Restructuring and Other Charges - Schedule of Restructuring and Other Activities by Major Type of Costs (Details) - CH2M Hill, KeyM, John Wood Group Acquisitions and ECR Sale - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 27, 2025	Jun. 28, 2024	Jun. 27, 2025	Jun. 28, 2024
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 22,254	$ 53,971	$ 47,657	$ 127,687
Lease Abandonments and Impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	0	0	0	49
Terminations
Restructuring Cost and Reserve [Line Items]
Restructuring costs	15,556	22,304	20,444	44,211
Outside Services
Restructuring Cost and Reserve [Line Items]
Restructuring costs	6,684	27,906	23,052	71,198
Other
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 14	$ 3,761	$ 4,161	$ 12,229