Changes in Equity and Comprehensive Income Changes in Equity and Comprehensive Income (Details of Accumulated Other Comprehensive Income Components) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Deferred Tax Assets, Other Comprehensive Loss	$ 114.4		$ 114.4		$ 115.8
Income Tax Expense (Benefit)	24.2	$ 30.4	39.9	$ 55.1
Amounts reclassified from accumulated other comprehensive income			4.8
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	3.2	3.2	6.2	6.5
Income Tax Expense (Benefit)	0.8	1.0	1.4	2.0
Amounts reclassified from accumulated other comprehensive income	$ 2.4	$ 2.2	$ 4.8	$ 4.5