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%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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