Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	$ 2,827,772	$ 2,654,885
Securities available-for-sale, Gross Unrealized Holding Gains	49,824	84,855
Securities available-for-sale, Gross Unrealized Holding Losses	(16,759)	(5,841)
Securities available-for-sale, Estimated Fair Value	2,860,837	2,733,899
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	10,649	10,792
Securities available-for-sale, Gross Unrealized Holding Gains	19	5
Securities available-for-sale, Gross Unrealized Holding Losses		(2)
Securities available-for-sale, Estimated Fair Value	10,668	10,795
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	113,450	148,393
Securities available-for-sale, Gross Unrealized Holding Gains	253	268
Securities available-for-sale, Gross Unrealized Holding Losses		(107)
Securities available-for-sale, Estimated Fair Value	113,703	148,554
Obligations of State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	1,534,095	1,379,879
Securities available-for-sale, Gross Unrealized Holding Gains	40,194	71,382
Securities available-for-sale, Gross Unrealized Holding Losses	(10,013)	(134)
Securities available-for-sale, Estimated Fair Value	1,564,276	1,451,127
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	51,920	86,182
Securities available-for-sale, Gross Unrealized Holding Gains	476	1,778
Securities available-for-sale, Gross Unrealized Holding Losses	(3)	(5)
Securities available-for-sale, Estimated Fair Value	52,393	87,955
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	848,614	781,648
Securities available-for-sale, Gross Unrealized Holding Gains	8,260	10,993
Securities available-for-sale, Gross Unrealized Holding Losses	(5,513)	(3,759)
Securities available-for-sale, Estimated Fair Value	851,361	788,882
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-Sale, Amortized Cost Basis, Total	269,044	247,991
Securities available-for-sale, Gross Unrealized Holding Gains	622	429
Securities available-for-sale, Gross Unrealized Holding Losses	(1,230)	(1,834)
Securities available-for-sale, Estimated Fair Value	$ 268,436	$ 246,586