Restructuring and Other Charges - Summary 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. 30, 2022	Dec. 31, 2021
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 34,443	$ 69,359
Lease Abandonments and Impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	26,831	65,542
Voluntary and Involuntary Terminations
Restructuring Cost and Reserve [Line Items]
Restructuring costs	6,570	563
Outside Services
Restructuring Cost and Reserve [Line Items]
Restructuring costs	675	4,676
Other
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 367	$ (1,422)