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Fair value measurements (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Marketable securities: |  |
Available-for-sale securities, Debt Securities, Current, Total | $ 84,652 | $ 124,218
Recurring basis |  |
Marketable securities: |  |
Corporate debt securities | 82,668 |
Commercial paper | 1,984 |
Available-for-sale securities, Debt Securities, Current, Total | 84,652 |
Recurring basis | Level 1 |  |
Marketable securities: |  |
Corporate debt securities | 82,668 |
Available-for-sale securities, Debt Securities, Current, Total | 82,668 |
Recurring basis | Level 2 |  |
Marketable securities: |  |
Commercial paper | 1,984 |
Available-for-sale securities, Debt Securities, Current, Total | $ 1,984 |

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26626-111562 + Details
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- Definition Unsecured short-term debt instrument issued by corporations which are highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months. + References No definition available. + Details
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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562 + Details
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