Income Taxes (Schedule Of Income Tax Provision) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income before provision for income taxes	$ 122.4	$ 160.1	$ 157.1	$ 144.2	$ 141.6	$ 136.5	$ 134.7	$ 118.4	$ 583.8	$ 531.2	$ 474.5
Current provision for income taxes, Federal									184.6	157.2	127.9
Current provision for income taxes, State									13.5	15.8	10.6
Current provision for income taxes									198.1	173.0	138.5
Deferred provision for income taxes, Federal									(88.4)	22.4	33.7
Deferred provision for income taxes, State									2.7	1.8	1.9
Deferred provision for income taxes									(85.7)	24.2	35.6
Interest									1.2	1.2	0.7
Unrecognized Tax Benefits, Income Tax Penalties Expense									0.0	0.0	0.0
Interest and penalties expense (benefit)									1.2	1.2	0.7
Provision for income taxes									$ 113.6	$ 198.4	$ 174.8