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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands | Sep. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Restricted cash: |  |
Amortized Cost | $ 270 | $ 269
Marketable securities: |  |
Amortized Cost | 138,025 | 171,479
Gross Unrealized Gains | 262 | 29
Gross Unrealized Losses | (23) | (212)
Estimated Fair Value | 138,264 | 171,296
Money market funds |  |
Included in cash and cash equivalents: |  |
Amortized Cost | 17,915 | 7,003
Estimated Fair Value | 17,915 | 7,003
Certificate of deposit |  |
Restricted cash: |  |
Amortized Cost | 270 | 269
Estimated Fair Value | 270 | 269
Government-sponsored enterprise securities (due in one to two years) |  |
Marketable securities: |  |
Amortized Cost | 10,008 |
Gross Unrealized Gains | 2 |
Gross Unrealized Losses | (15) |
Estimated Fair Value | 9,995 |
Commercial paper (due in less than one year) |  |
Marketable securities: |  |
Amortized Cost | 35,695 | 57,594
Gross Unrealized Gains | 58 | 22
Gross Unrealized Losses |  | (29)
Estimated Fair Value | 35,753 | 57,587
Corporate notes (due in less than one year) |  |
Marketable securities: |  |
Amortized Cost | 81,928 | 95,238
Gross Unrealized Gains | 184 | 7
Gross Unrealized Losses | (5) | (118)
Estimated Fair Value | 82,107 | 95,127
Corporate notes (due in one to two years) |  |
Marketable securities: |  |
Amortized Cost | 10,394 | 18,647
Gross Unrealized Gains | 18 |
Gross Unrealized Losses | (3) | (65)
Estimated Fair Value | $ 10,409 | $ 18,582

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