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Securities - Additional Information (Detail) | 12 Months Ended
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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

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- Definition Impairment losses related to federal home loan stock. + References No definition available. + Details
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