Revenues - Disaggregated Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2021	Sep. 27, 2020	Sep. 26, 2021	Sep. 27, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 689,514	$ 666,240	$ 2,185,279	$ 1,926,795
United States
Disaggregation of Revenue [Line Items]
Revenues	486,626	418,675	1,502,243	1,237,134
International
Disaggregation of Revenue [Line Items]
Revenues	202,888	247,565	683,036	689,661
Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues	308,100	338,451	1,038,320	975,628
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	276,817	223,592	779,629	666,527
Time and materials
Disaggregation of Revenue [Line Items]
Revenues	104,597	104,197	367,330	284,640
GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	448,078	521,346	1,480,982	1,486,643
GMS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	246,903	275,785	801,944	802,230
GMS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	201,175	245,561	679,038	684,413
GMS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	218,152	302,795	764,625	868,780
GMS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	191,530	161,024	532,789	481,309
GMS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	38,396	57,527	183,568	136,554
NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	241,436	144,894	704,297	440,152
NSS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	239,723	142,890	700,299	434,904
NSS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	1,713	2,004	3,998	5,248
NSS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	89,948	35,656	273,695	106,848
NSS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	85,287	62,568	246,840	185,218
NSS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	66,201	46,670	183,762	148,086
DoD
Disaggregation of Revenue [Line Items]
Revenues	217,147	269,047	737,998	790,562
DoD | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	129,261	203,693	486,292	595,933
DoD | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	87,886	65,354	251,706	194,629
Other U.S. government agencies
Disaggregation of Revenue [Line Items]
Revenues	419,755	327,723	1,208,075	958,539
Other U.S. government agencies | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	288,543	270,344	824,920	781,004
Other U.S. government agencies | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	131,212	57,379	383,155	177,535
Commercial and non-U.S. customers
Disaggregation of Revenue [Line Items]
Revenues	52,612	69,470	239,206	177,694
Commercial and non-U.S. customers | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	30,274	47,309	169,770	109,706
Commercial and non-U.S. customers | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 22,338	$ 22,161	$ 69,436	$ 67,988