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Securities - Additional Information (Detail) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2017 Security | Sep. 30, 2016 Security | Sep. 30, 2017 USD ($) Security | Sep. 30, 2016 USD ($) Security | Dec. 31, 2016 Security
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Percentage of investment portfolio |  |  | 100.00% |  | 100.00%
Weighted-average period of re-pricing of portfolio |  |  | 4 years 3 months 12 days |  | 4 years 8 months 12 days
Securities sold from the held to maturity portfolio | 0 | 0 | 0 | 1 |
Proceeds from sale | $ |  |  |  | $ 657,000 |
Impairment recognized | $ |  |  | $ 0 |  |
U.S. agency and mortgage-backed securities |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Number of securities in an unrealized loss position | 50 |  | 50 |  | 64
Total number of securities | 86 |  | 86 |  | 83
Obligations of states and political subdivisions |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Number of securities in an unrealized loss position | 11 |  | 11 |  | 50
Total number of securities | 82 |  | 82 |  | 78
Corporate debt securities |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |
Number of securities in an unrealized loss position | 1 |  | 1 |  | 1

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