Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Comprehensive Income [Abstract]
Net Income	$ 24.1	$ 38.8	$ 89.2	$ 120.2
Other Comprehensive Income (Loss) [Abstract]
Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Tax Expense of $0.1, $0.1, $0.4 and $0.2	0.4	0.2	1.2	0.5
Total Other Comprehensive Income	0.4	0.2	1.2	0.5
Total Comprehensive Income	24.5	39.0	90.4	120.7
Net Loss Attributable to Non-Controlling Interest	(3.5)	(1.9)	(18.1)	(6.9)
Total Comprehensive Income Attributable to ALLETE	$ 28.0	$ 40.9	$ 108.5	$ 127.6