Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Financial Data (Unaudited) [Abstract]
Operating Revenue	$ 239.2	$ 226.9	$ 219.9	$ 242.2	$ 238.1	$ 224.1	$ 211.2	$ 233.6	$ 928.2	$ 907.0	$ 759.1
Operating Income	34.2	38.9	26.1	50.8	22.7	35.3	31.7	46.1	150.0	135.8	106.0
Net Income Attributable to ALLETE	$ 19.1	$ 20.5	$ 17.0	$ 37.2	$ 13.3	$ 19.6	$ 19.4	$ 23.0	$ 93.8	$ 75.3	$ 61.0
Basic Earnings Per Share of Common Stock	$ 0.53	$ 0.57	$ 0.49	$ 1.07	$ 0.38	$ 0.57	$ 0.57	$ 0.68	$ 2.66	$ 2.20	$ 1.89
Diluted Earnings Per Share of Common Stock	$ 0.53	$ 0.57	$ 0.48	$ 1.07	$ 0.38	$ 0.56	$ 0.57	$ 0.68	$ 2.65	$ 2.19	$ 1.89