Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 3,275,277	$ 3,042,618	$ 2,975,990
Securities available-for-sale, Gross Unrealized Holding Gains	36,161	57,358	56,649
Securities available-for-sale, Gross Unrealized Holding Losses	(35,245)	(12,503)	(14,360)
Securities available-for-sale, Estimated Fair Value	3,276,193	3,087,473	3,018,279
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total			10,614
Securities available-for-sale, Gross Unrealized Holding Gains			5
Securities available-for-sale, Estimated Fair Value			10,619
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	50,467	60,516	79,777
Securities available-for-sale, Gross Unrealized Holding Gains			130
Securities available-for-sale, Gross Unrealized Holding Losses	(252)	(186)	(2)
Securities available-for-sale, Estimated Fair Value	50,215	60,330	79,905
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,289,886	1,369,295	1,524,010
Securities available-for-sale, Gross Unrealized Holding Gains	33,014	52,491	47,902
Securities available-for-sale, Gross Unrealized Holding Losses	(2,767)	(936)	(7,399)
Securities available-for-sale, Estimated Fair Value	1,320,133	1,420,850	1,564,513
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	9,382	11,421	47,601
Securities available-for-sale, Gross Unrealized Holding Gains		43	333
Securities available-for-sale, Gross Unrealized Holding Losses	(55)	(5)	(3)
Securities available-for-sale, Estimated Fair Value	9,327	11,459	47,931
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,449,482	1,223,452	989,272
Securities available-for-sale, Gross Unrealized Holding Gains	2,912	4,561	7,718
Securities available-for-sale, Gross Unrealized Holding Losses	(24,622)	(8,916)	(5,437)
Securities available-for-sale, Estimated Fair Value	1,427,772	1,219,097	991,553
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	476,060	377,934	324,716
Securities available-for-sale, Gross Unrealized Holding Gains	235	263	561
Securities available-for-sale, Gross Unrealized Holding Losses	(7,549)	(2,460)	(1,519)
Securities available-for-sale, Estimated Fair Value	$ 468,746	$ 375,737	$ 323,758