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FAIR VALUE MEASUREMENTS - RECURRING BASIS (Details) - Recurring basis - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Fair Value on a Recurring Basis
Total	$ 160,314	$ 178,884
Money market funds
Fair Value on a Recurring Basis
Total	15,457	7,003
Commercial paper
Fair Value on a Recurring Basis
Total	42,082	57,587
Corporate notes
Fair Value on a Recurring Basis
Total	102,193	113,709
Equity Investment
Fair Value on a Recurring Basis
Total	582	585
Level 1
Fair Value on a Recurring Basis
Total	15,457	7,003
Level 1 | Money market funds
Fair Value on a Recurring Basis
Total	15,457	7,003
Level 2
Fair Value on a Recurring Basis
Total	144,857	171,881
Level 2 | Commercial paper
Fair Value on a Recurring Basis
Total	42,082	57,587
Level 2 | Corporate notes
Fair Value on a Recurring Basis
Total	102,193	113,709
Level 2 | Equity Investment
Fair Value on a Recurring Basis
Total	$ 582	$ 585

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- Definition

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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