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Commitments and Letters of Credit - Additional Information (Detail) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Loss Contingencies [Line Items]    
Notional amount of financial standby letters of credit $ 1,500,000  
Notional amount of performance standby letters of credit 5,100,000  
Notional amount of commercial letters of credit 313,000  
Financial instrument of credit risk 198,000 $ 200,000
Unused commitments and letters of credit [Member]    
Loss Contingencies [Line Items]    
Commitment liability $ 5,200,000 $ 4,100,000