Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 2,975,990	$ 2,827,772	$ 2,657,185
Securities available-for-sale, Gross Unrealized Holding Gains	56,649	49,824	106,916
Securities available-for-sale, Gross Unrealized Holding Losses	(14,360)	(16,759)	(1,062)
Securities available-for-sale, Estimated Fair Value	3,018,279	2,860,837	2,763,039
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	10,614	10,649	10,756
Securities available-for-sale, Gross Unrealized Holding Gains	5	19	73
Securities available-for-sale, Estimated Fair Value	10,619	10,668	10,829
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	79,777	113,450	131,484
Securities available-for-sale, Gross Unrealized Holding Gains	130	253	1,061
Securities available-for-sale, Gross Unrealized Holding Losses	(2)
Securities available-for-sale, Estimated Fair Value	79,905	113,703	132,545
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,524,010	1,534,095	1,388,369
Securities available-for-sale, Gross Unrealized Holding Gains	47,902	40,194	82,693
Securities available-for-sale, Gross Unrealized Holding Losses	(7,399)	(10,013)	(28)
Securities available-for-sale, Estimated Fair Value	1,564,513	1,564,276	1,471,034
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	47,601	51,920	68,836
Securities available-for-sale, Gross Unrealized Holding Gains	333	476	1,896
Securities available-for-sale, Gross Unrealized Holding Losses	(3)	(3)
Securities available-for-sale, Estimated Fair Value	47,931	52,393	70,732
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	989,272	848,614	793,509
Securities available-for-sale, Gross Unrealized Holding Gains	7,718	8,260	17,180
Securities available-for-sale, Gross Unrealized Holding Losses	(5,437)	(5,513)	(963)
Securities available-for-sale, Estimated Fair Value	991,553	851,361	809,726
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	324,716	269,044	264,231
Securities available-for-sale, Gross Unrealized Holding Gains	561	622	4,013
Securities available-for-sale, Gross Unrealized Holding Losses	(1,519)	(1,230)	(71)
Securities available-for-sale, Estimated Fair Value	$ 323,758	$ 268,436	$ 268,173