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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 12 Months Ended
Sep. 27, 2024
Sep. 27, 2024
Sep. 29, 2023
Sep. 30, 2022
Restructuring Cost and Reserve [Line Items]        
Recorded third-party environmental recoveries receivable       $ 27,000
Interest Rate Swap        
Restructuring Cost and Reserve [Line Items]        
Derivative, gain on derivative $ 35,200 $ 35,200    
CH2M HILL Companies, Ltd.        
Restructuring Cost and Reserve [Line Items]        
Restructuring costs   134,911 $ 126,219 159,130
Pre-tax settlement less insurance recoveries       91,300
Lease Abandonments and Impairments | CH2M HILL Companies, Ltd.        
Restructuring Cost and Reserve [Line Items]        
Restructuring costs   49 44,788 67,458
Voluntary and Involuntary Terminations | CH2M HILL Companies, Ltd.        
Restructuring Cost and Reserve [Line Items]        
Restructuring costs   47,881 37,235 2,612
Outside Services | CH2M HILL Companies, Ltd.        
Restructuring Cost and Reserve [Line Items]        
Restructuring costs   100,593 35,099 22,068
Other | CH2M HILL Companies, Ltd.        
Restructuring Cost and Reserve [Line Items]        
Restructuring costs   $ (13,612) $ 9,097 $ 66,992