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Asset Purchases - Additional Information (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 13, 2013
Business Acquisition [Line Items] |  |  |  |
Number of years amortized over an estimated remaining economic useful life of deposits | 7 years |  |  |
Amortization expense | $ 126 | $ 203 | $ 245 |
Amortization expense | 167 |  |  |
Hicksville [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Amortization expense |  |  | $ 78 |
Custar [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Amortization expense | $ 167 |  |  |
Custar [Member] | Core Deposits [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Recognition of core deposit intangible asset with acquired purchase |  |  |  | $ 1,170

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