Income Taxes - Components of Income Tax Expense (Benefit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Federal income taxes:
Current	$ 83.9	$ 15.3	$ 129.2
Deferred	31.1	69.6	(239.3)
Total federal income taxes	115.0	84.9	(110.1)
State income taxes:
Current	20.5	6.0	14.8
Deferred	(1.5)	14.1	(0.9)
Total state income taxes	19.0	20.1	13.9
Foreign income taxes:
Current	2.8	(1.4)	1.2
Deferred	(0.5)	2.1	0.5
Total foreign income taxes	2.3	0.7	1.7
Income tax expense (benefit)	$ 136.3	$ 105.7	$ (94.5)