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	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	$ 1,227,515	$ 352,292
Available-for-Sale, Less than 12 Months, Unrealized Losses	(46,301)	(2,587)
Available-for-Sale, 12 months or longer, Fair Value	20,689	21,713
Available-for-Sale, 12 months or longer, Unrealized Losses	(15,109)	(17,293)
Available-for-Sale, Fair Value	1,248,204	374,005
Available-for-Sale, Unrealized Losses	(61,410)	(19,880)
Held-to-Maturity, Less than 12 Months, Fair Value	50,299
Held-to-Maturity, Less than 12 Months, Unrealized Losses	(1,351)
Held-to-Maturity, 12 months or longer, Fair Value
Held-to-Maturity, 12 months or longer, Unrealized Losses
Held-to-maturity, Fair Value	50,299
Held-to-Maturity, Unrealized Losses	(1,351)
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	346,427	201,151
Available-for-Sale, Less than 12 Months, Unrealized Losses	(17,614)	(938)
Available-for-Sale, Fair Value	346,427	201,151
Available-for-Sale, Unrealized Losses	(17,614)	(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	708,751	64,213
Available-for-Sale, Less than 12 Months, Unrealized Losses	(22,773)	(447)
Available-for-Sale, Fair Value	708,751	64,213
Available-for-Sale, Unrealized Losses	(22,773)	(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	42,020	63,311
Available-for-Sale, Less than 12 Months, Unrealized Losses	(1,572)	(1,040)
Available-for-Sale, 12 months or longer, Fair Value	2,821
Available-for-Sale, 12 months or longer, Unrealized Losses	(105)
Available-for-Sale, Fair Value	44,841	63,311
Available-for-Sale, Unrealized Losses	(1,677)	(1,040)
Held-to-Maturity, Less than 12 Months, Fair Value	50,299
Held-to-Maturity, Less than 12 Months, Unrealized Losses	(1,351)
Held-to-Maturity, 12 months or longer, Fair Value
Held-to-Maturity, 12 months or longer, Unrealized Losses
Held-to-maturity, Fair Value	50,299
Held-to-Maturity, Unrealized Losses	(1,351)
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	9,405	9,359
Available-for-Sale, 12 months or longer, Unrealized Losses	(13,518)	(15,525)
Available-for-Sale, Fair Value	9,405	9,359
Available-for-Sale, Unrealized Losses	(13,518)	(15,525)	(476)
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-Sale, Less than 12 Months, Fair Value	130,317	23,617
Available-for-Sale, Less than 12 Months, Unrealized Losses	(4,342)	(162)
Available-for-Sale, 12 months or longer, Fair Value	8,463	6,658
Available-for-Sale, 12 months or longer, Unrealized Losses	(1,486)	(1,430)
Available-for-Sale, Fair Value	138,780	30,275
Available-for-Sale, Unrealized Losses	$ (5,828)	$ (1,592)