Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Comprehensive Income [Abstract]
Net Income	$ 155.8	$ 141.5	$ 125.5
Other Comprehensive Income (Loss) [Abstract]
Unrealized Loss on Securities - Net of Income Tax Benefit of $(0.2), $(0.3) and $(0.2)	(0.2)	(0.5)	(0.2)
Unrealized Gain on Derivatives - Net of Income Tax Expense of $–, $0.1 and $0.1	0.0	0.1	0.2
Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Tax Benefit of $(2.4), $(2.2) and $(2.8)	(3.5)	(3.0)	(4.0)
Total Other Comprehensive Loss	(3.7)	(3.4)	(4.0)
Total Comprehensive Income	152.1	138.1	121.5
Less: Non-Controlling Interest in Subsidiaries	0.5	0.4	0.7
Total Comprehensive Income Attributable to ALLETE	$ 151.6	$ 137.7	$ 120.8