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	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	$ 2,399,493	$ 2,436,769
Available-for-sale, Unrealized Gains	36,214	83,895
Available-for-sale, Unrealized Losses	(61,410)	(19,880)
Available-for-sale securities, Fair Value	2,374,297	2,500,784	2,312,789
Held-to-maturity, Amortized Cost	758,194	402,828	410,521
Held-to-maturity, Unrealized Gains	21,709	30,373
Held-to-maturity, Unrealized Losses	(1,351)
Held-to-maturity, Fair Value	778,552	433,201	440,014
U.S. Treasury [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	28,131	11,437
Available-for-sale, Unrealized Gains	170	404
Available-for-sale, Unrealized Losses
Available-for-sale securities, Fair Value	28,301	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	408,851	515,469
Available-for-sale, Unrealized Gains	500	2,794
Available-for-sale, Unrealized Losses	(17,614)	(938)
Available-for-sale securities, Fair Value	391,737	517,325
Held-to-maturity, Amortized Cost	171,472	173,936
Held-to-maturity, Unrealized Gains	8,548	14,327
Held-to-maturity, Unrealized Losses
Held-to-maturity, Fair Value	180,020	188,263
Mortgage-Backed Securities-Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	1,395,110	1,130,991
Available-for-sale, Unrealized Gains	18,467	33,244
Available-for-sale, Unrealized Losses	(22,773)	(447)
Available-for-sale securities, Fair Value	1,390,804	1,163,788
Held-to-maturity, Amortized Cost	39,254	56,612
Held-to-maturity, Unrealized Gains	1,234	2,307
Held-to-maturity, Unrealized Losses
Held-to-maturity, Fair Value	40,488	58,919
Mortgage-Backed Securities-Non-Agency [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	18,082	29,359
Available-for-sale, Unrealized Gains	331	1,175
Available-for-sale, Unrealized Losses		(338)
Available-for-sale securities, Fair Value	18,413	30,196
States and Political Subdivisions [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	266,306	542,559
Available-for-sale, Unrealized Gains	11,169	35,805
Available-for-sale, Unrealized Losses	(1,677)	(1,040)
Available-for-sale securities, Fair Value	275,798	577,324
Held-to-maturity, Amortized Cost	547,468	169,282
Held-to-maturity, Unrealized Gains	11,927	13,739
Held-to-maturity, Unrealized Losses	(1,351)
Held-to-maturity, Fair Value	558,044	183,021
Pooled Trust Preferred Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	22,923	24,884
Available-for-sale, Unrealized Gains
Available-for-sale, Unrealized Losses	(13,518)	(15,525)
Available-for-sale securities, Fair Value	9,405	9,359
Other Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Available-for-sale, Amortized Cost	260,090	182,070
Available-for-sale, Unrealized Gains	5,577	10,473
Available-for-sale, Unrealized Losses	(5,828)	(1,592)
Available-for-sale securities, Fair Value	259,839	190,951
Held-to-maturity, Amortized Cost		2,998
Held-to-maturity, Unrealized Losses
Held-to-maturity, Fair Value		$ 2,998