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Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share - Basic [Abstract]
Net Income Attributable ALLETE	$ 51.8	$ 66.3
Average Common Shares	52.1	51.7
Earnings Per Share	$ 0.99	$ 1.28
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 51.8	$ 66.3
Average Common Shares	52.2	51.8
Earnings Per Share	$ 0.99	$ 1.28
Dilutive Securities (Shares)	0.1	0.1

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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