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FINANCIAL INSTRUMENTS AND FAIR VALUE MEASUREMENTS (Cash, Investments and Restricted Funds) (Details) - USD ($) $ in Thousands | Mar. 31, 2016 | Dec. 31, 2015 | Mar. 31, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Fair value disclosure |  |  |  |
Cash and cash equivalents | $ 134,788 | $ 164,973 | $ 166,022 | $ 157,042
Short-term investments | 69,608 | 61,597 |  |
Restricted cash | 1,385 | 1,384 |  |
Concentrations of Credit Risk of Financial Instruments |  |  |  |
Cash and cash equivalents which are not FDIC-insured | 55,300 | 69,900 |  |
Cash deposits |  |  |  |
Fair value disclosure |  |  |  |
Cash and cash equivalents | 87,820 | 110,279 |  |
Restricted cash | 1,385 | 1,384 |  |
Variable rate demand notes |  |  |  |
Fair value disclosure |  |  |  |
Cash and cash equivalents | 19,863 | 29,790 |  |
Money market funds |  |  |  |
Fair value disclosure |  |  |  |
Cash and cash equivalents | 27,105 | 24,904 |  |
Certificates of deposit |  |  |  |
Fair value disclosure |  |  |  |
Short-term investments | $ 69,608 | $ 61,597 |  |

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