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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash and Cash Equivalents and Short-term Investments) (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Fair value disclosure |  |
Cash and cash equivalents | $ 201,909 | $ 190,186
Short-term investments | 116,579 | 106,806
Concentrations of Credit Risk of Financial Instruments |  |
Cash, cash equivalents and short-term investments not FDIC-insured | 66,200 | 94,700
Cash deposits |  |
Fair value disclosure |  |
Cash and cash equivalents | 166,619 | 124,938
Variable rate demand notes |  |
Fair value disclosure |  |
Cash and cash equivalents | 14,750 | 19,786
Money market funds |  |
Fair value disclosure |  |
Cash and cash equivalents | 20,540 | 42,470
Certificates of deposit |  |
Fair value disclosure |  |
Short-term investments | 69,314 | 82,949
U.S. Treasury securities |  |
Fair value disclosure |  |
Cash and cash equivalents |  | 2,992
Short-term investments | $ 47,265 | $ 23,857

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