Investment Securities - Amortized Cost and Fair Value of Available-for-Sale and Held-to-Maturity Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 2,584,942	$ 2,629,128
Available-for-sale, Unrealized Gains	26,448	33,309
Available-for-sale, Unrealized Losses	(38,869)	(34,606)
Available-for-Sale securities, Fair Value	2,572,521	2,627,831	$ 2,513,117
Held-to-Maturity, Amortized Cost	823,255	844,054	852,904
Held-to-maturity, Unrealized Gains	44,671	59,958
Held-to-maturity, Unrealized Losses	(581)	(77)
Held-to-Maturity, Fair Value	867,345	903,935	$ 899,007
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	11,973	14,978
Available-for-sale, Unrealized Gains	198	196
Available-for-sale, Unrealized Losses		(8)
Available-for-Sale securities, Fair Value	12,171	15,166
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	699,972	692,704
Available-for-sale, Unrealized Gains	1,522	1,533
Available-for-sale, Unrealized Losses	(6,420)	(8,286)
Available-for-Sale securities, Fair Value	695,074	685,951
Held-to-Maturity, Amortized Cost	144,501	167,207
Held-to-maturity, Unrealized Gains	4,889	6,279
Held-to-Maturity, Fair Value	149,390	173,486
Mortgage-Backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	1,101,161	1,233,811
Available-for-sale, Unrealized Gains	15,466	18,219
Available-for-sale, Unrealized Losses	(12,482)	(10,368)
Available-for-Sale securities, Fair Value	1,104,145	1,241,662
Held-to-Maturity, Amortized Cost	19,547	23,648
Held-to-maturity, Unrealized Gains	791	926
Held-to-Maturity, Fair Value	20,338	24,574
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	385,889	304,435
Available-for-sale, Unrealized Gains	7,436	11,023
Available-for-sale, Unrealized Losses	(5,286)	(917)
Available-for-Sale securities, Fair Value	388,039	314,541
Held-to-Maturity, Amortized Cost	659,207	653,199
Held-to-maturity, Unrealized Gains	38,991	52,753
Held-to-maturity, Unrealized Losses	(581)	(77)
Held-to-Maturity, Fair Value	697,617	705,875
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	17,460	17,965
Available-for-sale, Unrealized Losses	(10,364)	(11,358)
Available-for-Sale securities, Fair Value	7,096	6,607
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Amortized Cost	368,487	365,235
Available-for-sale, Unrealized Gains	1,826	2,338
Available-for-sale, Unrealized Losses	(4,317)	(3,669)
Available-for-Sale securities, Fair Value	$ 365,996	$ 363,904