Income Taxes - Summary of Reconciliation of Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Gross Unrecognized Tax Benefits
Beginning balance	$ 16,144	$ 19,892	$ 18,647
Additions based on tax positions related to prior years	1,121	3,958	1,245
Settlements with taxing authorities	(13,027)	(4,820)	0
Reductions due to expiration of statute of limitations	0	(2,886)	0
Ending balance	$ 4,238	$ 16,144	$ 19,892