Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Recurring basis
Financial instruments carried at fair value (asset position):
Total financial assets	$ 1,637,948	$ 1,108,253
Financial instruments carried at fair value (liability position):
Total financial liabilities	(2,247)	(14,508)
Recurring basis \| Level 1
Financial instruments carried at fair value (asset position):
Total financial assets	1,238,312	704,753
Financial instruments carried at fair value (liability position):
Total financial liabilities	0	0
Recurring basis \| Level 2
Financial instruments carried at fair value (asset position):
Total financial assets	399,636	403,500
Financial instruments carried at fair value (liability position):
Total financial liabilities	(2,247)	(14,508)
Recurring basis \| Level 3
Financial instruments carried at fair value (asset position):
Total financial assets	0	0
Financial instruments carried at fair value (liability position):
Total financial liabilities	0	0
Prepaid and other current assets \| Foreign currency forward contracts \| Recurring basis
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	10,812	13
Prepaid and other current assets \| Foreign currency forward contracts \| Recurring basis \| Level 1
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	0	0
Prepaid and other current assets \| Foreign currency forward contracts \| Recurring basis \| Level 2
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	10,812	13
Prepaid and other current assets \| Foreign currency forward contracts \| Recurring basis \| Level 3
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	0	0
Other assets \| Foreign currency forward contracts \| Recurring basis
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	1,451
Other assets \| Foreign currency forward contracts \| Recurring basis \| Level 1
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	0
Other assets \| Foreign currency forward contracts \| Recurring basis \| Level 2
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	1,451
Other assets \| Foreign currency forward contracts \| Recurring basis \| Level 3
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	0
Other liabilities, current portion \| Foreign currency forward contracts \| Recurring basis
Financial instruments carried at fair value (liability position):
Derivative liability current	(1,948)	(13,642)
Other liabilities, current portion \| Foreign currency forward contracts \| Recurring basis \| Level 1
Financial instruments carried at fair value (liability position):
Derivative liability current	0	0
Other liabilities, current portion \| Foreign currency forward contracts \| Recurring basis \| Level 2
Financial instruments carried at fair value (liability position):
Derivative liability current	(1,948)	(13,642)
Other liabilities, current portion \| Foreign currency forward contracts \| Recurring basis \| Level 3
Financial instruments carried at fair value (liability position):
Derivative liability current	0	0
Other liabilities, excluding current portion \| Foreign currency forward contracts \| Recurring basis
Financial instruments carried at fair value (liability position):
Derivative liability, noncurrent	(299)	(866)
Other liabilities, excluding current portion \| Foreign currency forward contracts \| Recurring basis \| Level 1
Financial instruments carried at fair value (liability position):
Derivative liability, noncurrent	0	0
Other liabilities, excluding current portion \| Foreign currency forward contracts \| Recurring basis \| Level 2
Financial instruments carried at fair value (liability position):
Derivative liability, noncurrent	(299)	(866)
Other liabilities, excluding current portion \| Foreign currency forward contracts \| Recurring basis \| Level 3
Financial instruments carried at fair value (liability position):
Derivative liability, noncurrent	0	0
Money market funds \| Recurring basis
Financial instruments carried at fair value (asset position):
Cash equivalents	971,044	614,951
Money market funds \| Recurring basis \| Level 1
Financial instruments carried at fair value (asset position):
Cash equivalents	971,044	614,951
Money market funds \| Recurring basis \| Level 2
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Money market funds \| Recurring basis \| Level 3
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Government-sponsored enterprise securities \| Recurring basis
Financial instruments carried at fair value (asset position):
Cash equivalents	10,982	12,678
Marketable securities	2,491	2,303
Government-sponsored enterprise securities \| Recurring basis \| Level 1
Financial instruments carried at fair value (asset position):
Cash equivalents	10,982	12,678
Marketable securities	2,491	2,303
Government-sponsored enterprise securities \| Recurring basis \| Level 2
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Marketable securities	0	0
Government-sponsored enterprise securities \| Recurring basis \| Level 3
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Marketable securities	0	0
Corporate debt securities \| Recurring basis
Financial instruments carried at fair value (asset position):
Marketable securities	232,704	265,867
Corporate debt securities \| Recurring basis \| Level 1
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
Corporate debt securities \| Recurring basis \| Level 2
Financial instruments carried at fair value (asset position):
Marketable securities	232,704	265,867
Corporate debt securities \| Recurring basis \| Level 3
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
Commercial paper \| Recurring basis
Financial instruments carried at fair value (asset position):
Cash equivalents	21,263	57,357
Marketable securities	133,406	80,263
Commercial paper \| Recurring basis \| Level 1
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Marketable securities	0	0
Commercial paper \| Recurring basis \| Level 2
Financial instruments carried at fair value (asset position):
Cash equivalents	21,263	57,357
Marketable securities	133,406	80,263
Commercial paper \| Recurring basis \| Level 3
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Marketable securities	0	0
Corporate equity securities \| Recurring basis
Financial instruments carried at fair value (asset position):
Marketable securities	242,775	74,821
Corporate equity securities \| Recurring basis \| Level 1
Financial instruments carried at fair value (asset position):
Marketable securities	242,775	74,821
Corporate equity securities \| Recurring basis \| Level 2
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
Corporate equity securities \| Recurring basis \| Level 3
Financial instruments carried at fair value (asset position):
Marketable securities	0	$ 0
US Treasury Securities \| Recurring basis
Financial instruments carried at fair value (asset position):
Marketable securities	11,020
US Treasury Securities \| Recurring basis \| Level 1
Financial instruments carried at fair value (asset position):
Marketable securities	11,020
US Treasury Securities \| Recurring basis \| Level 2
Financial instruments carried at fair value (asset position):
Marketable securities	0
US Treasury Securities \| Recurring basis \| Level 3
Financial instruments carried at fair value (asset position):
Marketable securities	0
Variable Interest Entity, Primary Beneficiary \| Level 1
Financial instruments carried at fair value (liability position):
Cash and cash equivalents	$ 5,000

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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