Amortized Cost and Estimated Fair Values of investment securities Available-For-Sale by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Amortized Cost
Due in one year or less	$ 35,391	$ 35,544
Due after one year through five years	7,831	3,957
Due after five years through ten years	387,233	382,957
Due after ten years	397,344	368,708
Total debt securities available for sale	827,799	791,166
Estimated Fair Value
Due in one year or less	35,744	36,349
Due after one year through five years	8,060	3,994
Due after five years through ten years	390,124	399,180
Due after ten years	403,789	385,336
Estimated Fair Value	$ 838,440	$ 825,582