Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	$ 2,113,044	$ 2,035,126
Securities available-for-sale, Gross Unrealized Holding Gains	70,219	45,455
Securities available-for-sale, Gross Unrealized Holding Losses	(9,435)	(22,858)
Securities available-for-sale, Estimated Fair Value	2,173,828	2,057,723	1,974,604
Obligations of U.S. Government Sponsored-Enterprises and Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	118,907	136,416
Securities available-for-sale, Gross Unrealized Holding Gains	1,210	1,672
Securities available-for-sale, Estimated Fair Value	120,117	138,088
Obligations of States and Political Subdivisions [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	1,027,503	974,608
Securities available-for-sale, Gross Unrealized Holding Gains	49,171	27,980
Securities available-for-sale, Gross Unrealized Holding Losses	(2,016)	(11,319)
Securities available-for-sale, Estimated Fair Value	1,074,658	991,269
Corporate Bonds and Other [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	97,119	105,490
Securities available-for-sale, Gross Unrealized Holding Gains	3,621	3,550
Securities available-for-sale, Estimated Fair Value	100,740	109,040
Residential Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	734,301	706,289
Securities available-for-sale, Gross Unrealized Holding Gains	16,183	12,253
Securities available-for-sale, Gross Unrealized Holding Losses	(5,011)	(7,922)
Securities available-for-sale, Estimated Fair Value	745,473	710,620
Commercial Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	135,214	112,323
Securities available-for-sale, Gross Unrealized Holding Gains	34
Securities available-for-sale, Gross Unrealized Holding Losses	(2,408)	(3,617)
Securities available-for-sale, Estimated Fair Value	$ 132,840	$ 108,706