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Condensed Consolidated Statements of Changes in Shareholders' Equity (Parenthetical) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Statement of Stockholders' Equity [Abstract] |  |
Accumulated other comprehensive loss related to marketable securities | $ 981 | $ 903

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