Securities - Schedule of Fair Value and Amortized Cost of Available-for-sale and Held-to-maturity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	$ 1,004,636	$ 944,074
Available-for-sale, Gross Unrealized Gains	11,750	8,999
Available-for-sale, Gross Unrealized Losses	(10,307)	(18,687)
Available-for-sale, Estimated Fair Value	1,006,079	934,386
Held-to-maturity, Amortized Cost	607,695	598,520
Held-to-maturity, Gross Unrealized Gains	24,637	12,962
Held-to-maturity, Gross Unrealized Losses	(3,792)	(15,174)
Held-to-maturity securities, Fair value	628,540	596,308
Total securities, Amortized Cost	1,612,331	1,542,594
Total securities, Gross Unrealized Gains	36,387	21,961
Total securities, Gross Unrealized Losses	(14,099)	(33,861)
Total securities, Estimated Fair Value	1,634,619	1,530,694
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	772,918	707,000
Available-for-sale, Gross Unrealized Gains	4,114	3,191
Available-for-sale, Gross Unrealized Losses	(9,550)	(15,924)
Available-for-sale, Estimated Fair Value	767,482	694,267
Held-to-maturity, Amortized Cost	88,897	99,409
Held-to-maturity, Gross Unrealized Gains	3,705	2,804
Held-to-maturity, Gross Unrealized Losses	(501)	(1,023)
Held-to-maturity securities, Fair value	92,101	101,190
Total securities, Gross Unrealized Losses	(10,051)	(16,947)
Obligations of State and Political Subdivisions [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	104,553	112,536
Available-for-sale, Gross Unrealized Gains	5,700	4,165
Available-for-sale, Gross Unrealized Losses	(14)	(355)
Available-for-sale, Estimated Fair Value	110,239	116,346
Held-to-maturity, Amortized Cost	513,058	496,396
Held-to-maturity, Gross Unrealized Gains	20,866	10,158
Held-to-maturity, Gross Unrealized Losses	(3,206)	(13,906)
Held-to-maturity securities, Fair value	530,718	492,648
Total securities, Gross Unrealized Losses	(3,220)	(14,261)
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	36,731	38,777
Available-for-sale, Gross Unrealized Gains	260	174
Available-for-sale, Gross Unrealized Losses	(99)	(470)
Available-for-sale, Estimated Fair Value	36,892	38,481
Held-to-maturity, Amortized Cost	5,740	2,715
Held-to-maturity, Gross Unrealized Gains	66
Held-to-maturity, Gross Unrealized Losses	(85)	(245)
Held-to-maturity securities, Fair value	5,721	2,470
Total securities, Gross Unrealized Losses	(184)	(715)
Obligations of Government Agencies [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	79,164	75,164
Available-for-sale, Gross Unrealized Gains	382	6
Available-for-sale, Gross Unrealized Losses	(644)	(1,938)
Available-for-sale, Estimated Fair Value	78,902	73,232
Total securities, Gross Unrealized Losses	(644)	(1,938)
Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	993,366	933,477
Available-for-sale, Gross Unrealized Gains	10,456	7,536
Available-for-sale, Gross Unrealized Losses	(10,307)	(18,687)
Available-for-sale, Estimated Fair Value	993,515	922,326
Equity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-sale, Amortized Cost	11,270	10,597
Available-for-sale, Gross Unrealized Gains	1,294	1,463
Available-for-sale, Estimated Fair Value	$ 12,564	$ 12,060