Consolidated Statement of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Comprehensive Income [Abstract]
Net Income	$ 34.2	$ 31.3	$ 104.7	$ 82.3
Other Comprehensive Income (Loss) [Abstract]
Unrealized Gain (Loss) on Securities Net of Income Tax Expense of $0.1, $–, $0.1 and $–	0.1	0.0	0.2	(0.1)
Defined Benefit Pension and Other Postretirement Benefit Plans Net of Income Tax Expense of $–, 0.1, $0.1 and $0.2	0.2	0.3	0.2	0.7
Total Other Comprehensive Income	0.3	0.3	0.4	0.6
Total Comprehensive Income	$ 34.5	$ 31.6	$ 105.1	$ 82.9