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FAIR VALUE MEASUREMENTS (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total Investments as Fair Value | $ 240,553 | $ 192,088
Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total Investments as Fair Value | 195 | 1,916
Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total Investments as Fair Value | 240,358 | 190,172
Corporate securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 179,951 | 132,141
Corporate securities | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 0 | 0
Corporate securities | Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 179,951 | 132,141
Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 51,122 | 58,031
Commercial paper | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 0 | 0
Commercial paper | Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 51,122 | 58,031
Government securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 9,285 |
Government securities | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 0 |
Government securities | Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 9,285 |
Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 195 | 1,916
Money market funds | Quoted Prices in Active Markets for Identical Assets (Level 1) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 195 | 1,916
Money market funds | Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | $ 0 | $ 0

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