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Short-term Investments - Additional Information (Detail) - US Treasury Securities - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
Investment [Line Items] |  |
Available-for-sale securities, amortized cost | $ 292,300,000 | $ 466,600,000
Available-for-sale securities, fair value | 292,300,000 | 466,200,000
Available-for-sale securities, other-than-temporary impairments | $ 0 | $ 0
Minimum |  |
Investment [Line Items] |  |
Available-for-sale securities maturity range | 3 months |
Maximum |  |
Investment [Line Items] |  |
Available-for-sale securities maturity range | 1 year |

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