Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Consolidated Statement of Income [Abstract]
Operating Revenue	$ 462.5	$ 288.9	$ 1,105.8	$ 846.1
Operating Expenses [Abstract]
Fuel and Purchased Power	76.8	88.9	242.9	268.7
Transmission Services	13.9	11.9	40.1	33.2
Cost of Sales	149.8	16.8	233.3	59.2
Operating and Maintenance	81.3	65.7	246.4	214.3
Depreciation and Amortization	43.2	33.4	123.5	99.5
Taxes Other than Income Taxes	12.3	11.4	38.5	33.9
Total Operating Expenses	377.3	228.1	924.7	708.8
Operating Income	85.2	60.8	181.1	137.3
Other Income (Expense) [Abstract]
Interest Expense	(17.7)	(13.2)	(49.0)	(39.5)
Equity Earnings in ATC	5.5	5.3	14.1	15.6
Other	1.7	2.1	3.5	6.0
Total Other Expense	(10.5)	(5.8)	(31.4)	(17.9)
Income Before Non-Controlling Interest and Income Taxes	74.7	55.0	149.7	119.4
Income Tax Expense	14.4	13.4	27.0	27.1
Net Income	60.3	41.6	122.7	92.3
Less: Non-Controlling Interest in Subsidiaries	(0.1)	0.0	(0.1)	0.4
Net Income Attributable to ALLETE	$ 60.4	$ 41.6	$ 122.8	$ 91.9
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (in shares)	48.8	42.9	48.0	42.1
Diluted (in shares)	48.9	42.9	48.1	42.3
Basic Earnings Per Share of Common Stock	$ 1.24	$ 0.97	$ 2.56	$ 2.18
Diluted Earnings Per Share of Common Stock	1.23	0.97	2.55	2.17
Dividends Per Share of Common Stock	$ 0.505	$ 0.49	$ 1.515	$ 1.47