Loans - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Loans	Mar. 31, 2017 Loans	Dec. 31, 2017 USD ($) Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net unamortized loan fees | $	$ 3,211,000		$ 2,574,000
Percentage of comprised loan, commercial, industrial and agricultural loans	34.00%		35.00%
Percentage of comprised loan, commercial mortgage loans	31.00%		28.00%
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	32.00%		33.00%
Number of Loans | Loans	19		19
Minimum period to be considered for loan to have defaulted	90 days
Principal balances forgiven in connection with loan restructuring | $	$ 0		$ 0
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	0	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%