CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 61,364	$ 62,882	$ 62,567
Other comprehensive income (loss), net of tax:
Foreign currency translation adjustments	16,479	(18,925)	(12,902)
Unrealized holding gains (losses) on short-term investments arising during period	71	(62)	(103)
Other comprehensive income (loss), net of tax	16,550	(18,987)	(13,005)
Total comprehensive income, net of tax	$ 77,914	$ 43,895	$ 49,562