Securities - Amortized Cost and Estimated Fair Value of Available-for-Sale and Held-to-Maturity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Aug. 31, 2012
AMOUNTS MATURING IN:
One year or less, Amortized Cost	$ 2,681
One year through five years, Amortized Cost	55,679
Five years through ten years, Amortized Cost	88,137
After ten years, Amortized Cost	67,399
Amortized Costs, Total	213,896
One year or less, Fair Value	2,758
One year through five years, Fair Value	55,776
Five years through ten years, Fair Value	85,554
After ten years, Fair Value	65,554
Available-for-sale securities, Estimated Fair Value	209,642	197,354	18,800
AMOUNTS MATURING IN:
One year or less, Amortized Cost	0
One year through five years, Amortized Cost	722
Five years through ten years, Amortized Cost	13,073
After ten years, Amortized Cost	21,019
Amortized Costs, Total	34,814
One year or less, Fair Value	0
One year through five years, Fair Value	745
Five years through ten years, Fair Value	12,442
After ten years, Fair Value	18,715
Estimated Fair Value, Total	$ 31,902