Revenue and Contract Balances - Summary of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 29, 2025	Jun. 30, 2024	Jun. 29, 2025	Jun. 30, 2024
Disaggregation of Revenue [Line Items]
Revenue	$ 1,369,816	$ 1,344,323	$ 4,112,490	$ 3,824,205
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	633,508	548,452	1,678,280	1,478,915
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	595,082	589,301	1,799,288	1,730,796
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	141,226	206,570	634,922	614,494
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	452,629	432,242	1,367,899	1,221,320
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	189,181	146,401	599,606	444,877
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	221,351	241,838	666,027	665,675
International
Disaggregation of Revenue [Line Items]
Revenue	$ 506,655	$ 523,842	$ 1,478,958	$ 1,492,333