Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Comprehensive Income [Abstract]
Net Income	$ 40.3	$ 60.3	$ 111.5	$ 122.7
Other Comprehensive Income [Abstract]
Unrealized Gain (Loss) on Securities Net of Income Tax Expense (Benefit) of $0.2, $(0.4), $0.2, and $(0.3)	0.3	(0.7)	0.3	(0.6)
Unrealized Gain on Derivatives Net of Income Tax Expense of $–, $–, $–, and $0.1	0.0	0.0	0.0	0.1
Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $0.1, $0.3, $0.3, and $0.7	0.2	0.3	0.5	1.0
Total Other Comprehensive Income (Loss)	0.5	(0.4)	0.8	0.5
Total Comprehensive Income	40.8	59.9	112.3	123.2
Less: Non-Controlling Interest in Subsidiaries	0.0	(0.1)	0.5	(0.1)
Total Comprehensive Income Attributable to ALLETE	$ 40.8	$ 60.0	$ 111.8	$ 123.3