Income Taxes - Income Before Income Tax (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Domestic	$ 607.1	$ 635.5	$ 626.4
Foreign	53.2	36.9	20.6
Income before income taxes	$ 660.3	$ 672.4	$ 647.0