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Investment Securities - Additional Information (Detail)
 9 Months Ended
Sep. 30, 2017
USD ($)
Securities
Dec. 31, 2016
USD ($)
Investments, Debt and Equity Securities [Abstract]    
Number of securities with unrealized loss | Securities 65  
Percentage of gross unrealized loss 1.10%  
Percentage of amortized investment securities portfolio 0.60%  
Number of securities in continuous loss position | Securities 18  
Loss duration 12 months  
Unrealized loss on debt securities $ 441,000  
Amortized cost on debt securities in continuous loss 22,900,000  
Securities pledged to secure public deposits $ 102,401,000 $ 91,773,000