Income Taxes - Effective Income Tax Rate Reconciliation, Amount (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Statutory U.S. federal income tax	$ 37.9	$ 40.3	$ 39.6
State income taxes, net of federal benefit	9.1	10.9	12.1
Reserve adjustments	1.2	3.9	2.7
Tax credits and special deductions	(4.6)	(2.7)	(3.7)
Nondeductible compensation	3.4	1.0	0.8
Stock-based compensation excess tax benefits	(4.4)	(2.3)	(0.4)
Other	(0.1)	(0.2)	0.2
Income tax provision	$ 42.5	$ 50.9	$ 51.3