Revenue and Contract Balances - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jul. 02, 2023	Jun. 30, 2024	Jul. 02, 2023
Disaggregation of Revenue [Line Items]
Revenue	$ 1,344,323	$ 1,208,947	$ 3,824,205	$ 3,261,938
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	548,452	452,605	1,478,915	1,194,266
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	589,301	596,904	1,730,796	1,550,970
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	206,570	159,438	614,494	516,702
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	432,242	338,022	1,221,320	996,471
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	146,401	151,462	444,877	452,447
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	241,838	225,907	665,675	633,401
International
Disaggregation of Revenue [Line Items]
Revenue	$ 523,842	$ 493,556	$ 1,492,333	$ 1,179,619