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Marketable Securities - Additional Information (Detail) - USD ($) | 6 Months Ended
--- | ---
Jun. 30, 2018 | Jun. 30, 2017
Investments Debt And Equity Securities [Abstract] |  |
Realized losses | $ 0 | $ 0
Investments other-than-temporarily impaired | $ 0 | $ 0
Investments maturity period | 1 year |
Investments mature within 24 months | $ 34,200,000 |

X
- Definition Cash equivalents and available-for-sale securities mature within twenty four months. + References No definition available. + Details
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