Pension and Other Postretirement Benefit Plans - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended					9 Months Ended
	Jun. 26, 2020	Dec. 27, 2019	Jun. 28, 2019	Mar. 29, 2019	Dec. 28, 2018	Jun. 26, 2020	Jun. 28, 2019
Defined Benefit Plan Disclosure [Line Items]
Settlement gain (loss)	$ 0		$ 0			$ (2,651)	$ 34,621
Corporate | CH2M HILL Companies, Ltd. | Other Expense | United States
Defined Benefit Plan Disclosure [Line Items]
Gain (loss) related to settlement		$ 2,700	$ 0			$ (2,100)	$ 34,600
CH2M Hill and OMI Retiree Medical Plans
Defined Benefit Plan Disclosure [Line Items]
Settlement gain (loss)				$ 2,200	$ 32,400