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Accounting for Certain Loans Acquired in a Transfer - Amounts of Loans (Parenthetical) (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Allowance for loan losses $ 16,394 $ 14,837 $ 14,534 $ 16,501
Loans Purchased With Evidence of Credit Deterioration [Member]        
Accounts, Notes, Loans and Financing Receivable [Line Items]        
Allowance for loan losses $ 0 $ 0