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. 26, 2026	Jun. 27, 2025	Jun. 26, 2026	Jun. 27, 2025
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 21,395	$ 22,254	$ 39,565	$ 47,657
Voluntary and Involuntary Termination
Restructuring Cost and Reserve [Line Items]
Restructuring costs	11,084	15,556	20,850	20,444
Outside Services
Restructuring Cost and Reserve [Line Items]
Restructuring costs	3,973	6,684	7,014	23,052
Other
Restructuring Cost and Reserve [Line Items]
Restructuring costs	6,338	$ 14	11,701	$ 4,161
Professional Services Related to PA Consulting Transaction
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 4,000		$ 6,200

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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