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Securities - Narrative (Detail) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 USD ($) Security | Dec. 31, 2017 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 10 months 12 days | 4 years 8 months 12 days
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 | 86 | 68
Total number of securities | 90 | 82
Obligations of states and political subdivisions |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Number of securities in an unrealized loss position | 66 | 39
Total number of securities | 81 | 80
Corporate debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Number of securities in an unrealized loss position | 1 |

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