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Acquired SJB Assets and FDIC Loss Sharing Asset - Additional Information (Detail) - USD ($)
 9 Months Ended
Oct. 16, 2009
Sep. 30, 2015
Dec. 31, 2014
Business Combinations [Abstract]      
Acquisition date   Oct. 16, 2009  
After-tax gain included in earnings due to purchase method $ 12,300,000    
Remaining discount associated with SJB loans   $ 4,800,000  
Expected cash flows   $ 2,800,000  
Remaining average lives of respective pools loans   3 years 2 months 12 days  
Remaining average lives of respective individual loans   10 months 24 days  
Allowance for PCI loans losses   $ 0 $ 0