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Loans and Allowance for Credit Losses - Additional Information (Detail)
 12 Months Ended
Dec. 31, 2017
USD ($)
Contract
Dec. 31, 2016
USD ($)
Contract
Dec. 31, 2015
USD ($)
Contract
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Provision for credit losses $ 5,087,000 $ 18,480,000 $ 14,948,000
Charge-offs     $ 18,868,000
Number of loans modified | Contract 41 65 61
Total amount of loan modified $ 8,274,000 $ 10,275,000 $ 10,956,000
Off balance sheet reserve to nonperforming loans   12,000  
Loans held for sale 14,850,000 7,052,000  
Total gains or losses included in earnings 4,329,000 589,000  
Loans with modification to rate and payment due to amortization $ 300,000 6,600,000 $ 3,700,000
Minimum days required for conversion of trouble debt to be in default 90 days    
Impaired Loans [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Unfunded commitments related to nonperforming loans $ 2,400,000 1,779,000  
Consumer Loan [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Non Accrual Status Of Loans After Number Of Days Past Due 150 days    
Nonaccrual Commercial Loans [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Gain (loss) on sale of impaired loans $ 21,000 $ 0  
Reserve for Off-balance Sheet Activities [Member] | Impaired Loans [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Off balance sheet reserve to nonperforming loans $ 200,000