Income Taxes - Reconciliation of Income Tax Attributable to Pre-tax Income at Federal Statutory Tax Rates to Income Tax Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 8,448	$ 12,688	$ 9,699
Tax exempt income, net	(1,403)	(1,899)	(2,054)
Deferred tax adjustment related to reduction in U.S. federal statutory income tax rate	0	3,009	0
Bank owned life insurance	(296)	(581)	(379)
Merger costs	0	0	170
Other	(239)	(825)	(265)
Income tax expense	$ 6,510	$ 12,392	$ 7,171
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Tax at statutory rate	21.00%	35.00%	35.00%
Tax exempt income, net	(3.50%)	(5.20%)	(7.40%)
Deferred tax adjustment related to reduction in U.S. federal statutory income tax rate	0.00%	8.30%	0.00%
Bank owned life insurance	(0.70%)	(1.60%)	(1.30%)
Merger costs	0.00%	0.00%	0.60%
Other	(0.60%)	(2.30%)	(1.00%)
Income tax expense	16.20%	34.20%	25.90%