Investment in ATC (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	0 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Apr. 30, 2015
Investment in ATC [Abstract]
Ownership Percentage	8.00%
Approved Return on Common Equity	12.20%
Equity Method Investment, Expected Additional Investment	$ 1.1
Subsequent Event [Line Items]
Cash Investments	0.4
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2014	121.1us-gaap_EquityMethodInvestments
Cash Investments	0.4
Equity Earnings in ATC	3.9	5.1
Distributed ATC Earnings	(3.1)
Equity Investment Balance as of March 31, 2015	122.3us-gaap_EquityMethodInvestments
ATC Summarized Financial Data [Abstract]
Revenue	152.4	163.3
Operating Expense	80.0	78.6
Other Expense	24.4	21.6
Net Income	48.0	63.1
ALLETE's Equity in Net Income	3.9	5.1
Subsequent Event [Member]
Subsequent Event [Line Items]
Cash Investments			0.4
ALLETE's Investment in ATC [Roll Forward]
Cash Investments			$ 0.4

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

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This item represents disclosure of the amount of dividends or other distributions received from unconsolidated subsidiaries, certain corporate joint ventures, and certain noncontrolled corporation; these investments are accounted for under the equity method of accounting. This element excludes distributions that constitute a return of investment, which are classified as investing activities.

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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 the cost of sales reported by an equity method investment of the entity.

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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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