Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Statement [Abstract]
Operating Revenue	$ 349.6	$ 462.5	$ 998.2	$ 1,105.8
Operating Expenses [Abstract]
Fuel and Purchased Power	91.0	76.8	246.0	242.9
Transmission Services	16.6	13.9	49.5	40.1
Cost of Sales	46.4	149.8	113.1	233.3
Operating and Maintenance	80.8	81.3	240.9	246.4
Depreciation and Amortization	48.9	43.2	145.7	123.5
Taxes Other than Income Taxes	12.5	12.3	40.6	38.5
Total Operating Expenses	296.2	377.3	835.8	924.7
Operating Income	53.4	85.2	162.4	181.1
Other Income (Expense) [Abstract]
Interest Expense	(18.7)	(17.7)	(53.0)	(49.0)
Equity Earnings in ATC	6.1	5.5	15.0	14.1
Other	1.2	1.7	2.8	3.5
Total Other Expense	(11.4)	(10.5)	(35.2)	(31.4)
Income Before Non-Controlling Interest and Income Taxes	42.0	74.7	127.2	149.7
Income Tax Expense	1.7	14.4	15.7	27.0
Net Income	40.3	60.3	111.5	122.7
Less: Non-Controlling Interest in Subsidiaries	0.0	(0.1)	0.5	(0.1)
Net Income Attributable to ALLETE	$ 40.3	$ 60.4	$ 111.0	$ 122.8
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (in shares)	49.4	48.8	49.3	48.0
Diluted (in shares)	49.5	48.9	49.4	48.1
Basic Earnings Per Share of Common Stock	$ 0.82	$ 1.24	$ 2.25	$ 2.56
Diluted Earnings Per Share of Common Stock	0.81	1.23	2.25	2.55
Dividends Per Share of Common Stock	$ 0.52	$ 0.505	$ 1.56	$ 1.515