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Common Stock and Earnings Per Share - Reconciliation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 41.4	$ 44.9	$ 36.9	$ 49.0	$ 44.3	$ 40.3	$ 24.8	$ 45.9	$ 18.3	$ 60.4	$ 22.5	$ 39.9	$ 172.2	$ 155.3	$ 141.1
Average Common Shares													50.8	49.3	48.3
Earnings Per Share	$ 0.81	$ 0.88	$ 0.73	$ 0.97	$ 0.89	$ 0.82	$ 0.50	$ 0.93	$ 0.37	$ 1.24	$ 0.46	$ 0.85	$ 3.39	$ 3.15	$ 2.92
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE, Diluted													$ 172.2	$ 155.3	$ 141.1
Average Common Shares													51.0	49.5	48.4
Earnings Per Share	$ 0.81	$ 0.88	$ 0.72	$ 0.97	$ 0.89	$ 0.81	$ 0.50	$ 0.93	$ 0.37	$ 1.23	$ 0.46	$ 0.85	$ 3.38	$ 3.14	$ 2.92
Dilutive Securities (Shares)													0.2	0.2	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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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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