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Restructuring and Other Charges - Schedule of Restructuring and Other Activities by Major Type of Costs (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Jun. 30, 2023
Jul. 01, 2022
Jun. 30, 2023
Jul. 01, 2022
Restructuring Cost and Reserve [Line Items]        
Final pre-tax settlement   $ 91,300   $ 91,300
CH2M Hill, KeyM, John Wood Group Acquisitions and ECR Sale        
Restructuring Cost and Reserve [Line Items]        
Restructuring costs $ 30,912 4,577 $ 78,196 182,100
CH2M Hill, KeyM, John Wood Group Acquisitions and ECR Sale | Lease Abandonments and Impairments        
Restructuring Cost and Reserve [Line Items]        
Restructuring costs 803 (32) 38,076 67,805
CH2M Hill, KeyM, John Wood Group Acquisitions and ECR Sale | Voluntary and Involuntary Terminations        
Restructuring Cost and Reserve [Line Items]        
Restructuring costs 17,925 (76) 26,434 5,035
CH2M Hill, KeyM, John Wood Group Acquisitions and ECR Sale | Outside Services        
Restructuring Cost and Reserve [Line Items]        
Restructuring costs 12,168 4,337 13,646 20,513
CH2M Hill, KeyM, John Wood Group Acquisitions and ECR Sale | Other        
Restructuring Cost and Reserve [Line Items]        
Restructuring costs $ 16 348 $ 40 88,747
CH2M HILL Companies, Ltd.        
Restructuring Cost and Reserve [Line Items]        
Final pre-tax settlement   $ 91,300   $ 91,300