Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Short-term investments:
Trading securities held in Non-Qualified Deferred Compensation plan, or NQDC	$ 29,498	$ 26,622
Total Assets	523,120	267,403
Liabilities
Foreign currency exchange contracts	1,971	2,653
Money market funds [Member]
Assets
Cash equivalents	489,487	237,724
Marketable equity securities [Member]
Short-term investments:
Available-for-sale securities	4,135	3,057
Fair Value Measurements, Level 1 [Member]
Short-term investments:
Trading securities held in Non-Qualified Deferred Compensation plan, or NQDC	29,498	26,622
Total Assets	523,120	267,403
Liabilities
Foreign currency exchange contracts	0	0
Fair Value Measurements, Level 1 [Member] \| Money market funds [Member]
Assets
Cash equivalents	489,487	237,724
Fair Value Measurements, Level 1 [Member] \| Marketable equity securities [Member]
Short-term investments:
Available-for-sale securities	4,135	3,057
Fair Value Measurements, Level 2 [Member]
Short-term investments:
Trading securities held in Non-Qualified Deferred Compensation plan, or NQDC	0	0
Total Assets	0	0
Liabilities
Foreign currency exchange contracts	1,971	2,653
Fair Value Measurements, Level 2 [Member] \| Money market funds [Member]
Assets
Cash equivalents	0	0
Fair Value Measurements, Level 2 [Member] \| Marketable equity securities [Member]
Short-term investments:
Available-for-sale securities	0	0
Fair Value Measurements, Level 3 [Member]
Short-term investments:
Trading securities held in Non-Qualified Deferred Compensation plan, or NQDC	0	0
Total Assets	0	0
Liabilities
Foreign currency exchange contracts	0	0
Fair Value Measurements, Level 3 [Member] \| Money market funds [Member]
Assets
Cash equivalents	0	0
Fair Value Measurements, Level 3 [Member] \| Marketable equity securities [Member]
Short-term investments:
Available-for-sale 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://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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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Carrying amount as of the balance sheet date of assets held under deferred compensation agreements.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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