Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 72,187	$ 30,845	$ 36,138
Gross increases related to prior year tax positions	6,095	570	0
Gross decreases related to prior year tax positions	0	0	(8,645)
Gross increases related to current year tax positions	19,832	41,972	4,802
Decreases resulting from expiration of statute of limitation	(15,410)	(826)	(1,450)
Decreases resulting from settlements	(314)	(374)	0
Ending balance	$ 82,390	$ 72,187	$ 30,845