Income Taxes - Summary of Income Tax Reconciliation (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
Amount
Expected income tax expense									$ 137.0	$ 121.0	$ 163.5
State income tax, net of federal tax effect									22.6	23.2	12.2
Non-deductible FDIC insurance premiums									4.4	6.5	0.0
Tax-exempt interest									(21.4)	(19.4)	(26.6)
Federal income tax credits									(12.7)	(12.3)	(11.6)
Tax-exempt income from BOLI									(2.2)	(1.5)	(2.2)
The Tax Cuts and Jobs Act									0.0	(9.2)	(6.5)
Equity-based compensation									(0.1)	(1.3)	(1.1)
Other, net									4.4	1.2	2.2
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
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Expected income tax expense									21.00%	21.00%	35.00%
State income tax, net of federal tax effect									3.40%	4.10%	2.60%
Non-deductible FDIC insurance premiums									0.70%	1.10%	0.00%
Tax-exempt interest									(3.30%)	(3.40%)	(5.70%)
Federal income tax credits									(2.00%)	(2.10%)	(2.50%)
Tax-exempt income from BOLI									(0.30%)	(0.30%)	(0.50%)
The Tax Cuts and Jobs Act									0.00%	(1.60%)	(1.40%)
Equity-based compensation									0.00%	(0.20%)	(0.20%)
Other, net									0.70%	0.20%	0.50%
Effective income tax rate									20.20%	18.80%	27.80%