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FAIR VALUE MEASUREMENTS - RECURRING BASIS (Details) - Recurring basis - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value on a Recurring Basis
Total	$ 101,499	$ 107,613
Money market funds
Fair Value on a Recurring Basis
Total	5,383	11,030
Government-sponsored enterprise securities
Fair Value on a Recurring Basis
Total	9,970	12,460
Commercial paper
Fair Value on a Recurring Basis
Total	11,925	13,173
Corporate notes
Fair Value on a Recurring Basis
Total	73,157	70,950
Equity Investment
Fair Value on a Recurring Basis
Total	1,064
Level 1
Fair Value on a Recurring Basis
Total	5,383	11,030
Level 1 | Money market funds
Fair Value on a Recurring Basis
Total	5,383	11,030
Level 2
Fair Value on a Recurring Basis
Total	96,116	96,583
Level 2 | Government-sponsored enterprise securities
Fair Value on a Recurring Basis
Total	9,970	12,460
Level 2 | Commercial paper
Fair Value on a Recurring Basis
Total	11,925	13,173
Level 2 | Corporate notes
Fair Value on a Recurring Basis
Total	73,157	$ 70,950
Level 2 | Equity Investment
Fair Value on a Recurring Basis
Total	$ 1,064

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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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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