Amortized Cost and Estimated Fair Values of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012
Available-for-sale, Amortized Cost
Due in one year or less	$ 5,600
Due after one year through five years	17,018
Due after five years through ten years	152,756
Due after ten years	530,162
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Total	705,536
Available-for-sale, Fair Value
Due in one year or less	5,639
Due after one year through five years	17,611
Due after five years through ten years	158,907
Due after ten years	548,473
Available-for-sale Securities, Debt Securities, Total	$ 730,630