Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Selected Quarterly Financial Information [Abstract]
Operating Revenue	$ 320.4	$ 293.9	$ 243.2	$ 311.6	$ 304.6	$ 288.3	$ 290.4	$ 357.2	$ 448.3	$ 348.0	$ 344.1	$ 358.2	$ 1,169.1	$ 1,240.5	$ 1,498.6
Operating Income	36.4	41.6	12.7	60.2	49.8	37.0	36.2	56.8	64.0	43.3	36.5	57.4	150.9	179.8	201.2
Net Income Attributable to ALLETE	$ 47.1	$ 40.7	$ 20.1	$ 66.3	$ 49.7	$ 31.2	$ 34.2	$ 70.5	$ 61.1	$ 30.7	$ 31.3	$ 51.0	$ 174.2	$ 185.6	$ 174.1
Basic Earnings Per Share of Common Stock	$ 0.91	$ 0.78	$ 0.39	$ 1.28	$ 0.96	$ 0.60	$ 0.66	$ 1.37	$ 1.19	$ 0.59	$ 0.61	$ 1.00	$ 3.36	$ 3.59	$ 3.39
Diluted Earnings Per Share of Common Stock	$ 0.90	$ 0.78	$ 0.39	$ 1.28	$ 0.96	$ 0.60	$ 0.66	$ 1.37	$ 1.18	$ 0.59	$ 0.61	$ 0.99	$ 3.35	$ 3.59	$ 3.38