Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Data (Unaudited) [Abstract]
Operating Revenue	$ 290.7	$ 288.9	$ 260.7	$ 296.5	$ 268.0	$ 251.0	$ 235.6	$ 263.8
Operating Income	51.5	60.8	28.2	48.3	46.9	38.4	24.4	44.4	188.8	154.1	155.2
Net Income Attributable to ALLETE	$ 32.9	$ 41.6	$ 16.8	$ 33.5	$ 33.0	$ 25.2	$ 14.0	$ 32.5	$ 124.8	$ 104.7	$ 97.1
Basic Earnings Per Share of Common Stock	$ 0.73	$ 0.97	$ 0.40	$ 0.81	$ 0.82	$ 0.63	$ 0.36	$ 0.83	$ 2.91	$ 2.64	$ 2.59
Diluted Earnings Per Share of Common Stock	$ 0.73	$ 0.97	$ 0.40	$ 0.80	$ 0.82	$ 0.63	$ 0.35	$ 0.83	$ 2.90	$ 2.63	$ 2.58