Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012
Amortized Cost And Fair Value Debt Securities [Abstract]
Available-for-sale, Maturity, Within one year, Amortized Cost	$ 36,076
Available-for-sale, Maturity, One to five years, Amortized Cost	173,037
Available-for-sale, Maturity, Five to ten years, Amortized Cost	547,138
Available-for-sale, Maturity, Beyond ten years, Amortized Cost	1,643,242
Available-for-sale, Amortized Cost	2,399,493	2,436,769
Held-to-maturity, Maturity, Within one year, Amortized Cost	65
Held-to-maturity, Maturity, One to five years, Amortized Cost	14,601
Held-to-maturity, Maturity, Five to ten years, Amortized Cost	159,912
Held-to-maturity, Maturity, Beyond ten years, Amortized Cost	583,616
Held-to-maturity, Amortized Cost	758,194	402,828	410,521
Available-for-sale, Maturity, Within one year, Fair Value	36,290
Available-for-sale, Maturity, One to five years, Fair Value	179,404
Available-for-sale, Maturity, Five to ten years, Fair Value	535,569
Available-for-sale, Maturity, Beyond ten years, Fair Value	1,623,034
Available-for-sale securities, Fair Value	2,374,297	2,500,784	2,312,789
Held-to-maturity, Maturity, Within one year, Fair Value	66
Held-to-maturity, Maturity, One to five years, Fair Value	15,362
Held-to-maturity, Maturity, Five to ten years, Fair Value	165,571
Held-to-maturity, Maturity, Beyond ten years, Fair Value	597,553
Held-to-maturity, Fair Value	$ 778,552	$ 433,201	$ 440,014
Available-for-sale, Maturity, Within one year, Weighted Average Yield	2.47%
Available-for-sale, Maturity, One to five years, Weighted Average Yield	3.12%
Available-for-sale, Maturity, Five to ten years, Weighted Average Yield	2.43%
Available-for-sale, Maturity, Beyond ten years, Weighted Average Yield	2.48%
Available-for-sale, Weighted Average Yield	2.51%
Held-to-maturity, Maturity, Within one year, Weighted Average Yield	2.74%
Held-to-maturity, Maturity, One to five years, Weighted Average Yield	4.25%
Held-to-maturity, Maturity, Five to ten years, Weighted Average Yield	3.15%
Held-to-maturity, Maturity, Beyond ten years, Weighted Average Yield	5.54%
Held-to-maturity, Weighted Average Yield	5.01%