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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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