Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Oct. 02, 2020	Sep. 27, 2019	Sep. 28, 2018
Defined Benefit Plan Disclosure [Line Items]
(Gain) Loss on pension and retiree medical plan changes	$ (4,598)	$ 33,087	$ (5,414)
SG&A Expense
Defined Benefit Plan Disclosure [Line Items]
Service cost			24,200
U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Period of annual average returns used in return on plan assets simulation model (in years)	20 years
Service cost	$ 409	2,784	4,765
Gain on litigation settlement	(2,700)
Curtailments/settlements/plan amendments	18,557	9,751
Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	5,710	7,171	$ 8,269
Curtailments/settlements/plan amendments	4,782	5,814
CH2M HILL Companies, Ltd. | Other | Other Expense
Defined Benefit Plan Disclosure [Line Items]
Gain on litigation settlement		35,000
United Kingdom | Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, increase (decrease) due to pension ruling		38,200
United Kingdom | Non-U.S. Plans | Other Comprehensive Income (Loss)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, increase (decrease) due to pension ruling		1,500
Sverdrup and Ireland Pension Plans | CH2M Retiree Medical and Ireland | Non-U.S. Plans
Defined Benefit Plan Disclosure [Line Items]
(Gain) Loss on pension and retiree medical plan changes	$ 4,600	$ (33,100)