Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 2,825,560	$ 2,827,772	$ 2,623,934
Securities available-for-sale, Gross Unrealized Holding Gains	66,235	49,824	107,200
Securities available-for-sale, Gross Unrealized Holding Losses	(6,312)	(16,759)	(2,104)
Securities available-for-sale, Estimated Fair Value	2,885,483	2,860,837	2,729,030
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total		10,649	10,685
Securities available-for-sale, Gross Unrealized Holding Gains		19	54
Securities available-for-sale, Estimated Fair Value		10,668	10,739
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	73,583	113,450	114,918
Securities available-for-sale, Gross Unrealized Holding Gains	35	253	802
Securities available-for-sale, Gross Unrealized Holding Losses	(30)
Securities available-for-sale, Estimated Fair Value	73,588	113,703	115,720
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,379,117	1,534,095	1,419,737
Securities available-for-sale, Gross Unrealized Holding Gains	56,671	40,194	83,694
Securities available-for-sale, Gross Unrealized Holding Losses	(1,594)	(10,013)	(715)
Securities available-for-sale, Estimated Fair Value	1,434,194	1,564,276	1,502,716
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	19,439	51,920	68,285
Securities available-for-sale, Gross Unrealized Holding Gains	118	476	1,325
Securities available-for-sale, Gross Unrealized Holding Losses	(2)	(3)	(1)
Securities available-for-sale, Estimated Fair Value	19,555	52,393	69,609
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,024,615	848,614	750,673
Securities available-for-sale, Gross Unrealized Holding Gains	8,466	8,260	17,125
Securities available-for-sale, Gross Unrealized Holding Losses	(3,797)	(5,513)	(1,299)
Securities available-for-sale, Estimated Fair Value	1,029,284	851,361	766,499
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	328,806	269,044	259,636
Securities available-for-sale, Gross Unrealized Holding Gains	945	622	4,200
Securities available-for-sale, Gross Unrealized Holding Losses	(889)	(1,230)	(89)
Securities available-for-sale, Estimated Fair Value	$ 328,862	$ 268,436	$ 263,747