Investment in ATC (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2011 Percent	Sep. 30, 2010	Sep. 30, 2011 Percent	Sep. 30, 2010
Equity Method Investments and Joint Ventures [Abstract]
Ownership Percentage	8.00%		8.00%
Approved Return on Common Equity	12.20%		12.20%
Year-to-Date Investment			$ 2.0
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2010			93.3
Cash Investments			2.0	1.2
Equity in ATC Earnings	4.7	4.5	13.7	13.4
Distributed ATC Earnings			(11.1)
Equity Investment Balance as of September 30, 2011	97.9		97.9
ATC Summarized Financial Data [Abstract]
Revenue	142.8	136.9	420.6	414.1
Operating Expense	66.4	60.2	192.5	185.9
Other Expense	19.7	21.7	61.6	64.0
Net Income	56.7	55.0	166.5	164.2
Equity in ATC Earnings	$ 4.7	$ 4.5	$ 13.7	$ 13.4

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