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	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	$ 1,410,081	$ 352,292
Available-for-Sale, Less than 12 Months, Unrealized Losses	(58,501)	(2,587)
Available-for-Sale, 12 months or longer, Fair Value	16,951	21,713
Available-for-Sale, 12 months or longer, Unrealized Losses	(15,163)	(17,293)
Available-for-Sale, Fair Value	1,427,032	374,005
Available-for-Sale, Unrealized Losses	(73,664)	(19,880)
Held-to-Maturity, Less than 12 Months, Fair Value	28,600
Held-to-Maturity, Less than 12 Months, Unrealized Losses	(820)
Held-to-Maturity, 12 months or longer, Fair Value
Held-to-Maturity, 12 months or longer, Unrealized Losses
Held-to-maturity, Fair Value	28,600
Held-to-Maturity, Unrealized Losses	(820)
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	337,317	201,151
Available-for-Sale, Less than 12 Months, Unrealized Losses	(14,345)	(938)
Available-for-Sale, Fair Value	337,317	201,151
Available-for-Sale, Unrealized Losses	(14,345)	(938)
Mortgage-Backed Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	653,601	64,213
Available-for-Sale, Less than 12 Months, Unrealized Losses	(14,376)	(447)
Available-for-Sale, Fair Value	653,601	64,213
Available-for-Sale, Unrealized Losses	(14,376)	(447)
Residential Mortgage Backed Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, 12 months or longer, Fair Value		5,696
Available-for-Sale, 12 months or longer, Unrealized Losses		(338)
Available-for-Sale, Fair Value		5,696
Available-for-Sale, Unrealized Losses		(338)
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	321,483	63,311
Available-for-Sale, Less than 12 Months, Unrealized Losses	(27,301)	(1,040)
Available-for-Sale, 12 months or longer, Fair Value	949
Available-for-Sale, 12 months or longer, Unrealized Losses	(6)
Available-for-Sale, Fair Value	322,432	63,311
Available-for-Sale, Unrealized Losses	(27,307)	(1,040)
Held-to-Maturity, Less than 12 Months, Fair Value	28,600
Held-to-Maturity, Less than 12 Months, Unrealized Losses	(820)
Held-to-Maturity, 12 months or longer, Fair Value
Held-to-Maturity, 12 months or longer, Unrealized Losses
Held-to-maturity, Fair Value	28,600
Held-to-Maturity, Unrealized Losses	(820)
Pooled Trust Preferred Securities Group 1 [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, 12 months or longer, Fair Value	9,152	9,359
Available-for-Sale, 12 months or longer, Unrealized Losses	(13,909)	(15,525)
Available-for-Sale, Fair Value	9,152	9,359
Available-for-Sale, Unrealized Losses	(13,909)	(15,525)	475
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	97,680	23,617
Available-for-Sale, Less than 12 Months, Unrealized Losses	(2,479)	(162)
Available-for-Sale, 12 months or longer, Fair Value	6,850	6,658
Available-for-Sale, 12 months or longer, Unrealized Losses	(1,248)	(1,430)
Available-for-Sale, Fair Value	104,530	30,275
Available-for-Sale, Unrealized Losses	$ (3,727)	$ (1,592)