Income Taxes - Components of Consolidated Income Tax Provision (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income tax provision (benefit) -
U.S. federal	$ 209.3	$ 213.6	$ 205.1
State and local	34.9	29.1	20.5
Foreign	0.3	0.2	0.4
Total current provision for taxes	244.5	242.9	226.0
Deferred -
U.S. federal	(90.2)	(1.2)	(3.8)
State and local	5.7	(3.0)	5.6
Foreign		0.2	(0.1)
Total deferred provision (benefit) for taxes	(84.5)	(4.0)	1.7
Total provision (benefit) for taxes	$ 160.0	$ 238.9	$ 227.7