Revenues - Disaggregated Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 27, 2020	Sep. 29, 2019	Sep. 27, 2020	Sep. 29, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 666,240	$ 697,717	$ 1,926,795	$ 2,066,808
United States
Disaggregation of Revenue [Line Items]
Revenues	418,675	459,718	1,237,134	1,313,657
International
Disaggregation of Revenue [Line Items]
Revenues	247,565	237,999	689,661	753,151
Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues	338,451	301,640	975,628	942,721
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	223,592	286,690	666,527	786,275
Time and materials
Disaggregation of Revenue [Line Items]
Revenues	104,197	109,387	284,640	337,812
GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	521,346	535,635	1,486,643	1,566,138
GMS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	275,785	299,306	802,230	817,437
GMS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	245,561	236,329	684,413	748,701
GMS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	302,795	274,776	868,780	870,155
GMS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	161,024	220,142	481,309	571,132
GMS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	57,527	40,717	136,554	124,851
NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	144,894	162,082	440,152	500,670
NSS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	142,890	160,412	434,904	496,220
NSS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	2,004	1,670	5,248	4,450
NSS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	35,656	26,864	106,848	72,566
NSS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	62,568	66,548	185,218	215,143
NSS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	46,670	68,670	148,086	212,961
DoD
Disaggregation of Revenue [Line Items]
Revenues	269,047	267,421	790,562	797,940
DoD | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	203,693	220,480	595,933	635,531
DoD | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	65,354	46,941	194,629	162,409
Other U.S. government agencies
Disaggregation of Revenue [Line Items]
Revenues	327,723	378,323	958,539	1,116,688
Other U.S. government agencies | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	270,344	284,218	781,004	839,680
Other U.S. government agencies | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	57,379	94,105	177,535	277,008
Commercial and non-U.S. customers
Disaggregation of Revenue [Line Items]
Revenues	69,470	51,973	177,694	152,180
Commercial and non-U.S. customers | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	47,309	30,937	109,706	90,927
Commercial and non-U.S. customers | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 22,161	$ 21,036	$ 67,988	$ 61,253