Allowance for Loan Losses - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) SecurityLoan	Mar. 31, 2017 SecurityLoan	Dec. 31, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period for calculating Loss migration rates of portfolio segments	3 years
Allowance for loan losses	$ 12,814,000		$ 13,134,000
Number of days past due for loans to be considered as nonperforming	90 days
Reasonable period for nonperforming TDRs to be returned to performing status	6 months
Number of days reaching where loans are considered for nonaccrual status	90 days
Conditions where loans are considered for nonaccrual status	A loan may be returned to accruing status only if one of three conditions are met: the loan is well-secured and none of the principal and interest has been past due for a minimum of 90 days; the loan is a TDR and has made a minimum of six months payments; or the principal and interest payments are reasonably assured and a sustained period of performance has occurred, generally six months.
Loans modified in trouble debt restructuring	$ 0
Defaulted loans | SecurityLoan	0	0
Impaired loans	greater than $350
Allowance for loan losses recorded for acquired loans	$ 37,000		126,000
Foreclosed assets	11,000		16,000
Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Foreclosed assets	11,000
Residential mortgages in process of foreclosure	454,000		239,000
TDRs [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 216,000		$ 169,000