Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Comprehensive Income [Abstract]
Net Income	$ 44.9	$ 40.3	$ 130.8	$ 111.5
Other Comprehensive Income (Loss) [Abstract]
Currency Translation Adjustments	0.1	0.0	(0.1)	0.0
Unrealized Gain on Securities Net of Income Tax Expense of $0.1, $0.2, $0.6 and $0.2	0.1	0.3	0.8	0.3
Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $0.1, $0.1, $0.4, and $0.3	0.2	0.2	0.6	0.5
Total Other Comprehensive Income	0.4	0.5	1.3	0.8
Total Comprehensive Income	45.3	40.8	132.1	112.3
Less: Non-Controlling Interest in Subsidiaries	0.0	0.0	0.0	0.5
Total Comprehensive Income Attributable to ALLETE	$ 45.3	$ 40.8	$ 132.1	$ 111.8