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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, 2018 | Dec. 31, 2017
--- | --- | ---
Assets |  |
Total assets | $ 14,654 | $ 5,556
Cash [Member] |  |
Assets |  |
Total assets | 1,070 | 1,262
Money market securities (cash equivalents) [Member] |  |
Assets |  |
Total assets | 8,445 | 4,022
Restricted cash [Member] |  |
Assets |  |
Total assets | 50 | 272
Corporate bonds and commercial paper (short term investments) [Member] |  |
Assets |  |
Total assets | 5,089 |
Level 1 [Member] |  |
Assets |  |
Total assets | 9,565 | 5,556
Level 1 [Member] | Cash [Member] |  |
Assets |  |
Total assets | 1,070 | 1,262
Level 1 [Member] | Money market securities (cash equivalents) [Member] |  |
Assets |  |
Total assets | 8,445 | 4,022
Level 1 [Member] | Restricted cash [Member] |  |
Assets |  |
Total assets | 50 | $ 272
Level 2 [Member] |  |
Assets |  |
Total assets | 5,089 |
Level 2 [Member] | Corporate bonds and commercial paper (short term investments) [Member] |  |
Assets |  |
Total assets | $ 5,089 |

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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