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Asset Purchases - Additional Information (Detail) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Jul. 09, 2010
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 13, 2013
Business Acquisition [Line Items]          
Amortization expense   $ 203 $ 242 $ 323  
Amortization expense 2017   $ 245   $ 245  
Hicksville [Member]          
Business Acquisition [Line Items]          
Number of years amortized over an estimated remaining economic useful life of deposits   7 years      
Amortization expense 2017   $ 78      
Hicksville [Member] | Core Deposits [Member]          
Business Acquisition [Line Items]          
Recognition of core deposit intangible asset with acquired purchase $ 1,090        
Custar [Member]          
Business Acquisition [Line Items]          
Number of years amortized over an estimated remaining economic useful life of deposits         7 years
Amortization expense 2017   $ 167      
Custar [Member] | Core Deposits [Member]          
Business Acquisition [Line Items]          
Recognition of core deposit intangible asset with acquired purchase         $ 1,170