Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	$ 2,685,261	$ 2,436,769
Available-for-sale, Unrealized Gains	47,487	83,895
Available-for-sale, Unrealized Losses	(73,664)	(19,880)
Available-for-sale securities, Fair Value	2,659,084	2,500,784	2,208,262
Held-to-maturity, Amortized Cost	401,066	402,828	463,935
Held-to-maturity, Unrealized Gains	19,080	30,373
Held-to-maturity, Unrealized Losses	(820)
Held-to-maturity, Fair Value	419,326	433,201	493,704
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	14,166	11,437
Available-for-sale, Unrealized Gains	200	404
Available-for-sale securities, Fair Value	14,366	11,841
U.S. Government Sponsored Entities and Agencies [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	388,805	515,469
Available-for-sale, Unrealized Gains	496	2,794
Available-for-sale, Unrealized Losses	(14,345)	(938)
Available-for-sale securities, Fair Value	374,956	517,325
Held-to-maturity, Amortized Cost	172,299	173,936
Held-to-maturity, Unrealized Gains	8,925	14,327
Held-to-maturity, Fair Value	181,224	188,263
Mortgage-Backed Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	1,383,523	1,130,991
Available-for-sale, Unrealized Gains	23,365	33,244
Available-for-sale, Unrealized Losses	(14,376)	(447)
Available-for-sale securities, Fair Value	1,392,512	1,163,788
Held-to-maturity, Amortized Cost	44,286	56,612
Held-to-maturity, Unrealized Gains	1,699	2,307
Held-to-maturity, Fair Value	45,985	58,919
Residential Mortgage Backed Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	19,549	29,359
Available-for-sale, Unrealized Gains	808	1,175
Available-for-sale, Unrealized Losses		(338)
Available-for-sale securities, Fair Value	20,357	30,196
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	654,345	542,559
Available-for-sale, Unrealized Gains	16,849	35,805
Available-for-sale, Unrealized Losses	(27,307)	(1,040)
Available-for-sale securities, Fair Value	643,887	577,324
Held-to-maturity, Amortized Cost	184,481	169,282
Held-to-maturity, Unrealized Gains	8,456	13,739
Held-to-maturity, Unrealized Losses	(820)
Held-to-maturity, Fair Value	192,117	183,021
Pooled Trust Preferred Securities Group 1 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	23,061	24,884
Available-for-sale, Unrealized Gains
Available-for-sale, Unrealized Losses	(13,909)	(15,525)	475
Available-for-sale securities, Fair Value	9,152	9,359
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	201,812	182,070
Available-for-sale, Unrealized Gains	5,769	10,473
Available-for-sale, Unrealized Losses	(3,727)	(1,592)
Available-for-sale securities, Fair Value	203,854	190,951
Held-to-maturity, Amortized Cost		2,998
Held-to-maturity, Fair Value		$ 2,998