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, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax at statutory rate, value	$ 12,688	$ 9,699	$ 10,671
Tax exempt income, net value	(1,899)	(2,054)	(1,816)
Deferred tax adjustment related to reduction in U.S. federal statutory income tax rate, value	3,009	0	0
Bank owned life insurance, value	(581)	(379)	(418)
Merger costs, value	0	170	30
Other, value	(825)	(265)	(175)
Income tax expense	$ 12,392	$ 7,171	$ 8,292
Tax at statutory rate, percent	35.00%	35.00%	35.00%
Tax exempt income, net percent	(5.20%)	(7.40%)	(6.00%)
Deferred tax adjustment related to reduction in U.S. federal statutory income tax rate, percent	8.30%	0.00%	0.00%
Bank owned life insurance, percent	(1.60%)	(1.30%)	(1.40%)
Merger costs, percent	0.00%	0.60%	0.10%
Other, percent	(2.30%)	(1.00%)	(0.50%)
Income tax expense, percent	34.20%	25.90%	27.20%