Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Investments Debt And Equity Securities [Abstract]
Available-for-sale, Maturity, Within one year, Amortized Cost	$ 22,902
Available-for-sale, Maturity, One to five years, Amortized Cost	137,657
Available-for-sale, Maturity, Five to ten years, Amortized Cost	536,543
Available-for-sale, Maturity, Beyond ten years, Amortized Cost	1,988,159
Available-for-sale, Amortized Cost	2,685,261	2,436,769
Held-to-maturity, Maturity, Within one year, Amortized Cost	66
Held-to-maturity, Maturity, One to five years, Amortized Cost	15,452
Held-to-maturity, Maturity, Five to ten years, Amortized Cost	136,403
Held-to-maturity, Maturity, Beyond ten years, Amortized Cost	249,145
Held-to-maturity, Amortized Cost	401,066	402,828	463,935
Available-for-sale, Maturity, Within one year, Fair Value	23,082
Available-for-sale, Maturity, One to five years, Fair Value	143,052
Available-for-sale, Maturity, Five to ten years, Fair Value	528,020
Available-for-sale, Maturity, Beyond ten years, Fair Value	1,964,930
Available-for-sale securities, Fair Value	2,659,084	2,500,784	2,208,262
Held-to-maturity, Maturity, Within one year, Fair Value	66
Held-to-maturity, Maturity, One to five years, Fair Value	16,125
Held-to-maturity, Maturity, Five to ten years, Fair Value	141,789
Held-to-maturity, Maturity, Beyond ten years, Fair Value	261,346
Held-to-maturity, Fair Value	$ 419,326	$ 433,201	$ 493,704
Available-for-sale, Maturity, Within one year, Weighted Average Yield	3.12%
Available-for-sale, Maturity, One to five years, Weighted Average Yield	3.47%
Available-for-sale, Maturity, Five to ten years, Weighted Average Yield	2.48%
Available-for-sale, Maturity, Beyond ten years, Weighted Average Yield	2.89%
Available-for-sale, Weighted Average Yield	2.84%
Held-to-maturity, Maturity, Within one year, Weighted Average Yield	2.74%
Held-to-maturity, Maturity, One to five years, Weighted Average Yield	4.29%
Held-to-maturity, Maturity, Five to ten years, Weighted Average Yield	2.86%
Held-to-maturity, Maturity, Beyond ten years, Weighted Average Yield	4.49%
Held-to-maturity, Weighted Average Yield	3.93%