Investments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Investments [Abstract]
ALLETE Properties	$ 91.1	$ 91.1
Available-for-sale Securities	20.1	26.8
Cash Equivalents	22.7	20.7
Other	4.7	4.9
Total Other Investments	138.6	143.5
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