Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Comprehensive Income [Abstract]
Net Income	$ 131.3	$ 137.8	$ 152.8
Other Comprehensive Income (Loss) [Abstract]
Unrealized Gain (Loss) on Securities - Net of Income Tax Expense (Benefit) of $(0.2), $(0.1) and $0.1	(0.4)	(0.1)	0.1
Defined Benefit Pension and Other Postretirement Benefit Plans - Net of Income Tax Expense (Benefit) of $(0.1), $3.0 and $(3.1)	(0.2)	7.4	(7.6)
Total Other Comprehensive Income (Loss)	(0.6)	7.3	(7.5)
Total Comprehensive Income	130.7	145.1	145.3
Net Loss Attributable to Non-Controlling Interest	(58.0)	(31.4)	(12.6)
Total Comprehensive Income Attributable to ALLETE	$ 188.7	$ 176.5	$ 157.9