Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Financial instruments carried at fair value (asset positions):
Total financial assets	$ 2,017,245	$ 1,836,605
Financial instruments carried at fair value (liability positions):
Total financial liabilities	(378)	(448)
Level 1
Financial instruments carried at fair value (asset positions):
Total financial assets	1,579,826	1,410,155
Financial instruments carried at fair value (liability positions):
Total financial liabilities	0	0
Level 2
Financial instruments carried at fair value (asset positions):
Total financial assets	437,419	426,450
Financial instruments carried at fair value (liability positions):
Total financial liabilities	(378)	(448)
Level 3
Financial instruments carried at fair value (asset positions):
Total financial assets	0	0
Financial instruments carried at fair value (liability positions):
Total financial liabilities	0	0
Prepaid expenses and other current assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	19,210	19,023
Prepaid expenses and other current assets \| Foreign currency forward contracts \| Level 1
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	0	0
Prepaid expenses and other current assets \| Foreign currency forward contracts \| Level 2
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	19,210	19,023
Prepaid expenses and other current assets \| Foreign currency forward contracts \| Level 3
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	0	0
Other assets \| Foreign currency forward contracts
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	973
Total financial assets		1,514
Other assets \| Foreign currency forward contracts \| Level 1
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	0
Total financial assets		0
Other assets \| Foreign currency forward contracts \| Level 2
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	973
Total financial assets		1,514
Other assets \| Foreign currency forward contracts \| Level 3
Financial instruments carried at fair value (asset positions):
Foreign currency forward contracts	0
Total financial assets		0
Other current liabilities \| Foreign currency forward contracts
Financial instruments carried at fair value (liability positions):
Derivative liability current	(310)	(340)
Other current liabilities \| Foreign currency forward contracts \| Level 1
Financial instruments carried at fair value (liability positions):
Derivative liability current	0	0
Other current liabilities \| Foreign currency forward contracts \| Level 2
Financial instruments carried at fair value (liability positions):
Derivative liability current	(310)	(340)
Other current liabilities \| Foreign currency forward contracts \| Level 3
Financial instruments carried at fair value (liability positions):
Derivative liability current	0	0
Other long-term liabilities \| Foreign currency forward contracts
Financial instruments carried at fair value (liability positions):
Derivative liability, noncurrent	(68)	(108)
Other long-term liabilities \| Foreign currency forward contracts \| Level 1
Financial instruments carried at fair value (liability positions):
Derivative liability, noncurrent	0	0
Other long-term liabilities \| Foreign currency forward contracts \| Level 2
Financial instruments carried at fair value (liability positions):
Derivative liability, noncurrent	(68)	(108)
Other long-term liabilities \| Foreign currency forward contracts \| Level 3
Financial instruments carried at fair value (liability positions):
Derivative liability, noncurrent	0	0
Money market funds
Financial instruments carried at fair value (asset positions):
Cash equivalents	1,354,656	1,226,603
Money market funds \| Level 1
Financial instruments carried at fair value (asset positions):
Cash equivalents	1,354,656	1,226,603
Money market funds \| Level 2
Financial instruments carried at fair value (asset positions):
Cash equivalents	0	0
Money market funds \| Level 3
Financial instruments carried at fair value (asset positions):
Cash equivalents	0	0
U.S. Treasury securities
Financial instruments carried at fair value (asset positions):
Cash equivalents	5,996	5,966
Marketable securities		6,026
U.S. Treasury securities \| Level 1
Financial instruments carried at fair value (asset positions):
Cash equivalents	5,996	5,966
Marketable securities		6,026
U.S. Treasury securities \| Level 2
Financial instruments carried at fair value (asset positions):
Cash equivalents	0	0
Marketable securities		0
U.S. Treasury securities \| Level 3
Financial instruments carried at fair value (asset positions):
Cash equivalents	0	0
Marketable securities		0
Government-sponsored enterprise securities
Financial instruments carried at fair value (asset positions):
Cash equivalents	11,786	7,123
Marketable securities	15,181	10,704
Government-sponsored enterprise securities \| Level 1
Financial instruments carried at fair value (asset positions):
Cash equivalents	11,786	7,123
Marketable securities	15,181	10,704
Government-sponsored enterprise securities \| Level 2
Financial instruments carried at fair value (asset positions):
Cash equivalents	0	0
Marketable securities	0	0
Government-sponsored enterprise securities \| Level 3
Financial instruments carried at fair value (asset positions):
Cash equivalents	0	0
Marketable securities	0	0
Corporate debt securities
Financial instruments carried at fair value (asset positions):
Cash equivalents	4,519
Marketable securities	231,572	233,665
Corporate debt securities \| Level 1
Financial instruments carried at fair value (asset positions):
Cash equivalents	0
Marketable securities	0	0
Corporate debt securities \| Level 2
Financial instruments carried at fair value (asset positions):
Cash equivalents	4,519
Marketable securities	231,572	233,665
Corporate debt securities \| Level 3
Financial instruments carried at fair value (asset positions):
Cash equivalents	0
Marketable securities	0	0
Commercial paper
Financial instruments carried at fair value (asset positions):
Cash equivalents	35,955	58,268
Marketable securities	126,523	100,390
Commercial paper \| Level 1
Financial instruments carried at fair value (asset positions):
Cash equivalents	0	0
Marketable securities	0	0
Commercial paper \| Level 2
Financial instruments carried at fair value (asset positions):
Cash equivalents	35,955	58,268
Marketable securities	126,523	100,390
Commercial paper \| Level 3
Financial instruments carried at fair value (asset positions):
Cash equivalents	0	0
Marketable securities	0	0
Corporate equity securities
Financial instruments carried at fair value (asset positions):
Marketable securities	210,874	167,323
Corporate equity securities \| Level 1
Financial instruments carried at fair value (asset positions):
Marketable securities	192,207	153,733
Corporate equity securities \| Level 2
Financial instruments carried at fair value (asset positions):
Marketable securities	18,667	13,590
Corporate equity securities \| Level 3
Financial instruments carried at fair value (asset positions):
Marketable securities	$ 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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section 50
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Fair value as of the balance sheet date of all foreign currency derivatives designated as cash flow hedging instruments.

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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
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