Fair Value - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net carrying amount other real estate owned and other repossessed property	$ 200		$ 68
Other real estate owned property write-downs	0	$ 300
Valuation allowance for loan servicing rights with impairments	17	20	15	$ 29
Recoveries on loan servicing rights	$ 2	(9)
Past due period of mortgage loans held for sale, days	90 days
Interest income for residential loans held for sale	$ 188	22
States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3	4,000
Transfers into Level 3		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	31,400		49,300
Valuation allowance	9,500		$ 12,900
Provision for loan losses expensed	$ 1,200
Recapture of provision loan		$ 1,600