Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Investments Debt And Equity Securities [Abstract]
Available-for-sale, Maturity, Within one year, Amortized Cost	$ 22,438
Available-for-sale, Maturity, One to five years, Amortized Cost	397,946
Available-for-sale, Maturity, Five to ten years, Amortized Cost	577,480
Available-for-sale, Maturity, Beyond ten years, Amortized Cost	1,550,922
Available-for-sale, Amortized Cost	2,548,786	2,406,283
Held-to-maturity, Maturity, Within one year, Amortized Cost	1,514
Held-to-maturity, Maturity, One to five years, Amortized Cost	22,585
Held-to-maturity, Maturity, Five to ten years, Amortized Cost	173,189
Held-to-maturity, Maturity, Beyond ten years, Amortized Cost	655,616
Held-to-maturity, Amortized Cost	852,904	762,734	401,066
Available-for-sale, Maturity, Within one year, Fair Value	22,487
Available-for-sale, Maturity, One to five years, Fair Value	404,642
Available-for-sale, Maturity, Five to ten years, Fair Value	572,166
Available-for-sale, Maturity, Beyond ten years, Fair Value	1,536,762
Available-for-sale securities, Fair Value	2,536,057	2,372,201	2,659,084
Held-to-maturity, Maturity, Within one year, Fair Value	1,535
Held-to-maturity, Maturity, One to five years, Fair Value	23,755
Held-to-maturity, Maturity, Five to ten years, Fair Value	179,969
Held-to-maturity, Maturity, Beyond ten years, Fair Value	693,748
Held-to-maturity, Fair Value	$ 899,007	$ 780,758	$ 419,326
Available-for-sale, Maturity, Within one year, Weighted Average Yield	2.41%
Available-for-sale, Maturity, One to five years, Weighted Average Yield	2.31%
Available-for-sale, Maturity, Five to ten years, Weighted Average Yield	2.31%
Available-for-sale, Maturity, Beyond ten years, Weighted Average Yield	2.40%
Available-for-sale, Weighted Average Yield	2.37%
Held-to-maturity, Maturity, Within one year, Weighted Average Yield	3.13%
Held-to-maturity, Maturity, One to five years, Weighted Average Yield	3.90%
Held-to-maturity, Maturity, Five to ten years, Weighted Average Yield	3.25%
Held-to-maturity, Maturity, Beyond ten years, Weighted Average Yield	5.47%
Held-to-maturity, Weighted Average Yield	4.97%