Income Taxes - Benefit for Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Federal:
Current	$ 123		$ 20
Deferred	14,760	$ (16,024)	(893)
Gross Federal	14,883	(16,024)	(873)
State:
Current	184	328	255
Deferred	123	(152)	(24)
Gross State	307	176	231
Income tax expense (benefit)	$ 15,190	$ (15,848)	$ (642)