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Accounting for Certain Loans Acquired in a Transfer - Amounts of Loans (Parenthetical) (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Jun. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Allowance for loan losses $ 15,586 $ 15,027 $ 14,837 $ 14,524 $ 14,226 $ 14,534
Loans Purchased With Evidence of Credit Deterioration [Member]            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Allowance for loan losses $ 71   $ 0