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A sensitivity analysis using basis point reduction.

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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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The proposed return on common equity, which would be dedicated to the utility plant in the investee accounted for under the equity method of accounting.

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The quantified amount of the future effect on earnings.

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

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Investment in ATC (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended	0 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014 Sensitivity Analysis [Member]	Oct. 30, 2014 Subsequent Event [Member]
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
Subsequent Event [Line Items]
Cash Investments			3.1			0.8
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2013			114.6
Cash Investments			3.1			0.8
Equity Earnings in ATC	5.3	4.9	15.6	15.1
Distributed ATC Earnings			(12.6)
Equity Investment Balance as of September 30, 2014	120.7		120.7
ATC Summarized Financial Data [Abstract]
Revenue	163.6	160.5	487.0	464.3
Operating Expense	76.6	77.6	229.6	217.2
Other Expense	21.4	20.2	65.1	62.6
Net Income	65.6	62.7	192.3	184.5
ALLETE's Equity in Net Income	5.3	4.9	15.6	15.1
Equity Method Investment, Proposed Return on Common Equity	9.15%		9.15%
Sensitivity Analysis [Line Items]
Basis Point Reduction on Approved Rate of Return on Common Equity					50
Effect on Future Earnings, Amount					$ 0.5