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
	Dec. 29, 2023	Dec. 30, 2022
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 41,369	$ 34,443
Lease Abandonments and Impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	49	26,831
Voluntary and Involuntary Terminations
Restructuring Cost and Reserve [Line Items]
Restructuring costs	11,728	6,570
Outside Services
Restructuring Cost and Reserve [Line Items]
Restructuring costs	24,983	675
Other
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 4,609	$ 367