Consilidated Statement of Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Statement [Abstract]
Operating Revenue	$ 1,486.4	$ 1,136.8	$ 1,018.4
Operating Expenses [Abstract]
Fuel and Purchased Power	328.1	356.1	334.8
Transmission Services	54.1	45.6	32.3
Cost of Sales	302.3	77.9	71.2
Operating and Maintenance	333.5	287.1	267.7
Depreciation and Amortization	170.0	135.7	116.6
Taxes Other Than Income Taxes	51.4	45.6	41.7
Impairment of Real Estate	36.3	0.0	0.0
Total Operating Expenses	1,275.7	948.0	864.3
Operating Income	210.7	188.8	154.1
Other Income (Expense) [Abstract]
Interest Expense	(64.9)	(54.8)	(50.3)
Equity Earnings in ATC	16.3	19.6	20.3
Other	4.7	8.6	9.3
Total Other Expense	(43.9)	(26.6)	(20.7)
Income Before Non-Controlling Interest and Income Taxes	166.8	162.2	133.4
Income Tax Expense	25.3	36.7	28.7
Net Income	141.5	125.5	104.7
Less: Non-Controlling Interest in Subsidiaries	0.4	0.7	0.0
Net Income Attributable to ALLETE	$ 141.1	$ 124.8	$ 104.7
Average Shares of Common Stock and Per Share Data [Abstract]
Basic (Shares)	48.3	42.9	39.7
Diluted (Shares)	48.4	43.1	39.8
Basic Earnings Per Share of Common Stock	$ 2.92	$ 2.91	$ 2.64
Diluted Earnings Per Share of Common Stock	2.92	2.90	2.63
Dividends Per Share of Common Stock	$ 2.02	$ 1.96	$ 1.90