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Financial Instruments (Details Textual) | 6 Months Ended | 12 Months Ended
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Jun. 30, 2018 | Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |  |
Average maturity of marketable securities, months | 10 months | 17 months

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- Definition Average remaining time to maturity of marketable debt securities, available-for-sale securities. + References No definition available. + Details
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- References No definition available. + Details
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