Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Comprehensive Income [Abstract]
Net Income	$ 64.5	$ 70.5
Other Comprehensive Income (Loss) [Abstract]
Unrealized Gain (Loss) on Securities Net of Income Tax Expense of $(0.1) and $–	(0.4)	0.1
Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $0.1 and $0.1	0.2	0.0
Total Other Comprehensive Income (Loss)	(0.2)	0.1
Total Comprehensive Income	64.3	70.6
Net Loss Attributable to Non-Controlling Interest	(1.8)	0.0
Total Comprehensive Income Attributable to ALLETE	$ 66.1	$ 70.6