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Property and Equipment, Net - Additional Information (Detail) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Property Plant And Equipment [Line Items] |  |  |  |  |
Depreciation and amortization expense | $ 95,000 | $ 87,000 | $ 363,000 | $ 279,000 | $ 11,000
Maximum [Member] |  |  |  |  |
Property Plant And Equipment [Line Items] |  |  |  |  |
Depreciation and amortization expense |  |  |  |  | $ 100,000

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- Definition The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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