Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Comprehensive Income [Abstract]
Net Income	$ 21.7	$ 16.9	$ 65.1	$ 81.4
Other Comprehensive Income (Loss) [Abstract]
Unrealized Gain on Securities Net of Income Tax Benefit of $–, $0.1, $– and $–	0.0	0.4	0.0	0.0
Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $0.2, $–, $0.3 and $0.1	0.4	0.1	0.8	0.3
Total Other Comprehensive Income	0.4	0.5	0.8	0.3
Total Comprehensive Income	22.1	17.4	65.9	81.7
Net Loss Attributable to Non-Controlling Interest	(6.2)	(3.2)	(14.6)	(5.0)
Total Comprehensive Income Attributable to ALLETE	$ 28.3	$ 20.6	$ 80.5	$ 86.7