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. 28, 2025	Mar. 29, 2024	Mar. 28, 2025	Mar. 29, 2024
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 10,663	$ 35,362	$ 25,403	$ 73,716
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	4,503	10,765	4,888	21,907
Outside Services
Restructuring Cost and Reserve [Line Items]
Restructuring costs	4,956	19,499	16,368	43,292
Other
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 1,204	$ 5,098	$ 4,147	$ 8,468