Loans - Additional Information (Detail)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) Loan	Jun. 30, 2017 USD ($) Loan	Jun. 30, 2018 USD ($) Loan	Jun. 30, 2017 USD ($) Loan	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Accounts Notes And Loans Receivable [Line Items]
Allowance for loan losses	$ 21,405,000	$ 19,660,000	$ 21,405,000	$ 19,660,000	$ 21,610,000	$ 21,202,000	$ 18,970,000	$ 19,087,000
Amounts charged off	$ 254,000	435,000	$ 525,000	$ 2,305,000
Number of loans modified as troubled debt restructurings | Loan	5		6	2
Troubled debt restructuring period past due considered payment default				30 days
Period of cumulative homogeneous loans past due included in company analysis			90 days
Purchased Credit Impaired Loans [Member]
Accounts Notes And Loans Receivable [Line Items]
Allowance for loan losses	$ 55,000		$ 55,000			55,000
Increment in allowance for loan losses	0	$ 0	0	$ 0
Troubled Debt Restructuring [Member]
Accounts Notes And Loans Receivable [Line Items]
Recorded investment in trouble debt restructuring	18,200,000		18,200,000			19,800,000
Allowance for loan losses	1,200,000		1,200,000			1,600,000
Commitment to lend additional amounts	39,000		39,000			$ 37,000
Increment in allowance for loan losses	3,000		3,000	6,000
Amounts charged off	0		0	0
Number of loans modified as troubled debt restructurings | Loan		0
TDRs with Subsequent Default [Member]
Accounts Notes And Loans Receivable [Line Items]
Increment in allowance for loan losses	0	$ 0	0	0
Amounts charged off	$ 0	$ 0	$ 0	$ 0