Securities - Amortized Cost and Fair Value of Available for Sale Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013
Investments Debt And Equity Securities [Abstract]
Securities Available for Sale, Due in one year or less, Amortized Cost	$ 23,895
Securities Available for Sale, Due after one year through five years, Amortized Cost	130,419
Securities Available for Sale, Due after five years through ten years, Amortized Cost	154,962
Securities Available for Sale, Due after ten years, Amortized Cost	1,121,951
Total available for sale securities, Amortized Cost	1,431,227
Securities Available for Sale, Due in one year or less, Fair Value	24,010
Securities Available for Sale, Due after one year through five years, Fair Value	130,124
Securities Available for Sale, Due after five years through ten years, Fair Value	161,485
Securities Available for Sale, Due after ten years, Fair Value	1,137,151
Total available for sale securities, Fair Value	$ 1,452,770