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Securities - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2017
USD ($)
Security
Obligation
Dec. 31, 2016
USD ($)
Security
Obligation
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]    
Number of obligations of state and political subdivisions in an unrealized loss position | Obligation 39 50
Number of obligations of state and political subdivisions | Obligation 80 78
Percentage of investment portfolio 100.00% 100.00%
Weighted-average period of re-pricing of portfolio 4 years 8 months 12 days 4 years 8 months 12 days
Proceeds from maturities, calls, principal payments and sales of securities available for sale $ 28,157,000 $ 22,826,000
Gross realized gains on securities available for sale 49,000 22,000
Gross realized losses on securities available for sale $ 152,000 $ 14,000
Available-for-sale securities, number of securities donated to not-for-profit organization | Security 1 0
Gross gain on charitable donations of available for sale securities $ 13,000  
Proceeds from maturities, calls and principal payments of securities held to maturity $ 4,981,000 $ 12,821,000
Held to maturity securities, number of securities sold | Security 0 1
Gross gains or losses on held to maturity securities $ 0 $ 0
Proceeds from sales of securities held to maturity   657,000
Securities book value 35,200,000 $ 27,700,000
Impairment recognized $ 0  
U.S. agency and mortgage-backed securities    
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]    
Number of securities in unrealized loss position | Security 68 64
Number of securities | Security 82 83
Corporate debt securities    
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]    
Number of securities in unrealized loss position | Security   1