Income Taxes - Income Tax Effects Related to Items Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal									$ 90.8	$ 83.0	$ 88.9
State									26.9	23.3	15.2
Total current tax expense									117.7	106.3	104.1
Deferred tax expense									14.3	1.9	25.8
Total income tax expense	$ 36.4	$ 30.4	$ 35.2	$ 30.0	$ 15.8	$ 32.2	$ 30.8	$ 29.4	132.0	108.2	129.9
Increase (decrease) in valuation allowance for state deferred tax assets									$ (7.7)	$ 0.3	$ (0.6)