Income Taxes - Schedule of Changes in Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Unrecognized tax benefits, beginning balance	$ 151,100	$ 142,845	$ 140,889
Additions based on tax positions related to current	8,598	9,043	8,876
Additions for tax positions of prior years	427	68	82
Reductions for tax positions of prior years	(31)	(856)	(2,817)
Decreases related to settlements of prior year tax positions	(54)		(4,185)
Unrecognized tax benefits, ending balance	$ 160,040	$ 151,100	$ 142,845