Other Real Estate Owned and Other Repossessed Assets - Valuation Allowance Related to Real Estate Owned (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Real Estate And Foreclosed Assets [Abstract]
Beginning of year	$ 1,012,000	$ 1,229,000	$ 1,423,000
Additions (recoveries) charged to expense	37,000	(16,000)	287,000
Sales of real estate owned with a valuation allowance	(646,000)	(201,000)	(481,000)
End of year	$ 403,000	$ 1,012,000	$ 1,229,000