Segment Information - Schedule of Total Revenues, Segment Operating Profit and Total Asset for Reporting Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 29, 2019	Mar. 30, 2018	Mar. 29, 2019	Mar. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 3,091,596	$ 2,870,295	$ 6,175,384	$ 4,654,294
Total Segment Operating Profit	102,681	68,755	215,810	64,085
Restructuring and Other Charges	(93,938)	(76,473)	(141,171)	(92,200)
Total Other (Expense) Income, net	9,151	(20,104)	(11,789)	(22,137)
Earnings from Continuing Operations Before Taxes	111,832	48,651	204,021	41,948
Amortization of Intangible Assets			(37,963)	(36,048)
CH2M HILL Companies, Ltd.
Segment Reporting Information [Line Items]
Transaction Costs	0	(4,852)	0	(72,493)
CH2M HILL Companies, Ltd. | Other Expense
Segment Reporting Information [Line Items]
Litigation expenses included in segment profit	32,400
Amortization of deferred financing fees	500	500	1,000	700
Operating Segments
Segment Reporting Information [Line Items]
Total Segment Operating Profit	246,520	197,213	478,130	325,139
Corporate
Segment Reporting Information [Line Items]
Other Corporate Expenses (1)	(49,901)	(47,133)	(121,149)	(96,361)
Aerospace, Technology and Nuclear | Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,059,508	923,905	2,094,537	1,634,780
Total Segment Operating Profit	73,831	52,458	145,982	113,524
Buildings, Infrastructure and Advanced Facilities | Operating Segments
Segment Reporting Information [Line Items]
Revenues	2,032,088	1,946,390	4,080,847	3,019,514
Total Segment Operating Profit	172,689	144,755	332,148	211,615
ECR Business | Corporate | Other Expense
Segment Reporting Information [Line Items]
Other Corporate Expenses (1)	(6,400)	(6,400)	(12,800)
Amortization of Intangible Assets	$ (18,700)	$ 18,200	$ (37,300)	$ (29,800)