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Investment in ATC (Details) - USD ($) $ in Millions			3 Months Ended
		Apr. 30, 2018	Mar. 31, 2018	Mar. 31, 2017
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2017			$ 118.7
Cash Investments			1.6	$ 3.1
Equity in ATC Earnings			4.7	$ 6.1
Equity Investment Balance as of March 31, 2018			$ 120.1
FERC [Member] | Return on Equity Complaint 2 [Member]
ALLETE's Investment in ATC [Roll Forward]
Proposed Return on Equity			9.70%
Proposed Return on Equity, Including Incentive Adder			10.20%
ATC [Member]
Investment in ATC [Line Items]
Ownership Percentage			8.00%
Expected Additional Investment in 2018			$ 2.6
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2017			118.7
Cash Investments			1.6
Equity in ATC Earnings			4.7
Distributed ATC Earnings			(5.2)
Amortization of the Remeasurement of Deferred Income Taxes	[1]		0.3
Equity Investment Balance as of March 31, 2018			$ 120.1
Authorized Return on Equity			10.32%
Authorized Return on Equity, Including Incentive Adder			10.82%
ATC [Member] | Subsequent Event [Member]
ALLETE's Investment in ATC [Roll Forward]
Cash Investments		$ 2.3
[1] Amortization related to the impact of the remeasurement of deferred income tax assets and liabilities resulting from the TCJA.

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ALLETE's Investment in ATC [Roll Forward]

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Amortization from the Remeasurement of Deferred Income Taxes for Equity Method Investments

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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 approved percentage return on common equity, including an incentive adder for participation in a regional transmission organization 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 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 proposed return on common equity, including an incentive adder for participation in a regional transmission organization, which would be dedicated to the utility plant in the investee accounted for under the equity method of accounting.

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Amount of distribution received from equity method investee for return on investment, classified as operating activities. Excludes distribution for return of investment, 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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