Revenue and Contract Balances - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 30, 2025	Mar. 31, 2024	Mar. 30, 2025	Mar. 31, 2024
Disaggregation of Revenue [Line Items]
Revenue	$ 1,322,113	$ 1,251,616	$ 2,742,674	$ 2,479,883
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	524,950	459,022	1,044,772	930,464
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	605,258	591,844	1,204,206	1,141,495
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	191,905	200,750	493,696	407,924
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	413,422	407,002	915,270	789,078
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	207,438	147,551	410,425	298,476
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	211,085	201,407	444,676	423,837
International
Disaggregation of Revenue [Line Items]
Revenue	$ 490,168	$ 495,656	$ 972,303	$ 968,492