Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Comprehensive Income [Abstract]
Net Income	$ 38.8	$ 31.2	$ 120.2	$ 135.9
Other Comprehensive Income (Loss) [Abstract]
Unrealized Gain on Securities Net of Income Tax Expense of $–, $–, $– and $0.1	0.0	0.0	0.0	0.2
Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $0.1, $0.1, $0.2 and $0.2	0.2	0.1	0.5	0.3
Total Other Comprehensive Income	0.2	0.1	0.5	0.5
Total Comprehensive Income	39.0	31.3	120.7	136.4
Net Loss Attributable to Non-Controlling Interest	(1.9)	0.0	(6.9)	0.0
Total Comprehensive Income Attributable to ALLETE	$ 40.9	$ 31.3	$ 127.6	$ 136.4