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FAIR VALUE MEASUREMENTS - SECURITY TYPE (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Included in cash and cash equivalents: |  |
Amortized Cost | $ 15,022 |
Gross Unrealized Losses | (1) |
Estimated Fair Value | 15,021 |
Restricted cash: |  |
Amortized Cost | 268 | $ 268
Marketable securities: |  |
Amortized Cost | 92,798 | 116,042
Gross Unrealized Gains | 4 | 54
Gross Unrealized Losses | (210) | (107)
Estimated Fair Value | 92,592 | 115,989
Money market funds |  |
Included in cash and cash equivalents: |  |
Amortized Cost | 11,030 | 11,193
Estimated Fair Value | 11,030 | 11,193
Commercial paper |  |
Included in cash and cash equivalents: |  |
Amortized Cost | 2,242 |
Estimated Fair Value | 2,242 |
Corporate notes |  |
Included in cash and cash equivalents: |  |
Amortized Cost | 1,750 |
Gross Unrealized Losses | (1) |
Estimated Fair Value | 1,749 |
Certificate of deposit |  |
Restricted cash: |  |
Amortized Cost | 268 | 268
Estimated Fair Value | 268 | 268
Government-sponsored enterprise securities (due in less than one year) |  |
Marketable securities: |  |
Amortized Cost | 12,500 | 5,000
Gross Unrealized Losses | (40) | (3)
Estimated Fair Value | 12,460 | 4,997
Government-sponsored enterprise securities (due in one to two years) |  |
Marketable securities: |  |
Amortized Cost |  | 12,500
Gross Unrealized Losses |  | (42)
Estimated Fair Value |  | 12,458
Commercial paper (due in less than one year) |  |
Marketable securities: |  |
Amortized Cost | 10,928 | 31,024
Gross Unrealized Gains | 4 | 50
Gross Unrealized Losses | (1) | (5)
Estimated Fair Value | 10,931 | 31,069
Corporate notes (due in less than one year) |  |
Marketable securities: |  |
Amortized Cost | 55,067 | 66,012
Gross Unrealized Gains |  | 4
Gross Unrealized Losses | (107) | (47)
Estimated Fair Value | 54,960 | 65,969
Corporate notes (due in one to two years) |  |
Marketable securities: |  |
Amortized Cost | 14,303 | 1,506
Gross Unrealized Losses | (62) | (10)
Estimated Fair Value | $ 14,241 | $ 1,496

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