Income Taxes - Income Tax Expense by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ 258.9	$ 295.7	$ 258.5
State	29.8	34.9	28.6
Foreign	21.3	16.8	10.1
Total current	310.0	347.4	297.2
Deferred:
Domestic	(168.0)	(90.5)	(48.5)
Foreign	(4.7)	(8.9)	(4.8)
Total deferred	(172.7)	(99.4)	(53.3)
Income tax expense	$ 137.3	$ 248.0	$ 243.9