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FAIR VALUE MEASUREMENTS Fair Value Measurement (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total Investments at Fair Value | $ 192,286 | $ 191,121
Corporate securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 92,901 | 94,451
Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 99,196 | 96,366
Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 189 | 304
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 at Fair Value | 189 | 304
Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 0 | 0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 189 | 304
Significant Other Observable Inputs (Level 2) |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total Investments at Fair Value | 192,097 | 190,817
Significant Other Observable Inputs (Level 2) | Corporate securities |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 92,901 | 94,451
Significant Other Observable Inputs (Level 2) | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | 99,196 | 96,366
Significant Other Observable Inputs (Level 2) | Money market funds |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Investments at Fair Value | $ 0 | $ 0

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