Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Comprehensive Income [Abstract]
Net Income	$ 185.5	$ 174.1	$ 172.2
Other Comprehensive Income (Loss) [Abstract]
Unrealized Gain (Loss) on Securities - Net of Income Tax Expense (Benefit) of $0.1, $- and $0.7	0.2	(0.1)	0.9
Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Tax Expense (Benefit) of $1.4, $0.3 and $2.2	3.5	1.0	4.7
Total Other Comprehensive Income (Loss)	3.7	0.9	5.6
Total Comprehensive Income	189.2	175.0	177.8
Less: Non-Controlling Interest in Subsidiaries	(0.1)	0.0	0.0
Total Comprehensive Income Attributable to ALLETE	$ 189.3	$ 175.0	$ 177.8