Stockholders' Equity - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2015
Equity [Abstract]
Foreign currency translation adjustments, net of tax	$ (142,255)	$ (280,426)
Net unrealized gain (loss) on available for sale securities, net of tax	(6,678)	27
Accumulated other comprehensive loss	(148,933)	(280,399)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net unrealized loss on available-for-sale-securities before tax	9,000
2.5% Senior Notes Due 2022
Accumulated Other Comprehensive Income (Loss) [Line Items]
Foreign currency translation gains (losses), net of tax	45,000	(16,000)
Foreign currency translation gains (losses), before tax	$ 71,000	$ (25,000)
Senior notes, interest rate	2.50%	2.50%	2.50%	2.50%