Annual report pursuant to Section 13 and 15(d)

Off-Balance Sheet Commitments - Distribution of Undisbursed Loan Commitments (Detail)

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Off-Balance Sheet Commitments - Distribution of Undisbursed Loan Commitments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Loss Contingencies [Line Items]    
Total undisbursed loan commitments $ 347,236 $ 330,871
Commitments to extend credit [Member]    
Loss Contingencies [Line Items]    
Total undisbursed loan commitments 318,634 310,987
Standby letters of credit [Member]    
Loss Contingencies [Line Items]    
Total undisbursed loan commitments 19,294 15,669
Commercial letters of credit [Member]    
Loss Contingencies [Line Items]    
Total undisbursed loan commitments $ 9,308 $ 4,215