Equity Investments - Investment in ATC (Details) - USD ($) $ in Millions			3 Months Ended		6 Months Ended
		Jul. 31, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2019					$ 197.6
Cash Investments					66.6	$ 4.6
Equity in ATC Earnings (a)			$ 6.4	$ 4.8	11.6	$ 10.4
Equity Investment Balance as of June 30, 2020			$ 267.1		$ 267.1
ATC [Member]
Investment in ATC [Line Items]
Ownership Percentage			8.00%		8.00%
Expected Additional Investment in 2020			$ 0.8		$ 0.8
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2019					141.6
Cash Investments					1.1
Equity in ATC Earnings (a)	[1]				11.6
Distributed ATC Earnings					(9.4)
Amortization of the Remeasurement of Deferred Income Taxes					0.7
Equity Investment Balance as of June 30, 2020			$ 145.6		$ 145.6
Authorized Return on Equity					10.02%
Authorized Return on Equity, Including Incentive Adder			10.52%		10.52%
Additional Equity in Earnings During the Period					$ 1.2
ATC [Member] | Subsequent Event [Member]
ALLETE's Investment in ATC [Roll Forward]
Cash Investments		$ 0.8

[1] Equity in ATC Earnings includes $1.2 million of additional earnings resulting from the FERC’s May 21, 2020, order increasing ATC’s authorized return on equity to 10.52 percent including an incentive adder for participation in a regional transmission organization.