Revenue and Contract Balances - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 28, 2025	Jun. 29, 2025	Mar. 30, 2025	Dec. 29, 2024	Sep. 29, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Sep. 28, 2025	Sep. 29, 2024	Oct. 01, 2023
Disaggregation of Revenue [Line Items]
Revenue	$ 1,330,100	$ 1,369,816	$ 1,322,113	$ 1,420,561	$ 1,374,473	$ 1,344,323	$ 1,251,616	$ 1,228,267	$ 5,442,590	$ 5,198,679	$ 4,522,550
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue									2,365,680	2,016,638	1,643,849
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue									2,319,766	2,337,913	2,166,671
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue									757,144	844,128	712,030
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue									1,718,831	1,675,996	1,387,101
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue									789,968	613,185	607,074
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue									899,298	909,642	869,460
International
Disaggregation of Revenue [Line Items]
Revenue									$ 2,034,493	$ 1,999,856	$ 1,658,915