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Fair Values on Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Fair value of financial assets and liabilities |  |
Total financial assets | $ 93,475 | $ 50,771
Money market funds |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 2,487 | 3,758
United States government agency securities |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 16,914 | 11,228
United States government agency securities | Short-term Investments |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 16,914 | 11,228
Corporate debt securities |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 74,074 | 35,785
Level 1 |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 2,487 | 3,758
Level 1 | Money market funds |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 2,487 | 3,758
Level 2 |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 90,988 | 47,013
Level 2 | United States government agency securities | Short-term Investments |  |
Fair value of financial assets and liabilities |  |
Total financial assets | 16,914 | 11,228
Level 2 | Corporate debt securities |  |
Fair value of financial assets and liabilities |  |
Total financial assets | $ 74,074 | $ 35,785

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