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Investment in ATC (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2013	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Investment in ATC [Abstract]
Ownership Percentage		8.00%		8.00%
Approved Return on Common Equity		12.20%		12.20%
Expected Additional Investment		$ 0.7		$ 0.7
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2012	111.2			107.3
Cash Investments	0.8			1.6
Equity Earnings in ATC		5.0	4.8	10.2	9.4
Distributed ATC Earnings				(7.9)
Equity Investment Balance as of June 30, 2013		111.2		111.2
ATC Summarized Financial Data [Abstract]
Revenue		152.1	152.2	303.9	299.8
Operating Expense		69.9	71.8	139.7	141.3
Other Expense		20.9	21.1	42.4	41.1
Net Income		61.3	59.3	121.8	117.4
ALLETE's Equity in Net Income		$ 5.0	$ 4.8	$ 10.2	$ 9.4

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Additional investments expected to be made during the current fiscal year but after the filing date, not already separately disclosed as part of current period/year-to-date investments or as a subsequent event.

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