Summary of Significant Accounting Policies - Schedule of Components of Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ (7,573)
Foreign currency translation before reclassification			$ (3,505)
Foreign currency translation	5,593	$ (2,764)	(3,187)
Reclassification adjustment for write-off of foreign currency translation loss			318
Unrealized gain (loss) on investments, net of tax	(32)	17	(26)
Ending Balance	(2,012)	(7,573)
Beginning Balance	150,036	127,302	141,602
Ending Balance	141,189	150,036	127,302
Net Unrealized Gain (Loss) on Marketable Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(83)	(100)	(74)
Unrealized gain (loss) on investments, net of tax	(32)	17	(26)
Ending Balance	(115)	(83)	(100)
Cumulative Translation Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(7,490)	(4,726)	(1,539)
Foreign currency translation before reclassification			(3,505)
Foreign currency translation	5,593	(2,764)
Reclassification adjustment for write-off of foreign currency translation loss			318
Ending Balance	(1,897)	(7,490)	(4,726)
Total Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Foreign currency translation	5,593	(2,764)	(3,187)
Beginning Balance	(7,573)	(4,826)	(1,613)
Ending Balance	$ (2,012)	$ (7,573)	$ (4,826)