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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Deferred Tax Assets, Other Comprehensive Loss	$ 113.8		$ 113.8		$ 115.8
Income Tax Expense (Benefit)	20.7	$ 28.5	60.6	$ 83.6
Amounts reclassified from accumulated other comprehensive income			7.4
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	3.3	9.4	9.8
Income Tax Expense (Benefit)	0.6	1.0	2.0	3.0
Amounts reclassified from accumulated other comprehensive income	$ 2.6	$ 2.3	$ 7.4	$ 6.8