Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 3,208,412	$ 3,042,618	$ 2,905,245
Securities available-for-sale, Gross Unrealized Holding Gains	31,562	57,358	68,215
Securities available-for-sale, Gross Unrealized Holding Losses	(42,407)	(12,503)	(8,947)
Securities available-for-sale, Estimated Fair Value	3,197,567	3,087,473	2,964,513
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	9,957		10,578
Securities available-for-sale, Gross Unrealized Holding Gains	8
Securities available-for-sale, Gross Unrealized Holding Losses			(5)
Securities available-for-sale, Estimated Fair Value	9,965		10,573
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	304	60,516	73,666
Securities available-for-sale, Gross Unrealized Holding Gains			60
Securities available-for-sale, Gross Unrealized Holding Losses	(1)	(186)	(21)
Securities available-for-sale, Estimated Fair Value	303	60,330	73,705
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,257,547	1,369,295	1,476,714
Securities available-for-sale, Gross Unrealized Holding Gains	29,924	52,491	58,707
Securities available-for-sale, Gross Unrealized Holding Losses	(2,696)	(936)	(3,548)
Securities available-for-sale, Estimated Fair Value	1,284,775	1,420,850	1,531,873
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	4,861	11,421	30,079
Securities available-for-sale, Gross Unrealized Holding Gains		43	163
Securities available-for-sale, Gross Unrealized Holding Losses	(79)	(5)	(2)
Securities available-for-sale, Estimated Fair Value	4,782	11,459	30,240
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,464,345	1,223,452	971,557
Securities available-for-sale, Gross Unrealized Holding Gains	1,630	4,561	8,083
Securities available-for-sale, Gross Unrealized Holding Losses	(30,718)	(8,916)	(4,558)
Securities available-for-sale, Estimated Fair Value	1,435,257	1,219,097	975,082
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	471,398	377,934	342,651
Securities available-for-sale, Gross Unrealized Holding Gains		263	1,202
Securities available-for-sale, Gross Unrealized Holding Losses	(8,913)	(2,460)	(813)
Securities available-for-sale, Estimated Fair Value	$ 462,485	$ 375,737	$ 343,040