Investment in ATC (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended	9 Months Ended
Oct. 31, 2013	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Investment in ATC [Abstract]
Ownership Percentage		8.00%		8.00%
Approved Return on Common Equity		12.20%		12.20%
Equity Method Investment, Expected Additional Investment		$ 0		$ 0
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2012	112.7			107.3
Cash Investments	0.8			2.3
Equity Earnings in ATC		4.9	4.9	15.1	14.3
Distributed ATC Earnings				(12.0)
Equity Investment Balance as of September 30, 2013		112.7		112.7
ATC Summarized Financial Data [Abstract]
Revenue		160.5	150.3	464.3	450.1
Operating Expense		77.6	68.8	217.2	210.1
Other Expense		20.2	21.0	62.6	62.1
Net Income		62.7	60.5	184.5	177.9
ALLETE's Equity in Net Income		$ 4.9	$ 4.9	$ 15.1	$ 14.3

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The approved percentage return on common equity dedicated to the utility plant in the investee accounted for under the equity method of accounting.

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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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The amount of the operating expense reported by an equity method investment of the entity.

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The amount of other expense reported by an equity method investment of the entity.

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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The amount of net income (loss) reported by an equity method investment of the entity.

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The amount of revenue from sale of goods and services reduced by sales returns, allowances, and discounts reported by an equity method investment of the entity.

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