Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	$ 2,655,242	$ 2,338,683	$ 2,187,272
Securities available-for-sale, Gross Unrealized Holding Gains	84,454	81,202	75,057
Securities available-for-sale, Gross Unrealized Holding Losses	(2,343)	(4,029)	(8,567)
Securities available-for-sale, Estimated Fair Value	2,737,353	2,415,856	2,253,762
U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	10,828	518	520
Securities available-for-sale, Gross Unrealized Holding Gains	89	2
Securities available-for-sale, Estimated Fair Value	10,917	520	520
Obligations of U.S. Government Sponsored Enterprises and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	153,960	128,919	106,339
Securities available-for-sale, Gross Unrealized Holding Gains	1,237	864	964
Securities available-for-sale, Gross Unrealized Holding Losses		(28)	(2)
Securities available-for-sale, Estimated Fair Value	155,197	129,755	107,301
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	1,375,958	1,107,795	1,046,148
Securities available-for-sale, Gross Unrealized Holding Gains	62,005	60,083	56,398
Securities available-for-sale, Gross Unrealized Holding Losses	(812)	(250)	(577)
Securities available-for-sale, Estimated Fair Value	1,437,151	1,167,628	1,101,969
Corporate Bonds and Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	88,927	95,864	96,424
Securities available-for-sale, Gross Unrealized Holding Gains	2,382	2,894	3,269
Securities available-for-sale, Estimated Fair Value	91,309	98,758	99,693
Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	817,791	871,265	803,072
Securities available-for-sale, Gross Unrealized Holding Gains	16,566	16,804	14,395
Securities available-for-sale, Gross Unrealized Holding Losses	(1,498)	(2,858)	(5,600)
Securities available-for-sale, Estimated Fair Value	832,859	885,211	811,867
Commercial Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	207,778	134,322	134,769
Securities available-for-sale, Gross Unrealized Holding Gains	2,175	555	31
Securities available-for-sale, Gross Unrealized Holding Losses	(33)	(893)	(2,388)
Securities available-for-sale, Estimated Fair Value	$ 209,920	$ 133,984	$ 132,412