Income Taxes - Summary of Income Tax Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Current - federal	$ 8,978	$ 6,181	$ 7,777
Current - state	29	(33)	139
Deferred - federal	376	1,023	376
Deferred tax adjustment related to reduction in U.S. federal statutory income tax rate	3,009	0	0
Income tax expense	$ 12,392	$ 7,171	$ 8,292