Amortized Cost and Estimated Fair Values of investment securities Available-For-Sale by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Amortized Cost
Due in one year or less	$ 35,467	$ 35,544
Due after one year through five years	4,382	3,957
Due after five years through ten years	349,635	382,957
Due after ten years	345,171	368,708
Total debt securities available for sale	734,655	791,166
Estimated Fair Value
Due in one year or less	36,056	36,349
Due after one year through five years	4,463	3,994
Due after five years through ten years	364,619	399,180
Due after ten years	360,219	385,336
Estimated Fair Value	$ 765,357	$ 824,859