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Financial Instruments (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | --- | --- | ---
Cash and cash equivalents: |  |  |  |
Cash and cash equivalents | $ 16,369 | $ 23,596 | $ 62,780 | $ 14,668
Available-for-sale securities: |  |  |  |
Available-for-sale securities | 0 | 14,931 |  |
Corporate debt securities |  |  |  |
Available-for-sale securities: |  |  |  |
Available-for-sale securities |  | 13,930 |  |
U.S. government-sponsored securities |  |  |  |
Available-for-sale securities: |  |  |  |
Available-for-sale securities |  | 1,001 |  |
Recurring |  |  |  |
Assets: |  |  |  |
Total assets | 13,588 | 37,354 |  |
Recurring | Money market funds |  |  |  |
Cash and cash equivalents: |  |  |  |
Cash and cash equivalents | 13,588 | 22,423 |  |
Recurring | Corporate debt securities |  |  |  |
Available-for-sale securities: |  |  |  |
Available-for-sale securities |  | 13,930 |  |
Recurring | U.S. government-sponsored securities |  |  |  |
Available-for-sale securities: |  |  |  |
Available-for-sale securities |  | 1,001 |  |
Recurring | Level 1 |  |  |  |
Assets: |  |  |  |
Total assets | 13,588 | 22,423 |  |
Recurring | Level 1 | Money market funds |  |  |  |
Cash and cash equivalents: |  |  |  |
Cash and cash equivalents | $ 13,588 | 22,423 |  |
Recurring | Level 2 |  |  |  |
Assets: |  |  |  |
Total assets |  | 14,931 |  |
Recurring | Level 2 | Corporate debt securities |  |  |  |
Available-for-sale securities: |  |  |  |
Available-for-sale securities |  | 13,930 |  |
Recurring | Level 2 | U.S. government-sponsored securities |  |  |  |
Available-for-sale securities: |  |  |  |
Available-for-sale securities |  | $ 1,001 |  |

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