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Major Customers (Narrative) (Details) - number_of_largest_customers | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Major Customers [Abstract] |  |  |
Sales to largest customers (Percentage) | 46.00% | 43.00% | 47.00%
Number of customers | 10 | 10 | 10

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- Definition Major Customers Abstract + References No definition available. + Details
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- Definition Number of largest customers accounted for portion of revenue + References No definition available. + Details
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- Definition Percentage of revenue accounted by major customers + References No definition available. + Details
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