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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