Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Comprehensive Income [Abstract]
Net Income	$ 31.2	$ 30.7	$ 135.9	$ 113.0
Other Comprehensive Income (Loss) [Abstract]
Unrealized Gain (Loss) on Securities Net of Income Tax Expense of $–, $–, $0.1 and $–	0.0	0.0	0.2	(0.1)
Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $0.1, $0.1, $0.2 and $0.3	0.1	0.3	0.3	1.0
Total Other Comprehensive Income	0.1	0.3	0.5	0.9
Total Comprehensive Income	$ 31.3	$ 31.0	$ 136.4	$ 113.9