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Business Segments and Geographic Information (Customer Concentration Risk) (Details) - Customer Concentration Risk - Revenues | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Entity Wide Revenue Major Customer [Line Items] |  |  |
Percentage of revenue | 10.00% |  |
Various agencies of the U.S Federal Government: |  |  |
Entity Wide Revenue Major Customer [Line Items] |  |  |
Percentage of revenue | 50.00% | 48.00% | 48.00%

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