Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 3,176,244	$ 3,042,618	$ 2,825,560
Securities available-for-sale, Gross Unrealized Holding Gains	22,584	57,358	66,235
Securities available-for-sale, Gross Unrealized Holding Losses	(54,461)	(12,503)	(6,312)
Securities available-for-sale, Estimated Fair Value	3,144,367	3,087,473	2,885,483
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	9,964
Securities available-for-sale, Gross Unrealized Holding Losses	(35)
Securities available-for-sale, Estimated Fair Value	9,929
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	303	60,516	73,583
Securities available-for-sale, Gross Unrealized Holding Gains			35
Securities available-for-sale, Gross Unrealized Holding Losses	(1)	(186)	(30)
Securities available-for-sale, Estimated Fair Value	302	60,330	73,588
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,174,320	1,369,295	1,379,117
Securities available-for-sale, Gross Unrealized Holding Gains	21,664	52,491	56,671
Securities available-for-sale, Gross Unrealized Holding Losses	(4,097)	(936)	(1,594)
Securities available-for-sale, Estimated Fair Value	1,191,887	1,420,850	1,434,194
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	4,860	11,421	19,439
Securities available-for-sale, Gross Unrealized Holding Gains		43	118
Securities available-for-sale, Gross Unrealized Holding Losses	(121)	(5)	(2)
Securities available-for-sale, Estimated Fair Value	4,739	11,459	19,555
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,533,302	1,223,452	1,024,615
Securities available-for-sale, Gross Unrealized Holding Gains	912	4,561	8,466
Securities available-for-sale, Gross Unrealized Holding Losses	(39,908)	(8,916)	(3,797)
Securities available-for-sale, Estimated Fair Value	1,494,306	1,219,097	1,029,284
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	453,495	377,934	328,806
Securities available-for-sale, Gross Unrealized Holding Gains	8	263	945
Securities available-for-sale, Gross Unrealized Holding Losses	(10,299)	(2,460)	(889)
Securities available-for-sale, Estimated Fair Value	$ 443,204	$ 375,737	$ 328,862