Income Taxes - Schedule of Gross Unrecognized Tax Benefits (Excluding Interest and Penalties) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Beginning Balance	$ 18,384	$ 15,763	$ 15,597
Additions for current year tax positions	787	77	332
Increases (decreases) for prior year tax positions	3,361	2,551	(86)
Expiration of statutes	(10)	(7)	(80)
Ending Balance	$ 22,522	$ 18,384	$ 15,763