Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 3,950,510	$ 3,327,805	$ 3,172,535
Securities available-for-sale, Gross Unrealized Holding Gains	159,486	87,346	54,446
Securities available-for-sale, Gross Unrealized Holding Losses	(2,927)	(1,834)	(14,169)
Securities available-for-sale, Estimated Fair Value	4,107,069	3,413,317	3,212,812
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	10,039	9,997	9,977
Securities available-for-sale, Gross Unrealized Holding Gains	74	22	14
Securities available-for-sale, Estimated Fair Value	10,113	10,019	9,991
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,731,392	1,231,619	1,196,111
Securities available-for-sale, Gross Unrealized Holding Gains	78,996	57,764	44,155
Securities available-for-sale, Gross Unrealized Holding Losses	(2,717)	(400)	(536)
Securities available-for-sale, Estimated Fair Value	1,807,671	1,288,983	1,239,730
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	4,624	4,643	4,874
Securities available-for-sale, Gross Unrealized Holding Gains	133	65
Securities available-for-sale, Gross Unrealized Holding Losses	(6)		(15)
Securities available-for-sale, Estimated Fair Value	4,751	4,708	4,859
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,617,341	1,586,872	1,519,004
Securities available-for-sale, Gross Unrealized Holding Gains	62,659	23,139	8,506
Securities available-for-sale, Gross Unrealized Holding Losses	(20)	(1,148)	(11,222)
Securities available-for-sale, Estimated Fair Value	1,679,980	1,608,863	1,516,288
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	587,114	494,674	442,569
Securities available-for-sale, Gross Unrealized Holding Gains	17,624	6,356	1,771
Securities available-for-sale, Gross Unrealized Holding Losses	(184)	(286)	(2,396)
Securities available-for-sale, Estimated Fair Value	$ 604,554	$ 500,744	$ 441,944