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		6 Months Ended
	Mar. 29, 2024	Mar. 31, 2023	Mar. 29, 2024	Mar. 31, 2023
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 41,928	$ 12,840	$ 83,297	$ 47,284
Lease Abandonments and Impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	0	10,443	49	37,273
Terminations
Restructuring Cost and Reserve [Line Items]
Restructuring costs	11,823	1,939	23,551	8,509
Outside Services
Restructuring Cost and Reserve [Line Items]
Restructuring costs	22,083	802	47,066	1,478
Other
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 8,022	$ (344)	$ 12,631	$ 24