ALLETE's Investment in ATC [Roll Forward]

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- Definition

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

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No definition available.

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- Definition

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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No definition available.

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- Definition

Additional investments expected to be made in the next fiscal year in an equity method investment.

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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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- Definition

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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Amount of income (loss) for proportionate share of equity method investee's income (loss).

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The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

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Percentage of public utility's approved return on equity.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Equity Investments - Investment in ATC (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2020			$ 301.2
Cash Investments			17.1	$ 66.6
Equity in ATC Earnings	$ 5.1	$ 6.4	9.9	$ 11.6
Equity Investment Balance as of June 30, 2021	$ 317.8		$ 317.8
ATC [Member]
Investment in ATC [Line Items]
Ownership Percentage	8.00%		8.00%
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2020			$ 149.0
Equity in ATC Earnings			10.5
Distributed Earnings			(8.5)
Amortization of the Remeasurement of Deferred Income Taxes			0.6
Equity Investment Balance as of June 30, 2021	$ 151.6		$ 151.6
Authorized Return on Equity			10.02%
Authorized Return on Equity, Including Incentive Adder	10.52%		10.52%
Equity Method Investment, Expected Additional Investment Next Fiscal Year	$ 2.0		$ 2.0