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Loans - Additional Information (Detail)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
Loans
Sep. 30, 2016
USD ($)
Loans
Sep. 30, 2017
USD ($)
Loans
Sep. 30, 2016
USD ($)
Loans
Dec. 31, 2016
USD ($)
Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Net unamortized loan (fees) costs $ (2,744,000)   $ (2,744,000)   $ (1,507,000)
Percentage of comprised loan, commercial, industrial and agricultural loans 32.00%   32.00%   30.00%
Percentage of comprised loan, commercial mortgage loans 31.00%   31.00%   31.00%
Percentage of loan-to-value to business equipment     80.00%    
Percentage of loan-to-value to accounts receivable     75.00%    
Percentage of loan-to-value to business inventory     60.00%    
Percentage of loan-to-value to real estate     85.00%    
Percentage of residential real estate loan in portfolio 33.00%   33.00%   35.00%
Number of Loans | Loans     18   15
Minimum period to be considered for loan to have defaulted     90 days    
Allowance for loan losses, period increase (decrease) $ 169,000 $ 0 $ 1,324,000 $ 0  
Trouble Debt Restructuring [Member]          
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Number of Loans | Loans 4 0 4 0  
Maximum [Member]          
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Maximum percentage of consumer loan and credit card in portfolio 10.00%   10.00%   10.00%
Minimum [Member] | Commercial And Commercial Real Estate [Member]          
Accounts, Notes, Loans and Financing Receivable [Line Items]          
Individually evaluated for impairment threshold     $ 250,000