Investment Securities - Investment Securities with Unrealized Losses by Aggregated Major Security Type and Length of Time in Continuous Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	$ 200,852	$ 378,770
Available-for-Sale, Less than 12 Months, Unrealized Losses	(1,603)	(1,887)
Available-for-Sale, 12 months or longer, Fair Value	604,755	756,187
Available-for-Sale, 12 months or longer, Unrealized Losses	(22,541)	(32,719)
Available-for-Sale, Fair Value	805,607	1,134,957
Available-for-Sale, Unrealized Losses	(24,144)	(34,606)
Held-to-Maturity, Less than 12 Months, Fair Value	2,134	6,171
Held-to-Maturity, Less than 12 Months, Unrealized Losses	(4)	(77)
Held-to-Maturity, 12 months or longer, Fair Value	0	0
Held-to-Maturity, 12 months or longer, Unrealized Losses	0	0
Held-to-maturity, Fair Value	2,134	6,171
Held-to-Maturity, Unrealized Losses	(4)	(77)
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	12,900	9,524
Available-for-Sale, Less than 12 Months, Unrealized Losses		(8)
Available-for-Sale, Fair Value	12,900	9,524
Available-for-Sale, Unrealized Losses		(8)
U.S. Government-Sponsored Entities and Agencies [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	15,895	180,488
Available-for-Sale, Less than 12 Months, Unrealized Losses	(22)	(563)
Available-for-Sale, 12 months or longer, Fair Value	204,953	257,914
Available-for-Sale, 12 months or longer, Unrealized Losses	(2,095)	(7,723)
Available-for-Sale, Fair Value	220,848	438,402
Available-for-Sale, Unrealized Losses	(2,117)	(8,286)
Mortgage-backed Securities - Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	58,455	31,304
Available-for-Sale, Less than 12 Months, Unrealized Losses	(401)	(122)
Available-for-Sale, 12 months or longer, Fair Value	294,928	386,788
Available-for-Sale, 12 months or longer, Unrealized Losses	(6,069)	(10,246)
Available-for-Sale, Fair Value	353,383	418,092
Available-for-Sale, Unrealized Losses	(6,470)	(10,368)
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	35,751	41,481
Available-for-Sale, Less than 12 Months, Unrealized Losses	(350)	(288)
Available-for-Sale, 12 months or longer, Fair Value	6,561	9,534
Available-for-Sale, 12 months or longer, Unrealized Losses	(578)	(629)
Available-for-Sale, Fair Value	42,312	51,015
Available-for-Sale, Unrealized Losses	(928)	(917)
Held-to-Maturity, Less than 12 Months, Fair Value	2,134	6,171
Held-to-Maturity, Less than 12 Months, Unrealized Losses	(4)	(77)
Held-to-Maturity, 12 months or longer, Fair Value	0	0
Held-to-Maturity, 12 months or longer, Unrealized Losses	0	0
Held-to-maturity, Fair Value	2,134	6,171
Held-to-Maturity, Unrealized Losses	(4)	(77)
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, 12 months or longer, Fair Value	6,553	6,607
Available-for-Sale, 12 months or longer, Unrealized Losses	(11,153)	(11,358)
Available-for-Sale, Fair Value	6,553	6,607
Available-for-Sale, Unrealized Losses	(11,153)	(11,358)
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	77,851	115,973
Available-for-Sale, Less than 12 Months, Unrealized Losses	(830)	(906)
Available-for-Sale, 12 months or longer, Fair Value	91,760	95,344
Available-for-Sale, 12 months or longer, Unrealized Losses	(2,646)	(2,763)
Available-for-Sale, Fair Value	169,611	211,317
Available-for-Sale, Unrealized Losses	$ (3,476)	$ (3,669)