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Asset Purchases - Additional Information (Detail) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 13, 2013 | Jul. 09, 2010
Business Acquisition [Line Items] |  |  |  |  |  |
Amortization expense | $ 242 | $ 360 |  | $ 480 |  |
Scenario, Forecast [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Amortization expense |  |  | $ 323 |  |  |
Custar [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Recognition of core deposit intangible asset with acquired purchase |  |  |  |  | $ 1,170 |
Number of years amortized over an estimated remaining economic useful life of deposits | 7 years |  |  |  |  |
Hicksville [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Recognition of core deposit intangible asset with acquired purchase |  |  |  |  |  | $ 1,090
Number of years amortized over an estimated remaining economic useful life of deposits | 7 years |  |  |  |  |

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