Reconciliation of Income Tax Provision (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income tax provision at statutory rate	$ 111.2	$ 222.8	$ 245.8
Non-deductible expenses, net	4.9	5.9	4.6
State income taxes, net of federal benefit	22.8	19.7	21.7
Income tax provision, excluding other reconciling items	138.9	248.4	272.1
Change in tax rate	(5.0)	(44.2)	0.0
Change in valuation allowance, net	3.5	3.3	0.3
Adjustments and settlements	(0.7)	0.3	0.3
Federal and state tax credits	(1.0)	(3.7)	(1.9)
Other, net	(2.2)	(2.6)	(0.2)
Income tax provision	$ 133.5	$ 201.5	$ 270.6
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Income tax provision at statutory rate, %	21.00%	35.00%	35.00%
Non-deductible expenses, net, %	0.90%	0.90%	0.70%
State income taxes, net of federal benefit, %	4.30%	3.10%	3.10%
Income tax provision, excluding other reconciling items, %	26.20%	39.00%	38.80%
Change in tax rate, %	(0.90%)	(6.90%)	0.00%
Change in valuation allowance, net, %	0.70%	0.50%	0.00%
Adjustments and settlements, %	(0.10%)	0.10%	0.00%
Federal and state tax credits, %	(0.20%)	(0.60%)	(0.30%)
Other, net, %	(0.50%)	(0.40%)	0.00%
Income tax provision, %	25.20%	31.70%	38.50%