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Investment in ATC (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2012	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Investment in ATC [Abstract]
Ownership Percentage		8.00%		8.00%
Approved Return on Common Equity		12.20%		12.20%
Equity Method Investment - Additional Investment for Year		$ 0		$ 0
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Beginning Balance	105.5			98.9
Cash Investments	0.8			3.9
Equity Earnings in ATC		4.9	4.7	14.3	13.7
Distributed ATC Earnings				(11.6)
Equity Investment Balance as of September 30, 2012		105.5		105.5
ATC Summarized Financial Data [Abstract]
Revenue		150.3	142.8	450.1	420.6
Operating Expense		68.8	66.4	210.1	192.5
Other Expense		21.0	19.7	62.1	61.6
Net Income		60.5	56.7	177.9	166.5
ALLETE's Equity in Net Income		$ 4.9	$ 4.7	$ 14.3	$ 13.7

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