Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Comprehensive Income [Abstract]
Net Income	$ 36.9	$ 24.8	$ 85.9	$ 71.2
Other Comprehensive Income (Loss) [Abstract]
Currency Translation Adjustments	(0.2)	0.0	(0.2)	0.0
Unrealized Gain on Securities Net of Income Tax Expense of $0.2, $0.3, $0.5 and $-	0.4	0.4	0.7	0.0
Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $0.2, $0.1, $0.3, and $0.2	0.2	0.1	0.4	0.3
Total Other Comprehensive Income	0.4	0.5	0.9	0.3
Total Comprehensive Income	37.3	25.3	86.8	71.5
Less: Non-Controlling Interest in Subsidiaries	0.0	0.0	0.0	0.5
Total Comprehensive Income Attributable to ALLETE	$ 37.3	$ 25.3	$ 86.8	$ 71.0