Income Taxes - Components of Consolidated Income Tax Expense (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ 22,477	$ 35,602	$ 28,047
State	732	1,603	1,012
Total Current	23,209	37,205	29,059
Deferred:
Federal	12,965	(2,120)	1,343
State	(165)	(129)	103
Total Deferred	12,800	(2,249)	1,446
Income tax expense, Total	$ 36,009	$ 34,956	$ 30,505