Fair Value - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 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	$ 800		$ 800		$ 1,800
Other real estate owned property write-downs	100	$ 800	400	$ 1,600
Valuation allowance for loan servicing rights with impairments	15	75	15	75	29	$ 68
Additions (recoveries) on loan servicing rights	$ (4)	25	(14)	7
Past due period of mortgage loans held for sale, days	90 days
Interest income for residential loans held for sale	$ 40	41	62	69
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	37,100		37,100		38,600
Valuation allowance	12,700		12,700		$ 10,100
Provision for loan losses expensed	4,900	$ 5,100	6,500	$ 6,200
States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Pooled trust preferred securities	4,028		4,028
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