Components of Income Tax Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Expenses [Line Items]
Current income tax expense, Federal	$ 10,481	$ 10,202	$ 10,181
Current income tax expense, State	2,308	2,788	1,983
Current income tax expense, Total	12,789	12,990	12,164
Deferred income tax expense (benefit), Federal	(3,422)	1,088	(3,400)
Deferred income tax expense (benefit), State	(1,014)	(167)	(511)
Deferred income tax expense (benefit), Total	(4,436)	921	(3,911)
Income tax expense	$ 8,353	$ 13,911	$ 8,253