Income Taxes - Income Tax Provision (Benefit) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current income tax provision:
Federal	$ 41,177	$ 26,752	$ 25,105
State	5,420	2,798	2,560
Total	46,597	29,550	27,665
Deferred income tax provision:
Federal	1,177	5,217	987
State	(983)	216	37
Total	194	5,433	1,024
Total income tax provision	$ 46,791	$ 34,983	$ 28,689