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Earnings Per Share and Common Stock (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 31.3	$ 36.9	$ 82.3	$ 85.9
Average Common Shares	51.3	50.9	51.2	50.5
Earnings Per Share	$ 0.61	$ 0.73	$ 1.61	$ 1.70
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE	$ 31.3	$ 36.9	$ 82.3	$ 85.9
Average Common Shares	51.5	51.1	51.4	50.7
Earnings Per Share	$ 0.61	$ 0.72	$ 1.60	$ 1.69
Dilutive Securities (Shares)	0.2	0.2	0.2	0.2
Common Stock [Member]
Class of Stock [Line Items]
Options to Purchase Shares Excluded from Computation of Diluted Earnings Per Share			0.0	0.0

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