Consolidated Statement of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Consolidated Statement of Income [Abstract]
Operating Revenue	$ 323.3	$ 260.7	$ 643.3	$ 557.2
Operating Expenses [Abstract]
Fuel and Purchased Power	80.1	83.6	166.1	179.8
Transmission Services	11.3	10.5	26.2	21.3
Cost of Sales	52.3	18.9	83.5	42.4
Operating and Maintenance	85.4	74.3	165.1	148.6
Depreciation and Amortization	41.3	33.9	80.3	66.1
Taxes Other than Income Taxes	13.4	11.3	26.2	22.5
Total Operating Expenses	283.8	232.5	547.4	480.7
Operating Income	39.5	28.2	95.9	76.5
Other Income (Expense) [Abstract]
Interest Expense	(16.2)	(13.5)	(31.3)	(26.3)
Equity Earnings in ATC	4.7	5.2	8.6	10.3
Other	0.7	1.9	1.8	3.9
Total Other Expense	(10.8)	(6.4)	(20.9)	(12.1)
Income Before Non-Controlling Interest and Income Taxes	28.7	21.8	75.0	64.4
Income Tax Expense	6.4	4.9	12.6	13.7
Net Income	22.3	16.9	62.4	50.7
Less: Non-Controlling Interest in Subsidiaries	(0.2)	0.1	0.0	0.4
Net Income Attributable to ALLETE	$ 22.5	$ 16.8	$ 62.4	$ 50.3
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (in shares)	48.6	42.1	47.7	41.7
Diluted (in shares)	48.7	42.3	47.8	41.9
Basic Earnings Per Share of Common Stock	$ 0.46	$ 0.40	$ 1.31	$ 1.21
Diluted Earnings Per Share of Common Stock	0.46	0.40	1.30	1.20
Dividends Per Share of Common Stock	$ 0.505	$ 0.49	$ 1.01	$ 0.98