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Fair value (Details) - Fair value measurements on a recurring basis - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Assets: |  |  |
Total | $ 53,628 | $ 67,203 | $ 114,388
Level 1 |  |  |
Assets: |  |  |
Total | 24,601 | 15,703 | 43,930
Level 2 |  |  |
Assets: |  |  |
Total | 29,027 | 51,500 | 70,458
Cash and money market funds |  |  |
Assets: |  |  |
Total | 24,601 | 15,703 | 43,930
Cash and money market funds | Level 1 |  |  |
Assets: |  |  |
Total | 24,601 | 15,703 | 43,930
Corporate debt securities (including commercial paper) |  |  |
Assets: |  |  |
Total | 21,046 | 35,531 | 48,466
Corporate debt securities (including commercial paper) | Level 2 |  |  |
Assets: |  |  |
Total | 21,046 | 35,531 | $ 48,466
U.S. government securities |  |  |
Assets: |  |  |
Total | 7,981 | 15,969 |
U.S. government securities | Level 2 |  |  |
Assets: |  |  |
Total | $ 7,981 | $ 15,969 |

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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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