CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - UNAUDITED - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 16,101	$ 14,580	$ 45,232	$ 46,398
Other comprehensive income, net of tax:
Foreign currency translation adjustments	(109)	5,609	(8,630)	15,173
Unrealized holding gains on short-term investments arising during period	107	7	81	154
Other comprehensive income (loss), net of tax	(2)	5,616	(8,549)	15,327
Total comprehensive income, net of tax	$ 16,099	$ 20,196	$ 36,683	$ 61,725