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Fair Values on Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Fair value of financial assets and liabilities |  |
Total financial assets | $ 50,771 | $ 58,059
Money market funds |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 3,758 | 8,991
United States government agency securities |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 11,228 | 8,029
United States government agency securities | Short-term Investments |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 11,228 | 8,029
Corporate debt securities |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 35,785 | 37,087
Marketable equity securities |  |
Fair value of financial assets and liabilities |  |
Total financial assets |  | 3,952
Level 1 |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 3,758 | 12,943
Level 1 | Money market funds |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 3,758 | 8,991
Level 1 | Marketable equity securities |  |
Fair value of financial assets and liabilities |  |
Total financial assets |  | 3,952
Level 2 |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 47,013 | 45,116
Level 2 | United States government agency securities | Short-term Investments |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 11,228 | 8,029
Level 2 | Corporate debt securities |  |
Fair value of financial assets and liabilities |  |
Total financial assets | $ 35,785 | $ 37,087

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