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Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Jul. 02, 2021
Oct. 01, 2021
Oct. 02, 2020
Sep. 27, 2019
Mar. 02, 2021
Dec. 15, 2017
Defined Benefit Plan Disclosure [Line Items]            
(Gain) Loss on pension and retiree medical plan changes   $ (2,783) $ (4,598) $ 33,087    
Gain on litigation settlement   $ 19,500        
Pension Plan            
Defined Benefit Plan Disclosure [Line Items]            
Period of annual average returns used in return on plan assets simulation model (in years)   20 years        
PA Consulting Group Limited            
Defined Benefit Plan Disclosure [Line Items]            
Percentage of outstanding shares of common and preferred stock acquired         65.00%  
CH2M HILL Companies, Ltd.            
Defined Benefit Plan Disclosure [Line Items]            
Percentage of outstanding shares of common and preferred stock acquired           100.00%
CH2M HILL Companies, Ltd. | Other | Other Expense            
Defined Benefit Plan Disclosure [Line Items]            
Gain on litigation settlement   $ (2,100)   35,000    
PA Consulting | Pension Plan            
Defined Benefit Plan Disclosure [Line Items]            
Net benefit obligation at the end of the year         $ 1,025,000  
Fair value of plan assets at the end of the year         $ 1,018,000  
Plan assets (decrease) for assets transferred from Plan $ 960,000          
Benefit obligation decrease for liabilities transferred from plan $ 960,000          
Non-U.S. Plans | Pension Plan            
Defined Benefit Plan Disclosure [Line Items]            
Net benefit obligation at the end of the year   2,375,640 2,388,077 2,258,129    
Fair value of plan assets at the end of the year   2,203,495 2,043,356 $ 1,916,637    
Sverdrup and Ireland Pension Plans | CH2M Retiree Medical and Ireland | Pension Plan            
Defined Benefit Plan Disclosure [Line Items]            
(Gain) Loss on pension and retiree medical plan changes   $ 2,800 $ 4,600