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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS - Cash and Cash Equivalents and Short-term Investments (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Fair value disclosure |  |
Cash and cash equivalents | $ 301,542 | $ 303,954
Short-term investments | 59,316 | 65,408
Concentrations of Credit Risk of Financial Instruments |  |
Cash, cash equivalents and short-term investments not FDIC-insured | 152,900 | 156,400
Cash deposits |  |
Fair value disclosure |  |
Cash and cash equivalents | 236,380 | 240,687
Variable rate demand notes |  |
Fair value disclosure |  |
Cash and cash equivalents | 29,161 | 29,066
Money market funds |  |
Fair value disclosure |  |
Cash and cash equivalents | 36,001 | 34,201
Certificates of deposit |  |
Fair value disclosure |  |
Short-term investments | $ 59,316 | 53,297
U.S. Treasury securities |  |
Fair value disclosure |  |
Short-term investments |  | $ 12,111

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