Fair Value - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net carrying amount other real estate owned and other repossessed property	$ 1,800	$ 1,700
Other real estate owned property write-downs	2,500	3,000
Valuation allowance for loan servicing rights with impairments	29	68	$ 34
(Additions)/recoveries on loan servicing rights	$ (39)	34
Past due period of mortgage loans held for sale, days	90 days
Interest income for residential loans held for sale	$ 200	100
Impaired Commercial and Commercial Real Estate Loans [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Principal amount of impaired commercial and commercial real estate loans	38,600	26,400
Valuation allowance	10,100	8,000
Provision for loan losses expensed	$ 5,200
Recapture of provision loan		$ 1,100