Fair Value - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net carrying amount other real estate owned and other repossessed property	$ 700		$ 700			$ 1,800
Other real estate owned property write-downs	100	$ 300	500	$ 2,000
Valuation allowance for loan servicing rights with impairments	12	54	12	54	$ 15	29	$ 75	$ 68
Additions (recoveries) on loan servicing rights	(3)	(21)	$ (17)	(14)
Past due period of mortgage loans held for sale, days			90 days
Interest income for residential loans held for sale	200	56	$ 300	100
States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Pooled trust preferred securities	3,979		3,979
States and Political Subdivisions [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Pooled trust preferred securities	4,100		4,100
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	48,200		48,200			38,600
Valuation allowance	10,600		10,600			$ 10,100
Provision for loan losses expensed	$ 1,200		$ 7,700	$ 5,400
Recapture of provision loan		$ 800