Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Operating Revenue [Abstract]
Utility	$ 264.9	$ 234.9	$ 546.5	$ 487.2
Non-utility	88.4	79.9	172.4	161.4
Total Operating Revenue	353.3	314.8	718.9	648.6
Operating Expenses [Abstract]
Fuel, Purchased Power and Gas – Utility	93.1	79.0	189.7	158.9
Transmission Services – Utility	17.6	16.1	34.2	32.9
Cost of Sales – Non-utility	38.4	32.5	70.0	62.8
Operating and Maintenance	84.9	82.0	168.2	160.1
Depreciation and Amortization	50.1	48.7	100.6	96.8
Taxes Other than Income Taxes	14.2	14.3	28.6	28.1
Total Operating Expenses	298.3	272.6	591.3	539.6
Operating Income	55.0	42.2	127.6	109.0
Other Income (Expense) [Abstract]
Interest Expense	(16.7)	(17.4)	(33.9)	(34.3)
Equity Earnings in ATC	5.3	4.1	11.4	8.9
Other	0.6	0.6	1.2	1.6
Total Other Expense	(10.8)	(12.7)	(21.3)	(23.8)
Income Before Non-Controlling Interest and Income Taxes	44.2	29.5	106.3	85.2
Income Tax Expense	7.3	4.7	20.4	14.0
Net Income	36.9	24.8	85.9	71.2
Less: Non-Controlling Interest in Subsidiaries	0.0	0.0	0.0	0.5
Net Income Attributable to ALLETE	$ 36.9	$ 24.8	$ 85.9	$ 70.7
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (Shares)	50.9	49.3	50.5	49.2
Diluted (Shares)	51.1	49.5	50.7	49.3
Basic Earnings Per Share of Common Stock	$ 0.73	$ 0.50	$ 1.70	$ 1.44
Diluted Earnings Per Share of Common Stock	0.72	0.50	1.69	1.43
Dividends Per Share of Common Stock	$ 0.535	$ 0.52	$ 1.07	$ 1.04