FINANCIAL INSTRUMENTS - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Cash equivalents		$ 195,902
Marketable debt securities	$ 24,947
Total	330,151
Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents		189,207
Total	137,359
Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents		6,695
Total	192,792
Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents		0
Total	0
Money market funds
Assets:
Cash equivalents	137,359	189,207
Money market funds \| Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	137,359	189,207
Money market funds \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	0	0
Money market funds \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Treasury discount notes
Assets:
Cash equivalents	99,914	500
Marketable debt securities	24,947
Treasury discount notes \| Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	0	0
Marketable debt securities	0
Treasury discount notes \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	99,914	500
Marketable debt securities	24,947
Treasury discount notes \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0	0
Marketable debt securities	0
Commercial paper
Assets:
Cash equivalents	52,931
Commercial paper \| Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	0
Commercial paper \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	52,931
Commercial paper \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	0
Time deposits
Assets:
Cash equivalents	15,000
Time deposits \| Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents	0
Time deposits \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents	15,000
Time deposits \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents	$ 0
Certificate of deposit
Assets:
Cash equivalents		6,195
Certificate of deposit \| Quoted Market Prices in Active Markets for Identical Assets (Level 1)
Assets:
Cash equivalents		0
Certificate of deposit \| Significant Other Observable Inputs (Level 2)
Assets:
Cash equivalents		6,195
Certificate of deposit \| Significant Unobservable Inputs (Level 3)
Assets:
Cash equivalents		$ 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
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-Section 50
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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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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