Amortized Cost and Estimated Fair Values of investment securities Available-For-Sale by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Amortized Cost
Due in one year or less	$ 20,284	$ 65,569
Due after one year through five years	37,844	62,993
Due after five years through ten years	311,158	342,813
Due after ten years	404,086	446,552
Total debt securities available for sale	773,372	917,927
Estimated Fair Value
Due in one year or less	20,381	65,972
Due after one year through five years	39,045	64,656
Due after five years through ten years	324,495	354,238
Due after ten years	424,237	466,335
Estimated Fair Value	$ 808,158	$ 951,201