Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	$ 1,338,191	$ 1,093,628	$ 1,096,183
Other comprehensive (loss) income before reclassifications	(35,390)	28,996	5,633
Amounts reclassified from accumulated other comprehensive income (loss)	2,645	(9,647)	(4,726)
Total changes in other comprehensive (loss) income	(32,745)	19,349	907
Balance	2,042,306	1,338,191	1,093,628
Foreign currency translation adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	(7,862)	(2,080)	(971)
Other comprehensive (loss) income before reclassifications	(13,169)	(5,782)	(1,109)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0
Total changes in other comprehensive (loss) income	(13,169)	(5,782)	(1,109)
Balance	(21,031)	(7,862)	(2,080)
Unrealized holding gains (losses) on marketable securities, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	17,521	126	(123)
Other comprehensive (loss) income before reclassifications	6,954	17,395	249
Amounts reclassified from accumulated other comprehensive income (loss)	0	0	0
Total changes in other comprehensive (loss) income	6,954	17,395	249
Balance	24,475	17,521	126
Unrealized (losses) gains on foreign currency forward contracts, net of tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	11,514	3,778	2,011
Other comprehensive (loss) income before reclassifications	(29,175)	17,383	6,493
Amounts reclassified from accumulated other comprehensive income (loss)	2,645	(9,647)	(4,726)
Total changes in other comprehensive (loss) income	(26,530)	7,736	1,767
Balance	(15,016)	11,514	3,778
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance	21,173	1,824	917
Total changes in other comprehensive (loss) income	(32,745)	19,349	907
Balance	$ (11,572)	$ 21,173	$ 1,824