Revenues - Disaggregated Revenues (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 27, 2020	Jun. 28, 2020	Mar. 29, 2020	Dec. 31, 2019	Sep. 29, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 787,833	$ 666,240	$ 643,303	$ 617,253	$ 697,085	$ 697,717	$ 695,607	$ 673,484	$ 2,714,628	$ 2,763,893	$ 2,608,562
United States
Disaggregation of Revenue [Line Items]
Revenues									1,731,497	1,748,471	1,648,988
International
Disaggregation of Revenue [Line Items]
Revenues									983,131	1,015,422	959,574
Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues									1,348,891	1,312,070	1,319,768
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues									946,244	1,003,961	876,986
Time and materials
Disaggregation of Revenue [Line Items]
Revenues									419,493	447,862	411,808
GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									2,080,474	2,099,737	1,968,843
GMS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									1,104,536	1,090,765	1,013,830
GMS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									975,938	1,008,972	955,013
GMS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									1,182,865	1,206,710	1,229,707
GMS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									676,714	717,595	595,127
GMS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									220,895	175,432	144,009
NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									634,154	664,156	639,719
NSS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									626,961	657,706	635,158
NSS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									7,193	6,450	4,561
NSS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									166,026	105,360	90,061
NSS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									269,530	286,366	281,859
NSS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									198,598	272,430	267,799
DoD
Disaggregation of Revenue [Line Items]
Revenues									1,084,856	1,081,601	989,936
DoD | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									807,722	858,623	784,532
DoD | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									277,134	222,978	205,404
Other U.S. government agencies
Disaggregation of Revenue [Line Items]
Revenues									1,343,530	1,483,140	1,456,170
Other U.S. government agencies | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									1,079,345	1,123,459	1,096,193
Other U.S. government agencies | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									264,185	359,681	359,977
Commercial and non-U.S. customers
Disaggregation of Revenue [Line Items]
Revenues									286,242	199,152	162,456
Commercial and non-U.S. customers | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									193,407	117,655	88,118
Commercial and non-U.S. customers | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues									$ 92,835	$ 81,497	$ 74,338