Income Taxes - Summary of Differences in Taxes from Continuing Operations Computed at Statutory Rate (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Provision at statutory rate of 35%	$ 17,791	$ 15,987	$ 51,363	$ 53,291
Tax-exempt income	(4,290)	(4,297)	(13,036)	(12,698)
State income taxes	506	749	1,553	3,396
Interim period effective rate adjustment	(861)	(1,418)	(1,602)	(1,603)
Tax credit investments-federal	(1,684)	(80)	(5,431)	(240)
ONI nondeductible goodwill		23		8,328
Other, net	(3)	5	(313)	978
Income tax expense	$ 11,459	$ 10,969	$ 32,534	$ 51,452
Effective tax rate	22.50%	24.00%	22.20%	33.80%