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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Included in cash and cash equivalents: |  |
Amortized Cost | $ 10,661 |
Estimated Fair Value | 10,661 |
Restricted cash: |  |
Amortized Cost | 270 | $ 269
Marketable securities: |  |
Amortized Cost | 145,200 | 171,479
Gross Unrealized Gains | 157 | 29
Gross Unrealized Losses | (25) | (212)
Estimated Fair Value | 145,332 | 171,296
Money market funds |  |
Included in cash and cash equivalents: |  |
Amortized Cost | 6,671 | 7,003
Estimated Fair Value | 6,671 | 7,003
Commercial paper |  |
Included in cash and cash equivalents: |  |
Amortized Cost | 3,990 |
Estimated Fair Value | 3,990 |
Certificate of deposit |  |
Restricted cash: |  |
Amortized Cost | 270 | 269
Estimated Fair Value | 270 | 269
Government-sponsored enterprise securities (due in less than one year) |  |
Marketable securities: |  |
Amortized Cost | 6,506 |
Gross Unrealized Gains | 6 |
Estimated Fair Value | 6,512 |
Government-sponsored enterprise securities (due in one to two years) |  |
Marketable securities: |  |
Amortized Cost | 6,999 |
Gross Unrealized Gains | 1 |
Estimated Fair Value | 7,000 |
Commercial paper (due in less than one year) |  |
Marketable securities: |  |
Amortized Cost | 40,110 | 57,594
Gross Unrealized Gains | 33 | 22
Gross Unrealized Losses | (3) | (29)
Estimated Fair Value | 40,140 | 57,587
Corporate notes (due in less than one year) |  |
Marketable securities: |  |
Amortized Cost | 78,926 | 95,238
Gross Unrealized Gains | 116 | 7
Gross Unrealized Losses | (13) | (118)
Estimated Fair Value | 79,029 | 95,127
Corporate notes (due in one to two years) |  |
Marketable securities: |  |
Amortized Cost | 12,659 | 18,647
Gross Unrealized Gains | 1 |
Gross Unrealized Losses | (9) | (65)
Estimated Fair Value | $ 12,651 | $ 18,582

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