Reconciliation of Income Tax Expense at Federal Statutory Rate to Actual Tax Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of federal statutory to actual tax expense
Federal statutory income tax at 34%	$ 3,359	$ 1,297	$ 2,797
Low income housing credits	(283)	(445)	(592)
Tax-exempt income	(587)	(615)	(723)
Other	143	92	194
Total income tax expense	$ 2,632	$ 329	$ 1,676
Effective tax rate	26.64%	8.63%	20.37%