Income Taxes - Tax Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal tax provision at statutory rate	$ 136.2	$ 78.8	$ 70.9
State taxes, net of federal income tax benefit	40.4	23.6	16.1
Non-deductible items	2.8	2.6	1.5
Permanent differences related to stock compensation	(0.5)	0.2	(0.1)
Net change in valuation allowance	0.5	(0.5)	0.5
General business credits	(1.3)	(0.9)	(1.1)
Deferred remeasurement for change in statutory tax rate	0.0	0.0	(15.8)
Other	0.1	0.1	(0.2)
Total	$ 178.2	$ 103.9	$ 71.8