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FAIR VALUE MEASUREMENTS - RECURRING BASIS (Details) - Recurring basis - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value on a Recurring Basis
Total	$ 168,489	$ 178,884
Money market funds
Fair Value on a Recurring Basis
Total	4,750	7,003
Commercial paper
Fair Value on a Recurring Basis
Total	56,500	57,587
Corporate notes
Fair Value on a Recurring Basis
Total	106,551	113,709
Equity Investment
Fair Value on a Recurring Basis
Total	688	585
Level 1
Fair Value on a Recurring Basis
Total	4,750	7,003
Level 1 | Money market funds
Fair Value on a Recurring Basis
Total	4,750	7,003
Level 2
Fair Value on a Recurring Basis
Total	163,739	171,881
Level 2 | Commercial paper
Fair Value on a Recurring Basis
Total	56,500	57,587
Level 2 | Corporate notes
Fair Value on a Recurring Basis
Total	106,551	113,709
Level 2 | Equity Investment
Fair Value on a Recurring Basis
Total	$ 688	$ 585

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

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)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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