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Allowance for Loan Losses - Additional Information (Detail)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
SecurityLoan
Dec. 31, 2016
USD ($)
SecurityLoan
Dec. 31, 2015
USD ($)
SecurityLoan
Accounts Notes And Loans Receivable [Line Items]      
Period for calculating Loss migration rates of portfolio segments 3 years   3 years
Allowance for loan losses $ 13,134 $ 13,305  
Proceeds from payoff on previously charged-off loans   1,303  
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 the borrower 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.    
Gross interest income recorded on nonaccrual loans $ 712 701 $ 1,761
Interest income on nonaccrual loans recognized on cash basis $ 139 $ 1,138 $ 766
Defaulted loans | SecurityLoan 0 0 1
Total loans modified and considered TDRs     $ 107
Impaired loans greater than $350    
Foreclosed assets $ 16 $ 37  
Allowance for loan losses recorded for acquired loans 126 175  
Residential Mortgage [Member]      
Accounts Notes And Loans Receivable [Line Items]      
Foreclosed assets 16    
Residential mortgages in process of foreclosure 239 710  
TDRs [Member]      
Accounts Notes And Loans Receivable [Line Items]      
Allowance for loan losses $ 169 $ 278 286
Commercial and Agriculture [Member]      
Accounts Notes And Loans Receivable [Line Items]      
Decreases allowance for loans     $ (625)
Commercial Real Estate Owner Occupied [Member]      
Accounts Notes And Loans Receivable [Line Items]      
Allowance for loan losses look back period     3 years
Commercial Real Estate Non Owner Occupied [Member]      
Accounts Notes And Loans Receivable [Line Items]      
Allowance for loan losses look back period     3 years
Residential Real Estate [Member]      
Accounts Notes And Loans Receivable [Line Items]      
Allowance for loan losses look back period     3 years