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FAIR VALUE MEASUREMENTS - RECURRING BASIS (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value on a Recurring Basis
Total	$ 107,613	$ 127,182
Money market funds
Fair Value on a Recurring Basis
Total	11,030	11,193
Government-sponsored enterprise securities
Fair Value on a Recurring Basis
Total	12,460	17,455
Commercial paper
Fair Value on a Recurring Basis
Total	13,173	31,069
Corporate notes
Fair Value on a Recurring Basis
Total	70,950	67,465
Level 1
Fair Value on a Recurring Basis
Total	11,030	11,193
Level 1 | Money market funds
Fair Value on a Recurring Basis
Total	11,030	11,193
Level 2
Fair Value on a Recurring Basis
Total	96,583	115,989
Level 2 | Government-sponsored enterprise securities
Fair Value on a Recurring Basis
Total	12,460	17,455
Level 2 | Commercial paper
Fair Value on a Recurring Basis
Total	13,173	31,069
Level 2 | Corporate notes
Fair Value on a Recurring Basis
Total	$ 70,950	$ 67,465

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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://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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