Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Comprehensive Income [Abstract]
Net Income	$ 174.1	$ 172.2	$ 155.8
Other Comprehensive Income (Loss) [Abstract]
Unrealized Gain (Loss) on Securities - Net of Income Tax Expense (Benefit) of $-, $0.7 and $(0.2)	(0.1)	0.9	(0.2)
Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Tax Expense (Benefit) of $0.3, $2.2 and $(2.4)	1.0	4.7	(3.5)
Total Other Comprehensive Income (Loss)	0.9	5.6	(3.7)
Total Comprehensive Income	175.0	177.8	152.1
Less: Non-Controlling Interest in Subsidiaries	0.0	0.0	0.5
Total Comprehensive Income Attributable to ALLETE	$ 175.0	$ 177.8	$ 151.6