Loans - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Jan. 31, 2017 USD ($) Loan	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) Loan	Dec. 31, 2015 USD ($) Loan	Dec. 31, 2014 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Outstanding standby letters of credit		$ 239,000	$ 1,000,000
Outstanding commitments to fund the Overdraft Privilege Program		$ 54,400,000	$ 50,500,000
Evaluation period of net charge-off history		66 months	54 months
Total unpaid principal balance outstanding		$ 19,083,000	$ 24,114,000
Protracted litigation and reserve				$ 546,000
Financing receivables, asset written down through other expenses		2,300,000
Amounts charged off		$ 3,598,000	5,634,000	3,826,000
Reduction in stated interest rate of loan, minimum outstanding period		6 months
Reduction in stated interest rate of loan, maximum outstanding period		2 years
Allowance for loan losses		$ 21,202,000	19,087,000	17,712,000	$ 17,687,000
Aggregate nonaccrual TDR loans		11,703,000	12,439,000
Loans modified as TDRs which are payment defaults		$ 0	$ 3,606,000	$ 77,000
Troubled debt restructuring period past due considered payment default		30 days
Number of loans | Loan			2	4
Period of cumulative homogeneous loans past due included in company analysis		90 days
Maximum duration under which loans may be housed under Special Mention category		No Longer Than 12 Months
Purchased Credit Impaired Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance for loan losses		$ 55,000
purchased credit impaired loans allowance for loan losses		55,000
TDRs [Member]
Accounts Notes And Loans Receivable [Line Items]
Increment in allowance for loan losses		7,000	$ 39,000	$ 135,000
Amounts charged off		0	0	0
Recorded investment in trouble debt restructuring		19,800,000	26,600,000
Allowance for loan losses		1,600,000	3,000,000
Commitment to lend additional amounts		37,000	31,000
Aggregate nonaccrual TDR loans		2,400,000	6,600,000
Aggregate accrual TDR loans		17,900,000	20,000,000
TDRs with Subsequent Default [Member]
Accounts Notes And Loans Receivable [Line Items]
Increment in allowance for loan losses			820,000	0
Amounts charged off			$ 350,000	0
Loans modified as TDRs which are payment defaults	$ 0
Number of loans | Loan	0
Secured [Member] | Nonresidential [Member]
Accounts Notes And Loans Receivable [Line Items]
Total unpaid principal balance outstanding		$ 4,100,000		$ 7,000,000
Minimum [Member]
Accounts Notes And Loans Receivable [Line Items]
Interest rate of fixed rate loan		2.75%
Period of maturity of fixed rate loan		3 months
Minimum [Member] | TDRs [Member]
Accounts Notes And Loans Receivable [Line Items]
Extension of the maturity date		6 months
Maximum [Member]
Accounts Notes And Loans Receivable [Line Items]
Interest rate of fixed rate loan		18.00%
Period of maturity of fixed rate loan		30 years
Maximum [Member] | TDRs [Member]
Accounts Notes And Loans Receivable [Line Items]
Extension of the maturity date		10 years