Revenues - Disaggregated Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 29, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 617,253	$ 673,484
United States
Disaggregation of Revenue [Line Items]
Revenues	407,048	427,871
International
Disaggregation of Revenue [Line Items]
Revenues	210,205	245,613
Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues	302,983	312,913
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	230,996	251,913
Time and materials
Disaggregation of Revenue [Line Items]
Revenues	83,274	108,658
GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	457,444	504,480
GMS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	248,909	260,088
GMS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	208,535	244,392
GMS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	266,080	290,691
GMS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	162,038	175,399
GMS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	29,326	38,390
NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	159,809	169,004
NSS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	158,139	167,783
NSS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	1,670	1,221
NSS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	36,903	22,222
NSS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	68,958	76,514
NSS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	53,948	70,268
DoD
Disaggregation of Revenue [Line Items]
Revenues	254,783	259,154
DoD | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	187,605	200,402
DoD | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	67,178	58,752
Other U.S. government agencies
Disaggregation of Revenue [Line Items]
Revenues	320,764	363,321
Other U.S. government agencies | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	250,042	274,112
Other U.S. government agencies | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	70,722	89,209
Commercial and non-U.S. customers
Disaggregation of Revenue [Line Items]
Revenues	41,706	51,009
Commercial and non-U.S. customers | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	19,797	29,966
Commercial and non-U.S. customers | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 21,909	$ 21,043