Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 3,153,568	$ 3,153,568	$ 3,042,618
Securities available-for-sale, Gross Unrealized Holding Gains	34,611		57,358
Securities available-for-sale, Gross Unrealized Holding Losses	(29,402)		(12,503)
Securities available-for-sale, Estimated Fair Value	3,158,777	$ 3,158,777	3,087,473
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	9,970
Securities available-for-sale, Gross Unrealized Holding Losses	(8)
Securities available-for-sale, Estimated Fair Value	9,962
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	301		60,516
Securities available-for-sale, Gross Unrealized Holding Losses			(186)
Securities available-for-sale, Estimated Fair Value	301		60,330
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,229,828		1,369,295
Securities available-for-sale, Gross Unrealized Holding Gains	30,013		52,491
Securities available-for-sale, Gross Unrealized Holding Losses	(1,970)		(936)
Securities available-for-sale, Estimated Fair Value	1,257,871		1,420,850
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	4,875		11,421
Securities available-for-sale, Gross Unrealized Holding Gains			43
Securities available-for-sale, Gross Unrealized Holding Losses	(77)		(5)
Securities available-for-sale, Estimated Fair Value	4,798		11,459
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,472,228		1,223,452
Securities available-for-sale, Gross Unrealized Holding Gains	3,928		4,561
Securities available-for-sale, Gross Unrealized Holding Losses	(21,611)		(8,916)
Securities available-for-sale, Estimated Fair Value	1,454,545		1,219,097
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	436,366		377,934
Securities available-for-sale, Gross Unrealized Holding Gains	670		263
Securities available-for-sale, Gross Unrealized Holding Losses	(5,736)		(2,460)
Securities available-for-sale, Estimated Fair Value	$ 431,300		$ 375,737