Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 3,182,772	$ 3,153,568	$ 3,208,412
Securities available-for-sale, Gross Unrealized Holding Gains	80,341	34,611	31,562
Securities available-for-sale, Gross Unrealized Holding Losses	(3,621)	(29,402)	(42,407)
Securities available-for-sale, Estimated Fair Value	3,259,492	3,158,777	3,197,567
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	9,984	9,970	9,957
Securities available-for-sale, Gross Unrealized Holding Gains	36		8
Securities available-for-sale, Gross Unrealized Holding Losses		(8)
Securities available-for-sale, Estimated Fair Value	10,020	9,962	9,965
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	304
Securities available-for-sale, Gross Unrealized Holding Losses			(1)
Securities available-for-sale, Estimated Fair Value		301	303
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,103,682	1,229,828	1,257,547
Securities available-for-sale, Gross Unrealized Holding Gains	53,593	30,013	29,924
Securities available-for-sale, Gross Unrealized Holding Losses	(61)	(1,970)	(2,696)
Securities available-for-sale, Estimated Fair Value	1,157,214	1,257,871	1,284,775
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	4,643	4,875	4,861
Securities available-for-sale, Gross Unrealized Holding Gains	51
Securities available-for-sale, Gross Unrealized Holding Losses		(77)	(79)
Securities available-for-sale, Estimated Fair Value	4,694	4,798	4,782
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,579,544	1,472,228	1,464,345
Securities available-for-sale, Gross Unrealized Holding Gains	19,732	3,928	1,630
Securities available-for-sale, Gross Unrealized Holding Losses	(3,101)	(21,611)	(30,718)
Securities available-for-sale, Estimated Fair Value	1,596,175	1,454,545	1,435,257
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	484,919	436,366	471,398
Securities available-for-sale, Gross Unrealized Holding Gains	6,929	670	0
Securities available-for-sale, Gross Unrealized Holding Losses	(459)	(5,736)	(8,913)
Securities available-for-sale, Estimated Fair Value	$ 491,389	$ 431,300	$ 462,485