Segment Information - Schedule of Total Revenues, Segment Operating Profit and Total Asset for Reporting Segment (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Jun. 26, 2020	Dec. 27, 2019	Jun. 28, 2019	Jun. 26, 2020	Jun. 28, 2019
Segment Reporting Information [Line Items]
Revenues	$ 3,260,057		$ 3,169,622	$ 10,047,286	$ 9,345,005
Total Segment Operating Profit	194,376		89,954	513,500	305,763
Restructuring, Transaction and Other Charges	(20,472)		(105,145)	(115,539)	(246,317)
Total Other Income (Expense), net	109,305		3,445	(132,453)	(8,344)
Earnings from Continuing Operations Before Taxes	303,681		93,399	381,047	297,419
Other expense			2,000		14,800
Amortization of intangible assets				67,074
Fair value adjustments related to investment in Worley stock	122,900			(119,000)
Operating Segments
Segment Reporting Information [Line Items]
Total Segment Operating Profit	280,061		259,624	822,187	737,754
Corporate
Segment Reporting Information [Line Items]
Other Corporate Expenses	(65,213)		(64,525)	(193,148)	(185,674)
Corporate | Other Expense | CH2M HILL Companies, Ltd.
Segment Reporting Information [Line Items]
Amortization of deferred financing fees			500	700	1,500
Corporate | Other Expense | CH2M HILL Companies, Ltd. | United States
Segment Reporting Information [Line Items]
Gain (loss) related to settlement		$ 2,700	0	(2,100)	34,600
Critical Mission Solutions | Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,211,143		1,156,488	3,636,978	3,251,024
Total Segment Operating Profit	89,608		76,306	264,323	222,289
People & Places Solutions | Operating Segments
Segment Reporting Information [Line Items]
Revenues	2,048,914		2,013,134	6,410,308	6,093,981
Total Segment Operating Profit	190,453		183,318	557,864	515,465
Worley
Segment Reporting Information [Line Items]
TSA related income	1,000			15,200
Worley | Corporate | Other Expense
Segment Reporting Information [Line Items]
Amortization of intangible assets	$ 23,100		18,400	$ 67,100	55,700
Worley | Corporate | Revenue
Segment Reporting Information [Line Items]
Other Corporate Expenses			$ (14,100)		$ (14,100)