Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Comprehensive Income [Abstract]
Net Income	$ 161.6	$ 185.5	$ 174.1
Other Comprehensive Income (Loss) [Abstract]
Unrealized Gain (Loss) on Securities - Net of Income Tax Expense (Benefit) of $0.1, $0.1 and $-	0.1	0.2	(0.1)
Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Tax Expense (Benefit) of $(3.1), $1.4 and $0.3	(7.6)	3.5	1.0
Total Other Comprehensive Income (Loss)	(7.5)	3.7	0.9
Total Comprehensive Income	154.1	189.2	175.0
Net Loss Attributable to Non-Controlling Interest	(12.6)	(0.1)	0.0
Total Comprehensive Income Attributable to ALLETE	$ 166.7	$ 189.3	$ 175.0