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Business Segments and Geographic Information (Customer Concentration Risk) (Details) - Customer Concentration Risk - Revenues
12 Months Ended
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%