Investments (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Investments [Abstract]
ALLETE Properties	$ 91.1	$ 91.3
Available-for-sale Securities	27.9	24.7
Other	20.7	16.3
Total Investments	139.7	132.3
ALLETE Properties [Rollforward]
Land Inventory Beginning Balance (January 1, 2012 and 2011, respectively)	86.0	86.0
Deeds to Collateralized Property	0.5	1.8
Land Impairment	0	(1.7)
Capitalized Improvements and Other	0.1	0.2
Cost of Real Estate Sold	(0.2)	(0.3)
Land Inventory Ending Balance	86.4	86.0
Long-Term Finance Receivables (net of allowances of $0.6 and $0.6)	1.4	2.0
Other	3.3	3.3
Total Real Estate Assets	91.1	91.3
Long-Term Finance Receivables, Allowance	$ 0.6	$ 0.6