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		9 Months Ended
	Jun. 26, 2020	Jun. 28, 2019	Jun. 26, 2020	Jun. 28, 2019
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 19,681	$ 93,239	$ 112,309	$ 205,119
Lease Abandonments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	(11,776)	22,982	(7,860)	72,296
Involuntary Terminations
Restructuring Cost and Reserve [Line Items]
Restructuring costs	(49)	12,020	18,222	22,944
Outside Services
Restructuring Cost and Reserve [Line Items]
Restructuring costs	20,865	39,853	74,223	95,130
Other
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 10,641	$ 18,384	$ 27,724	$ 14,749