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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Restricted cash: |  |
Amortized Cost | $ 270 | $ 269
Marketable securities: |  |
Amortized Cost | 162,952 | 171,479
Gross Unrealized Gains | 136 | 29
Gross Unrealized Losses | (37) | (212)
Estimated Fair Value | 163,051 | 171,296
Money market funds |  |
Included in cash and cash equivalents: |  |
Amortized Cost | 4,750 | 7,003
Estimated Fair Value | 4,750 | 7,003
Certificate of deposit |  |
Restricted cash: |  |
Amortized Cost | 270 | 269
Estimated Fair Value | 270 | 269
Commercial paper (due in less than one year) |  |
Marketable securities: |  |
Amortized Cost | 56,474 | 57,594
Gross Unrealized Gains | 45 | 22
Gross Unrealized Losses | (19) | (29)
Estimated Fair Value | 56,500 | 57,587
Corporate notes (due in less than one year) |  |
Marketable securities: |  |
Amortized Cost | 88,648 | 95,238
Gross Unrealized Gains | 45 | 7
Gross Unrealized Losses | (13) | (118)
Estimated Fair Value | 88,680 | 95,127
Corporate notes (due in one to two years) |  |
Marketable securities: |  |
Amortized Cost | 17,830 | 18,647
Gross Unrealized Gains | 46 |
Gross Unrealized Losses | (5) | (65)
Estimated Fair Value | $ 17,871 | $ 18,582

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