Allowance for Loan Losses - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Originated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Interest income on non accrual loans	$ 338,000	$ 244,000	$ 964,000	$ 617,000
Interest income on non accrual loans ,recognized	59,000	33,000	133,000	49,000
Impaired TDR Loans	8,845,000	13,352,000	8,845,000	13,352,000	$ 12,517,000
Obligations to lend additional funds on TDR	0	209,000	0	209,000	1,000
PNCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Interest income on non accrual loans	39,000	90,000	93,000	188,000
Interest income on non accrual loans ,recognized	12,000	2,000	23,000	14,000
Impaired TDR Loans	840,000	1,611,000	840,000	1,611,000	1,352,000
Obligations to lend additional funds on TDR	$ 0	$ 3,000	$ 0	$ 3,000	$ 0