Income Taxes - Summary of Income Tax Reconciliation (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Federal income tax at expected statutory rate									$ 88,458	$ 81,683	$ 44,413
State income tax expense, net of federal tax effect									21,046	19,197	15,274
Tax-exempt interest, net of interest incurred to carry tax-exempt securities									(5,245)	(5,308)	(4,864)
Bank owned life insurance									(2,568)	(2,343)	(2,385)
Tax credits from securities and other investments									(27,037)	(25,954)	(28,988)
Impact of the Tax Act									15,441	0	0
Other, net									736	(2,041)	488
Income tax expense	$ 34,958	$ 17,088	$ 20,714	$ 18,071	$ 18,336	$ 17,049	$ 15,460	$ 14,389	$ 90,831	$ 65,234	$ 23,938