Income Taxes - Effective Tax Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Statutory federal income tax rate, amount	$ 176.5	$ 231.1	$ 235.5
State taxes, net of federal effect, amount	31.1	18.3	17.7
Equity based compensation, amount	(19.7)	(36.2)	(1.6)
Effect of rates different than statutory, amount	0.6	(6.3)	(4.6)
Foreign withholding tax, amount	2.8	1.0	0.8
Effect of U.K. tax rate change on deferred taxes, amount	0.0	0.0	(1.5)
Effect of US Tax Cuts and Jobs Act on Deferred Taxes, amount	(1.9)	(75.5)	0.0
Other, amount	8.1	5.2	1.8
Income tax expense	$ 197.5	$ 137.6	$ 248.1
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory federal income tax rate, percent	21.00%	35.00%	35.00%
State taxes, net of federal effect, percent	3.70%	2.80%	2.60%
Equity based compensation, percent	(2.30%)	(5.50%)	(0.20%)
Effect of rates different than statutory, percent	0.10%	(1.00%)	(0.70%)
Foreign withholding tax, percent	0.30%	0.10%	0.10%
Effect of U.K. tax rate change on deferred taxes, percent	0.00%	0.00%	(0.20%)
Effect of US Tax Cuts and Jobs Act on Deferred Taxes, percent	(0.20%)	(11.40%)	0.00%
Other, percent	0.90%	0.80%	0.30%
Income tax expense, rate	23.50%	20.80%	36.90%