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Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Assets |  |
Total assets | $ 5,556 | $ 15
Cash [Member] |  |
Assets |  |
Total assets | 1,262 | 15
Money market securities (cash equivalents) [Member] |  |
Assets |  |
Total assets | 4,022 |
Restricted cash [Member] |  |
Assets |  |
Total assets | 272 |
Level 1 [Member] |  |
Assets |  |
Total assets | 5,556 | 15
Level 1 [Member] | Cash [Member] |  |
Assets |  |
Total assets | 1,262 | $ 15
Level 1 [Member] | Money market securities (cash equivalents) [Member] |  |
Assets |  |
Total assets | 4,022 |
Level 1 [Member] | Restricted cash [Member] |  |
Assets |  |
Total assets | $ 272 |

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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