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Operations by Business Segment and Geographic Area - Concentrations (Details) - Customer concentration - Consolidated revenue - Single customer
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Energy & Chemicals      
Concentration risk      
Concentration risk (as a percentage) 13.00% 10.00% 11.00%
Government Segment      
Concentration risk      
Concentration risk (as a percentage) 15.00% 13.00% 12.00%