Amortized Cost and Estimated Fair Values of investment securities Available-For-Sale by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Amortized Cost
Due in one year or less	$ 10,275	$ 65,569
Due after one year through five years	32,700	62,993
Due after five years through ten years	368,273	342,813
Due after ten years	385,018	446,552
Total debt securities available for sale	796,266	917,927
Estimated Fair Value
Due in one year or less	10,406	65,972
Due after one year through five years	33,600	64,656
Due after five years through ten years	385,574	354,238
Due after ten years	404,362	466,335
Estimated Fair Value	$ 833,942	$ 951,201