Components of Income Tax Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expenses [Line Items]
Current income tax expense, Federal	$ 10,202	$ 10,181	$ 2,519
Current income tax expense, State	2,788	1,983	136
Current income tax expense, Total	12,990	12,164	2,655
Deferred income tax expense (benefit), Federal	1,088	(3,400)	5,589
Deferred income tax expense (benefit), State	(167)	(511)	117
Deferred income tax expense (benefit), Total	921	(3,911)	5,706
Income tax expense	$ 13,911	$ 8,253	$ 8,361