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Credit Quality and Allowance for Loan Losses - Summary of Information Pertaining to Non Accrual Non Covered Loans (Parenthetical) (Detail) - USD ($)
Sep. 30, 2017
Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Nonaccrual loans $ 17,701,000 $ 13,753,000
Originated Loans [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Nonaccrual loans 16,481,000 12,290,000
Nonaccrual loans restructured 8,900,000 10,000,000
Acquired Loans [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Nonaccrual loans 1,220,000 1,463,000
Nonaccrual loans restructured 457,000 435,000
Acquired Loans [Member] | ASC 310-30 [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Nonaccrual loans $ 1,500,000 $ 2,700,000