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Regulatory Matters - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2019
Jan. 01, 2018
Jan. 01, 2017
Jan. 01, 2016
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]            
Capital conservation buffer period 4 years          
Cash dividends on common stock $ 692 $ 591        
Capital conservation buffer percentage 1.25% 0.625%     0.625% 0.625%
Capital conservation buffer percentage maintained at bank 5.12% 5.47%        
Scenario, Forecast            
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]            
Capital conservation buffer percentage     0.625% 0.625%    
First Bank            
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]            
Cash dividends on common stock $ 5,500