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Revenue - Revenue by Contract Relationship (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disaggregation of Revenue [Line Items]      
Total revenue $ 1,279,304 $ 1,114,788 $ 1,190,519
Prime contractor      
Disaggregation of Revenue [Line Items]      
Total revenue 1,200,726 1,083,485 1,131,773
Subcontractor      
Disaggregation of Revenue [Line Items]      
Total revenue $ 78,578 $ 31,303 $ 58,746