Loans - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Loans	Dec. 31, 2016 USD ($) Loans	Dec. 31, 2015 USD ($) Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net unamortized loan (fees) costs	$ (2,574,000)	$ (1,507,000)
Percentage of comprised loan, commercial, industrial and agricultural loans	35.00%	30.00%
Percentage of comprised loan, commercial mortgage loans	28.00%	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%	35.00%
Allowance for loan losses, period increase (decrease)	$ 4,024,000	$ 0	$ 0
Minimum period to be considered for loan to have defaulted	90 days
Principal balances forgiven in connection with loan restructuring	$ 0	0
Loans modified in a troubled debt restructuring	$ 17,303,000	$ 11,831,000
Number of Loans | Loans	19	15
Charge-offs	$ 4,077,000	$ 4,961,000	$ 3,718,000
Outstanding balance of semi annual loans	Greater than $1 million
Outstanding balance of annual loans	Less than $1 million
Trouble Debt Restructuring [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | Loans	6	1	1
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maximum percentage of consumer loan and credit card in portfolio	10.00%	10.00%
Reduction in stated interest rate modification having loan range period	15 years
Extension of maturity date modification having loan range period	18 years
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reduction in stated interest rate modification having loan range period	4 years
Extension of maturity date modification having loan range period	4 years
Commercial and Industrial [Member] | Trouble Debt Restructuring [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans modified in a troubled debt restructuring	$ 2,750,000	$ 109,000	$ 1,327,000
Number of Loans | Loans	4	1	1
Additional provision for loan losses	$ 129,000	$ 0	$ 0
Charge-offs	0	0	0
Commercial Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans modified in a troubled debt restructuring	$ 12,090,000	$ 9,191,000
Number of Loans | Loans	9	8
Charge-offs	$ 116,000	$ 201,000	$ 486,000
Commercial Mortgages [Member] | Trouble Debt Restructuring [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans modified in a troubled debt restructuring	$ 6,421,000
Number of Loans | Loans	2	0	0
Additional provision for loan losses	$ 3,895,000
Charge-offs	$ 0