Securities - Amortized Cost and Fair Value of Held to Maturity Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investments Debt And Equity Securities [Abstract]
Securities Held to Maturity, Due in one year or less, Amortized Cost	$ 107,697
Securities Held to Maturity, Due after one year through five years, Amortized Cost	669,896
Securities Held to Maturity, Due after five years through ten years, Amortized Cost	121,588
Securities Held to Maturity, Due after ten years, Amortized Cost	1,766,901
Total Held to Maturity securities, Amortized Cost	2,666,082	1,668,018
Securities Held to Maturity, Due in one year or less, Fair Value	108,407
Securities Held to Maturity, Due after one year through five years, Fair Value	655,554
Securities Held to Maturity, Due after five years through ten years, Fair Value	117,864
Securities Held to Maturity, Due after ten years, Fair Value	1,772,042
Total Held to Maturity Securities, Fair Value	$ 2,653,867	$ 1,710,465