Allowance for Loan Losses - Additional Information (Detail) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) SecurityLoan	Sep. 30, 2017 SecurityLoan	Sep. 30, 2018 USD ($) SecurityLoan	Sep. 30, 2017 SecurityLoan	Dec. 31, 2017 USD ($)	Sep. 14, 2018 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Period for calculating Loss migration rates of portfolio segments			3 years
Allowance for loan losses	$ 13,331		$ 13,331		$ 13,134
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.
Defaulted loans | SecurityLoan	0	0	0	0
Impaired loans			greater than $350
Loans acquired with credit deterioration						$ 878
Allowance for loan losses recorded for acquired loans			$ 30		126
Foreclosed assets	$ 0		0		16
Residential Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Residential mortgages in process of foreclosure	286		286		239
TDRs [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for loan losses	$ 171		$ 171		$ 169