Investments (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Dec. 31, 2011
Investments [Abstract]      
ALLETE Properties $ 91.3 $ 91.3 $ 91.3
Available-for-sale Securities 27.7 24.7 24.7
Other 16.7 16.3 16.3
Total Investments 135.7 132.3 132.3
ALLETE Properties [Rollforward]      
Land Inventory Beginning Balance (January 1, 2012 and 2011, respectively) 86.0   86.0
Deeds to Collateralized Property 0   1.8
Land Impairment 0 1.7 (1.7)
Capitalized Improvements and Other 0.1   0.2
Cost of Real Estate Sold 0   (0.3)
Land Inventory Ending Balance 86.1 86.0 86.0
Long-Term Finance Receivables (net of allowances of $0.6 and $0.6) 2.0 2.0 2.0
Other 3.2 3.3 3.3
Total Real Estate Assets 91.3 91.3 91.3
Long-Term Finance Receivables, Allowance $ 0.6 $ 0.6 $ 0.6