Equity Investments - Investment in ATC (Details) - USD ($) $ in Millions			3 Months Ended		9 Months Ended
		Oct. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
ALLETE's Investment in ATC [Roll Forward]
Equity Investment Balance as of December 31, 2019					$ 197.6
Cash Investments					91.0	$ 26.4
Equity in ATC Earnings (a)			$ 5.1	$ 4.9	16.7	$ 15.3
Equity Investment Balance as of September 30, 2020			$ 291.8		$ 291.8
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, 2019					$ 141.6
Cash Investments					2.0
Equity in ATC Earnings (a)	[1]				16.7
Distributed ATC Earnings					(14.4)
Amortization of the Remeasurement of Deferred Income Taxes					0.9
Equity Investment Balance as of September 30, 2020			$ 146.8		$ 146.8
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.