Income Taxes - Reconciliation of Change in Tax Liability for Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 14,393	$ 13,070
Adjustments for tax positions related to the current year	1,921	2,359
Adjustments for tax positions related to prior years	(766)	(856)
Statute expiration	(493)	(180)
Ending balance	$ 15,055	$ 14,393