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Investment in ATC (Details) - USD ($) $ in Millions			3 Months Ended		6 Months Ended
		Jul. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2017					$ 118.7
Cash Investments					3.9	$ 5.0
Equity in ATC Earnings			$ 4.3	$ 5.3	9.0	$ 11.4
Equity Investment Balance as of June 30, 2018			$ 123.6		$ 123.6
FERC [Member] | Return on Equity Complaint [Member]
ALLETE's Investment in ATC [Roll Forward]
Proposed Return on Equity			9.70%		9.70%
Proposed Return on Equity, Including Incentive Adder			10.20%		10.20%
ATC [Member]
Investment in ATC [Line Items]
Ownership Percentage			8.00%		8.00%
Expected Additional Investment in 2018			$ 1.3		$ 1.3
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2017					118.7
Cash Investments					3.9
Equity in ATC Earnings					9.0
Distributed ATC Earnings					(8.6)
Amortization of the Remeasurement of Deferred Income Taxes	[1]				0.6
Equity Investment Balance as of June 30, 2018			$ 123.6		$ 123.6
Authorized Return on Equity			10.32%		10.32%
Authorized Return on Equity, Including Incentive Adder			10.82%		10.82%
ATC [Member] | Subsequent Event [Member]
ALLETE's Investment in ATC [Roll Forward]
Cash Investments		$ 1.2
[1] Amortization related to the impact of the remeasurement of deferred income tax assets and liabilities resulting from the TCJA.

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