Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	$ 2,657,185	$ 2,654,885	$ 2,598,678
Securities available-for-sale, Gross Unrealized Holding Gains	106,916	84,855	92,762
Securities available-for-sale, Gross Unrealized Holding Losses	(1,062)	(5,841)	(2,110)
Securities available-for-sale, Estimated Fair Value	2,763,039	2,733,899	2,689,330
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	10,756	10,792	10,899
Securities available-for-sale, Gross Unrealized Holding Gains	73	5	84
Securities available-for-sale, Gross Unrealized Holding Losses		(2)
Securities available-for-sale, Estimated Fair Value	10,829	10,795	10,983
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	131,484	148,393	173,777
Securities available-for-sale, Gross Unrealized Holding Gains	1,061	268	1,340
Securities available-for-sale, Gross Unrealized Holding Losses		(107)
Securities available-for-sale, Estimated Fair Value	132,545	148,554	175,117
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	1,388,369	1,379,879	1,238,354
Securities available-for-sale, Gross Unrealized Holding Gains	82,693	71,382	65,922
Securities available-for-sale, Gross Unrealized Holding Losses	(28)	(134)	(784)
Securities available-for-sale, Estimated Fair Value	1,471,034	1,451,127	1,303,492
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	68,836	86,182	95,044
Securities available-for-sale, Gross Unrealized Holding Gains	1,896	1,778	3,089
Securities available-for-sale, Gross Unrealized Holding Losses		(5)
Securities available-for-sale, Estimated Fair Value	70,732	87,955	98,133
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	793,509	781,648	900,831
Securities available-for-sale, Gross Unrealized Holding Gains	17,180	10,993	20,752
Securities available-for-sale, Gross Unrealized Holding Losses	(963)	(3,759)	(1,217)
Securities available-for-sale, Estimated Fair Value	809,726	788,882	920,366
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	264,231	247,991	179,773
Securities available-for-sale, Gross Unrealized Holding Gains	4,013	429	1,575
Securities available-for-sale, Gross Unrealized Holding Losses	(71)	(1,834)	(109)
Securities available-for-sale, Estimated Fair Value	$ 268,173	$ 246,586	$ 181,239