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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 + Details
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