Summary of Significant Accounting Policies - Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets and liabilities carried at fair value measured on a recurring basis
Money Market Funds, at Carrying Value	$ 0	$ 585,000
Time Deposits, at Carrying Value	16,678	24,482
Fair value measured on recurring basis \| Quoted prices in active markets (Level 1)
Assets and liabilities carried at fair value measured on a recurring basis
Money Market Funds, at Carrying Value		0
Time Deposits, at Carrying Value	0	0
Trading securities	10,739	11,279
Foreign Currency Contract, Asset, Fair Value Disclosure		0
Foreign Currency Contracts, Liability, Fair Value Disclosure	0	0
Interest Rate Swap	0
Fair value measured on recurring basis \| Significant other observable inputs (Level 2)
Assets and liabilities carried at fair value measured on a recurring basis
Money Market Funds, at Carrying Value		585,000
Time Deposits, at Carrying Value	16,678	24,482
Trading securities	390	505
Foreign Currency Contract, Asset, Fair Value Disclosure		1,579
Foreign Currency Contracts, Liability, Fair Value Disclosure	1,102	2,329
Interest Rate Swap	185
Fair value measured on recurring basis \| Significant unobservable inputs (Level 3)
Assets and liabilities carried at fair value measured on a recurring basis
Money Market Funds, at Carrying Value		0
Time Deposits, at Carrying Value	0	0
Trading securities	0	0
Foreign Currency Contract, Asset, Fair Value Disclosure		0
Foreign Currency Contracts, Liability, Fair Value Disclosure	0	0
Interest Rate Swap	0
Estimate of Fair Value Measurement [Member] \| Fair value measured on recurring basis
Assets and liabilities carried at fair value measured on a recurring basis
Money Market Funds, at Carrying Value		585,000
Time Deposits, at Carrying Value	16,678	24,482
Trading securities	11,129	11,784
Foreign Currency Contract, Asset, Fair Value Disclosure		1,579
Foreign Currency Contracts, Liability, Fair Value Disclosure	1,102	$ 2,329
Interest Rate Swap	$ 185

Fair value of all liability derivatives designated as cash flow hedging instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624163-113959

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Fair value portion of asset contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward contracts, and swaps.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

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Any certificate of deposit or savings account held by a bank or other financial institution for a short-term specified period of time. Because of their short-term, time deposits are considered 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. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Cash Equivalents
-URI http://asc.fasb.org/extlink&oid=6507016

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26626-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

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