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Short-term Investments - Additional Information (Detail) - US Treasury Securities - USD ($) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2019 | Dec. 31, 2018
Investment [Line Items] |  |
Available-for-sale securities, amortized cost | $ 203,200,000 | $ 466,600,000
Available-for-sale securities, fair value | 203,200,000 | 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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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454 + Details
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