Accumulated Other Comprehensive Loss - Activity within Accumulated Other Comprehensive 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	$ 754,917	$ 654,400	$ 581,405
Other comprehensive income (loss)	16,744	(22,524)	(51,693)
Ending Balance	764,597	754,917	654,400
Unrealized Gain on Available-for-Sale Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			22,977
Other comprehensive income (loss)			(22,977)
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(94,100)	(71,576)	(42,860)
Other comprehensive income (loss)	16,744	(22,524)	(28,716)
Ending Balance	(77,356)	(94,100)	(71,576)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	(94,100)	(71,576)	(19,883)
Other comprehensive income (loss)	16,744	(22,524)	(51,693)
Ending Balance	$ (77,356)	$ (94,100)	$ (71,576)