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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash and Cash Equivalents and Short-term Investments) (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Fair value disclosure |  |
Cash and cash equivalents | $ 190,186 | $ 120,772
Short-term investments | 106,806 | 56,401
Concentrations of Credit Risk of Financial Instruments |  |
Cash, cash equivalents and short-term deposits not FDIC-insured | 94,700 | 61,100
Cash deposits |  |
Fair value disclosure |  |
Cash and cash equivalents | 124,938 | 86,510
Variable rate demand notes |  |
Fair value disclosure |  |
Cash and cash equivalents | 19,786 | 19,744
Money market funds |  |
Fair value disclosure |  |
Cash and cash equivalents | 42,470 | 14,518
Certificates of deposit |  |
Fair value disclosure |  |
Short-term investments | 82,949 | $ 56,401
U.S. Treasury securities |  |
Fair value disclosure |  |
Cash and cash equivalents | 2,992 |
Short-term investments | $ 23,857 |

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