Revenues - Disaggregated Revenues (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 28, 2021	Mar. 29, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 748,567	$ 617,253
United States
Disaggregation of Revenue [Line Items]
Revenues	514,642	407,048
International
Disaggregation of Revenue [Line Items]
Revenues	233,925	210,205
Cost-reimbursable
Disaggregation of Revenue [Line Items]
Revenues	351,013	302,983
Fixed-price
Disaggregation of Revenue [Line Items]
Revenues	263,821	230,996
Time and materials
Disaggregation of Revenue [Line Items]
Revenues	133,733	83,274
GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	521,561	457,444
GMS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	288,772	248,909
GMS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	232,789	208,535
GMS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	260,920	266,080
GMS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	181,304	162,038
GMS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	79,337	29,326
NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	227,006	159,809
NSS | United States | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	225,870	158,139
NSS | International | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	1,136	1,670
NSS | Cost-reimbursable | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	90,093	36,903
NSS | Fixed-price | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	82,517	68,958
NSS | Time and materials | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	54,396	53,948
DoD
Disaggregation of Revenue [Line Items]
Revenues	244,633	254,783
DoD | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	162,433	187,605
DoD | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	82,200	67,178
Other U.S. government agencies
Disaggregation of Revenue [Line Items]
Revenues	388,880	320,764
Other U.S. government agencies | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	268,785	250,042
Other U.S. government agencies | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	120,095	70,722
Commercial and non-U.S. customers
Disaggregation of Revenue [Line Items]
Revenues	115,054	41,706
Commercial and non-U.S. customers | GMS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	90,343	19,797
Commercial and non-U.S. customers | NSS | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	$ 24,711	$ 21,909