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Short-term Investments - Additional Information (Detail) - US Treasury Securities and Certificates of Deposit - USD ($) | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Investment [Line Items] |  |
Held to maturity securities, amortized cost | $ 609,200,000 | $ 254,900,000
Held to maturity securities, carrying value | 609,200,000 | 254,900,000
Held to maturity securities, fair value | 608,600,000 | 254,800,000
Other-than-temporary impairments related to these investments | $ 0 | $ 0
Minimum |  |
Investment [Line Items] |  |
Held to maturity securities maturity range | 3 months |
Maximum |  |
Investment [Line Items] |  |
Held to maturity securities maturity range | 1 year |

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