Revenues - Disaggregated Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 27, 2021	Jun. 28, 2020	Jun. 27, 2021	Jun. 28, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 747,198	$ 643,303	$ 1,495,765	$ 1,260,556
United States
Disaggregation of Revenue [Line Items]
Revenues	500,974	411,411	1,015,616	818,460
International
Disaggregation of Revenue [Line Items]
Revenues	246,224	231,892	480,149	442,096
Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues	379,205	334,194	730,219	637,178
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	238,991	211,940	502,812	442,935
Time and materials
Disaggregation of Revenue [Line Items]
Revenues	129,002	97,169	262,734	180,443
GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	511,343	507,854	1,032,904	965,298
GMS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	266,268	277,536	555,040	526,446
GMS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	245,075	230,318	477,864	438,852
GMS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	285,552	299,906	546,472	565,986
GMS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	159,955	158,247	341,259	320,285
GMS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	65,836	49,701	145,173	79,027
NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	235,855	135,449	462,861	295,258
NSS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	234,706	133,875	460,576	292,014
NSS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	1,149	1,574	2,285	3,244
NSS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	93,653	34,288	183,747	71,192
NSS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	79,036	53,693	161,553	122,650
NSS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	63,166	47,468	117,561	101,416
DoD
Disaggregation of Revenue [Line Items]
Revenues	276,219	266,730	520,852	521,515
DoD | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	194,598	204,634	357,031	392,240
DoD | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	81,621	62,096	163,821	129,275
Other U.S. government agencies
Disaggregation of Revenue [Line Items]
Revenues	399,440	310,148	788,320	630,816
Other U.S. government agencies | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	267,592	260,714	536,377	510,660
Other U.S. government agencies | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	131,848	49,434	251,943	120,156
Commercial and non-U.S. customers
Disaggregation of Revenue [Line Items]
Revenues	71,539	66,425	186,593	108,225
Commercial and non-U.S. customers | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	49,153	42,506	139,496	62,398
Commercial and non-U.S. customers | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 22,386	$ 23,919	$ 47,097	$ 45,827