Components of Income Before Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Domestic	$ 683.2	$ 665.0	$ 595.6
Foreign	9.5	40.9	39.8
Income before Income Taxes	$ 692.7	$ 705.9	$ 635.4