Restructuring and Other Charges - Summary of Restructuring and Other Activities by Major Type of Costs (Details) - CH2M HILL Companies, Ltd. - USD ($) $ in Thousands	12 Months Ended
	Oct. 02, 2020	Sep. 27, 2019	Sep. 28, 2018
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 324,183	$ 311,470	$ 153,640
Lease Abandonments and Impairments
Restructuring Cost and Reserve [Line Items]
Restructuring costs	151,150	99,976	61,526
Voluntary and Involuntary Terminations
Restructuring Cost and Reserve [Line Items]
Restructuring costs	53,484	33,742	29,056
Outside Services
Restructuring Cost and Reserve [Line Items]
Restructuring costs	88,476	133,148	35,987
Other
Restructuring Cost and Reserve [Line Items]
Restructuring costs	$ 31,073	$ 44,604	$ 27,071