Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 2,905,245	$ 2,827,772	$ 2,671,037
Securities available-for-sale, Gross Unrealized Holding Gains	68,215	49,824	124,967
Securities available-for-sale, Gross Unrealized Holding Losses	(8,947)	(16,759)	(648)
Securities available-for-sale, Estimated Fair Value	2,964,513	2,860,837	2,795,356
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	10,578	10,649	10,721
Securities available-for-sale, Gross Unrealized Holding Gains		19	90
Securities available-for-sale, Gross Unrealized Holding Losses	(5)
Securities available-for-sale, Estimated Fair Value	10,573	10,668	10,811
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	73,666	113,450	121,174
Securities available-for-sale, Gross Unrealized Holding Gains	60	253	1,178
Securities available-for-sale, Gross Unrealized Holding Losses	(21)
Securities available-for-sale, Estimated Fair Value	73,705	113,703	122,352
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,476,714	1,534,095	1,418,342
Securities available-for-sale, Gross Unrealized Holding Gains	58,707	40,194	97,980
Securities available-for-sale, Gross Unrealized Holding Losses	(3,548)	(10,013)	(10)
Securities available-for-sale, Estimated Fair Value	1,531,873	1,564,276	1,516,312
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	30,079	51,920	71,687
Securities available-for-sale, Gross Unrealized Holding Gains	163	476	1,723
Securities available-for-sale, Gross Unrealized Holding Losses	(2)	(3)
Securities available-for-sale, Estimated Fair Value	30,240	52,393	73,410
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	971,557	848,614	787,451
Securities available-for-sale, Gross Unrealized Holding Gains	8,083	8,260	18,928
Securities available-for-sale, Gross Unrealized Holding Losses	(4,558)	(5,513)	(606)
Securities available-for-sale, Estimated Fair Value	975,082	851,361	805,773
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	342,651	269,044	261,662
Securities available-for-sale, Gross Unrealized Holding Gains	1,202	622	5,068
Securities available-for-sale, Gross Unrealized Holding Losses	(813)	(1,230)	(32)
Securities available-for-sale, Estimated Fair Value	$ 343,040	$ 268,436	$ 266,698