Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Income Statement [Abstract]
Operating Revenue	$ 333.8	$ 320.0
Operating Expenses [Abstract]
Fuel and Purchased Power	76.9	86.0
Transmission Services	16.8	14.9
Cost of Sales	33.3	31.2
Operating and Maintenance	78.1	79.7
Depreciation and Amortization	48.1	39.0
Taxes Other than Income Taxes	13.8	12.8
Total Operating Expenses	267.0	263.6
Operating Income	66.8	56.4
Other Income (Expense) [Abstract]
Interest Expense	(16.9)	(15.1)
Equity Earnings in ATC	4.8	3.9
Other	1.0	1.1
Total Other Expense	(11.1)	(10.1)
Income Before Non-Controlling Interest and Income Taxes	55.7	46.3
Income Tax Expense	9.3	6.2
Net Income	46.4	40.1
Less: Non-Controlling Interest in Subsidiaries	0.5	0.2
Net Income Attributable to ALLETE	$ 45.9	$ 39.9
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (in shares)	49.2	46.9
Diluted (in shares)	49.2	47.1
Basic Earnings Per Share of Common Stock	$ 0.93	$ 0.85
Diluted Earnings Per Share of Common Stock	0.93	0.85
Dividends Per Share of Common Stock	$ 0.52	$ 0.505