Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financial instruments carried at fair value (asset position):
Total financial assets	$ 1,836,605	$ 1,108,253
Total financial liabilities	(448)	(14,508)
Money market funds
Financial instruments carried at fair value (asset position):
Cash equivalents	1,226,603	614,951
U.S. Treasury securities
Financial instruments carried at fair value (asset position):
Cash equivalents	5,966
Marketable securities	6,026
Government-sponsored enterprise securities
Financial instruments carried at fair value (asset position):
Cash equivalents	7,123	12,678
Marketable securities	10,704	2,303
Commercial paper
Financial instruments carried at fair value (asset position):
Cash equivalents	58,268	57,357
Marketable securities	100,390	80,263
Corporate equity securities
Financial instruments carried at fair value (asset position):
Marketable securities	167,323	74,821
Corporate debt securities
Financial instruments carried at fair value (asset position):
Marketable securities	233,665	265,867
Prepaid and other current assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Prepaid and other current assets	19,023	13
Other assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other assets	1,514
Other liabilities, current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	(340)	(13,642)
Other liabilities, excluding current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, excluding current portion	(108)	(866)
Level 1
Financial instruments carried at fair value (asset position):
Total financial assets	1,410,155	704,753
Total financial liabilities	0	0
Level 1 \| Money market funds
Financial instruments carried at fair value (asset position):
Cash equivalents	1,226,603	614,951
Level 1 \| U.S. Treasury securities
Financial instruments carried at fair value (asset position):
Cash equivalents	5,966
Marketable securities	6,026
Level 1 \| Government-sponsored enterprise securities
Financial instruments carried at fair value (asset position):
Cash equivalents	7,123	12,678
Marketable securities	10,704	2,303
Level 1 \| Commercial paper
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Marketable securities	0	0
Level 1 \| Corporate equity securities
Financial instruments carried at fair value (asset position):
Marketable securities	153,733	74,821
Level 1 \| Corporate debt securities
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
Level 1 \| Prepaid and other current assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Prepaid and other current assets	0	0
Level 1 \| Other assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other assets	0
Level 1 \| Other liabilities, current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	0	0
Level 1 \| Other liabilities, excluding current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, excluding current portion	0	0
Level 2
Financial instruments carried at fair value (asset position):
Total financial assets	426,450	403,500
Total financial liabilities	(448)	(14,508)
Level 2 \| Money market funds
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Level 2 \| U.S. Treasury securities
Financial instruments carried at fair value (asset position):
Cash equivalents	0
Marketable securities	0
Level 2 \| Government-sponsored enterprise securities
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Marketable securities	0	0
Level 2 \| Commercial paper
Financial instruments carried at fair value (asset position):
Cash equivalents	58,268	57,357
Marketable securities	100,390	80,263
Level 2 \| Corporate equity securities
Financial instruments carried at fair value (asset position):
Marketable securities	13,590	0
Level 2 \| Corporate debt securities
Financial instruments carried at fair value (asset position):
Marketable securities	233,665	265,867
Level 2 \| Prepaid and other current assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Prepaid and other current assets	19,023	13
Level 2 \| Other assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other assets	1,514
Level 2 \| Other liabilities, current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	(340)	(13,642)
Level 2 \| Other liabilities, excluding current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, excluding current portion	(108)	(866)
Level 3
Financial instruments carried at fair value (asset position):
Total financial assets	0	0
Total financial liabilities	0	0
Level 3 \| Money market funds
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Level 3 \| U.S. Treasury securities
Financial instruments carried at fair value (asset position):
Cash equivalents	0
Marketable securities	0
Level 3 \| Government-sponsored enterprise securities
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Marketable securities	0	0
Level 3 \| Commercial paper
Financial instruments carried at fair value (asset position):
Cash equivalents	0	0
Marketable securities	0	0
Level 3 \| Corporate equity securities
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
Level 3 \| Corporate debt securities
Financial instruments carried at fair value (asset position):
Marketable securities	0	0
Level 3 \| Prepaid and other current assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Prepaid and other current assets	0	0
Level 3 \| Other assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other assets	0
Level 3 \| Other liabilities, current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Foreign currency forward contracts	0	0
Level 3 \| Other liabilities, excluding current portion \| Foreign currency forward contracts
Financial instruments carried at fair value (asset position):
Other liabilities, excluding current portion	$ 0	$ 0

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
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Fair value as of the balance sheet date of all foreign currency derivatives designated as cash flow hedging instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
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Amount of investment in marketable security.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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