Loans - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Accounts Notes And Loans Receivable [Line Items]
Allowance for loan losses	$ 21,610,000	$ 18,970,000	$ 21,202,000	$ 19,087,000
Amounts charged off	$ 271,000	$ 1,870,000
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
Increment in allowance for loan losses	0
Troubled Debt Restructuring [Member]
Accounts Notes And Loans Receivable [Line Items]
Recorded investment in trouble debt restructuring	18,800,000		19,800,000
Allowance for loan losses	1,300,000		1,600,000
Commitment to lend additional amounts	38,000		$ 37,000
Increment in allowance for loan losses	0	$ 6,000
Amounts charged off	0	0
TDRs with Subsequent Default [Member]
Accounts Notes And Loans Receivable [Line Items]
Increment in allowance for loan losses	0	0
Amounts charged off	$ 0	$ 0