Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 2,671,037	$ 2,654,885	$ 2,669,891
Securities available-for-sale, Gross Unrealized Holding Gains	124,967	84,855	68,137
Securities available-for-sale, Gross Unrealized Holding Losses	(648)	(5,841)	(8,915)
Securities available-for-sale, Estimated Fair Value	2,795,356	2,733,899	2,729,113
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	10,721	10,792	10,864
Securities available-for-sale, Gross Unrealized Holding Gains	90	5	72
Securities available-for-sale, Gross Unrealized Holding Losses		(2)
Securities available-for-sale, Estimated Fair Value	10,811	10,795	10,936
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	121,174	148,393	164,351
Securities available-for-sale, Gross Unrealized Holding Gains	1,178	268	985
Securities available-for-sale, Gross Unrealized Holding Losses		(107)
Securities available-for-sale, Estimated Fair Value	122,352	148,554	165,336
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,418,342	1,379,879	1,340,163
Securities available-for-sale, Gross Unrealized Holding Gains	97,980	71,382	49,483
Securities available-for-sale, Gross Unrealized Holding Losses	(10)	(134)	(5,400)
Securities available-for-sale, Estimated Fair Value	1,516,312	1,451,127	1,384,246
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	71,687	86,182	94,485
Securities available-for-sale, Gross Unrealized Holding Gains	1,723	1,778	2,623
Securities available-for-sale, Gross Unrealized Holding Losses		(5)
Securities available-for-sale, Estimated Fair Value	73,410	87,955	97,108
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	787,451	781,648	856,546
Securities available-for-sale, Gross Unrealized Holding Gains	18,928	10,993	14,383
Securities available-for-sale, Gross Unrealized Holding Losses	(606)	(3,759)	(2,838)
Securities available-for-sale, Estimated Fair Value	805,773	788,882	868,091
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	261,662	247,991	203,482
Securities available-for-sale, Gross Unrealized Holding Gains	5,068	429	591
Securities available-for-sale, Gross Unrealized Holding Losses	(32)	(1,834)	(677)
Securities available-for-sale, Estimated Fair Value	$ 266,698	$ 246,586	$ 203,396