Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Comprehensive Income [Abstract]
Net Income	$ 16.9	$ 34.2	$ 81.4	$ 104.7
Other Comprehensive Income (Loss) [Abstract]
Unrealized Gain on Securities Net of Income Tax Expense of $0.1, $0.1, $– and $0.1	0.4	0.1	0.0	0.2
Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $–, $–, $0.1 and $0.1	0.1	0.2	0.3	0.2
Total Other Comprehensive Income	0.5	0.3	0.3	0.4
Total Comprehensive Income	17.4	34.5	81.7	105.1
Net Loss Attributable to Non-Controlling Interest	(3.2)	0.0	(5.0)	0.0
Total Comprehensive Income Attributable to ALLETE	$ 20.6	$ 34.5	$ 86.7	$ 105.1