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Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | $ 98,076 | $ 50,868
Gross Unrealized Loss | (417) | (97)
Fair Value | 97,659 | 50,771
Money market funds |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 54 | 3,758
Fair Value | 54 | 3,758
United States government agency securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 15,940 | 11,252
Gross Unrealized Loss | (42) | (24)
Fair Value | 15,898 | 11,228
Corporate debt securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Amortized Cost | 82,082 | 35,858
Gross Unrealized Loss | (375) | (73)
Fair Value | $ 81,707 | $ 35,785

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