Income Taxes - Components of Income Tax Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Federal income taxes:
Current	$ 91.9	$ 83.9	$ 15.3
Deferred	45.4	31.1	69.6
Total federal income taxes	137.3	115.0	84.9
State income taxes:
Current	21.0	20.5	6.0
Deferred	8.7	(1.5)	14.1
Total state income taxes	29.7	19.0	20.1
Foreign income taxes:
Current	1.2	2.8	(1.4)
Deferred		(0.5)	2.1
Total foreign income taxes	1.2	2.3	0.7
Income tax expense	$ 168.2	$ 136.3	$ 105.7