Securities - Amortized Cost and Fair Value of Held to Maturity Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investments Debt And Equity Securities [Abstract]
Securities Held to Maturity, Due in one year or less, Amortized Cost	$ 10,343
Securities Held to Maturity, Due after one year through five years, Amortized Cost	569,432
Securities Held to Maturity, Due after five years through ten years, Amortized Cost	240,743
Securities Held to Maturity, Due after ten years, Amortized Cost	884,733
Total Held to Maturity securities, Amortized Cost	1,705,251	1,668,018
Securities Held to Maturity, Due in one year or less, Fair Value	10,437
Securities Held to Maturity, Due after one year through five years, Fair Value	564,644
Securities Held to Maturity, Due after five years through ten years, Fair Value	241,613
Securities Held to Maturity, Due after ten years, Fair Value	895,022
Total Held to Maturity Securities, Fair Value	$ 1,711,716	$ 1,710,465