Investment Securities (Detail) - Amortized cost and estimated fair value of investment securities by contractual maturity (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Securities available for sale
Due within one year	$ 1,013
Due within one year	1,019
Due after one year through five years	1,219
Due after one year through five years	1,380
Due after five years through ten years
Due after five years through ten years
Due after ten years	21,480
Due after ten years	18,599
Total	23,712
Total	20,998
Securities available for sale, Amortized Cost	153,359	83,708
Securities available for sale, Fair Value	154,234	83,629
Total	315,069
Total	314,597
Securities held to maturity
Due within one year	400
Due within one year	400
Due after one year through five years	100
Due after one year through five years	100
Due after five years through ten years	1,986
Due after five years through ten years	2,057
Due after ten years	12,383
Due after ten years	14,385
Total	14,869
Total	16,942
Mortgage Backed Securities Excluding Collateralized Mortgage Obligations [Member]
Securities available for sale
Securities available for sale, Amortized Cost	143,543
Securities available for sale, Fair Value	145,308
Collateralized Mortgage Obligations [Member]
Securities available for sale
Securities available for sale, Amortized Cost	145,159
Securities available for sale, Fair Value	144,507
Other Securities [Member]
Securities available for sale
Other securities with no stated maturity	2,655
Other securities with no stated maturity	$ 3,784