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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share - Basic [Abstract]
Net Income Attributable to ALLETE	$ 44.3	$ 40.3	$ 24.8	$ 45.9	$ 18.3	$ 60.4	$ 22.5	$ 39.9	$ 155.3	$ 141.1	$ 124.8
Average Common Shares									49.3	48.3	42.9
Earnings Per Share	$ 0.89	$ 0.82	$ 0.50	$ 0.93	$ 0.37	$ 1.24	$ 0.46	$ 0.85	$ 3.15	$ 2.92	$ 2.91
Earnings Per Share - Diluted [Abstract]
Net Income Attributable to ALLETE									$ 155.3	$ 141.1	$ 124.8
Average Common Shares									49.5	48.4	43.1
Earnings Per Share	$ 0.89	$ 0.81	$ 0.50	$ 0.93	$ 0.37	$ 1.23	$ 0.46	$ 0.85	$ 3.14	$ 2.92	$ 2.90
Dilutive Securities (Shares)									0.2	0.1	0.2