Income Taxes - Schedule of Changes to 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	$ 203,936	$ 99,127	$ 41,377
Increases in balances related to prior year tax positions		28,677	6,626
Decrease in balances related to prior year tax positions	(31,493)
Increases in balances related to current year tax positions	84,352	62,805	51,124
Assumed uncertain tax positions through acquisition		13,327
Change in balances related to effect of U.S. tax law change	(58,050)
Unrecognized tax benefits, ending balance	$ 198,745	$ 203,936	$ 99,127