Revenue and Contract Balances - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2023	Jan. 01, 2023
Disaggregation of Revenue [Line Items]
Revenue	$ 1,228,267	$ 894,766
Fixed-price
Disaggregation of Revenue [Line Items]
Revenue	471,442	327,737
Time-and-materials
Disaggregation of Revenue [Line Items]
Revenue	549,651	420,570
Cost-plus
Disaggregation of Revenue [Line Items]
Revenue	207,174	146,459
U.S. federal government
Disaggregation of Revenue [Line Items]
Revenue	382,076	276,075
U.S. state and local government
Disaggregation of Revenue [Line Items]
Revenue	150,925	153,195
U.S. commercial
Disaggregation of Revenue [Line Items]
Revenue	222,430	198,956
International
Disaggregation of Revenue [Line Items]
Revenue	$ 472,836	$ 266,540