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Investment in ATC (Details) - USD ($) $ in Millions		3 Months Ended
	Apr. 30, 2019	Mar. 31, 2019	Mar. 31, 2018
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2018		$ 161.1
Cash Investments		0.5	$ 1.6
Equity in ATC Earnings		5.6	$ 4.7
Equity Investment Balance as of March 31, 2019		$ 154.8
FERC [Member] | Return on Equity Complaint [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 2019		$ 5.8
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2018		128.1
Cash Investments		0.4
Equity in ATC Earnings		5.6
Distributed ATC Earnings		(4.4)
Amortization of the Remeasurement of Deferred Income Taxes		0.3
Equity Investment Balance as of March 31, 2019		$ 130.0
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

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ALLETE's Investment in ATC [Roll Forward]

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