Securities - Additional Information (Detail) $ in Thousands	Sep. 30, 2018 USD ($) Security	Dec. 31, 2017 USD ($) Security
Schedule of Available-for-sale Securities [Line Items]
Fair value of securities	$ 18,300	$ 19,400
Debt Securities, Available-for-sale, Restriction Type [Extensible List]	us-gaap:CollateralPledgedMember	us-gaap:CollateralPledgedMember
Securities sold under repurchase agreements	$ 6,083	$ 9,498	[1]
Company's investment in Federal Home Loan Bank stock	4,300	3,700
Company's investment in Federal Reserve Bank stock	$ 2,300	$ 1,900
U.S. Government Agencies
Schedule of Available-for-sale Securities [Line Items]
Securities with unrealized loss positions | Security	56	36
State and Municipal Obligations
Schedule of Available-for-sale Securities [Line Items]
Securities with unrealized loss positions | Security	49	34
Corporate Bonds
Schedule of Available-for-sale Securities [Line Items]
Securities with unrealized loss positions | Security	2	1

[1] Derived from audited December 31, 2017 Consolidated Financial Statements.