Financial instruments - Fair value of assets and liabilities (Details) $ in Thousands |
Dec. 31, 2017
USD ($)
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|---|---|
| Marketable securities: | |
| Corporate debt securities | $ 124,218 |
| Recurring basis | Corporate debt securities | |
| Marketable securities: | |
| Corporate debt securities | 124,218 |
| Recurring basis | Level 1 | Corporate debt securities | |
| Marketable securities: | |
| Corporate debt securities | 124,218 |
| Recurring basis | Level 3 | Corporate debt securities | |
| Marketable securities: | |
| Corporate debt securities | $ 0 |
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- Definition Amount of investments in debt securities classified as available-for-sale, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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