Basis of Presentation (Details) - USD ($) shares in Millions, $ in Millions			3 Months Ended		6 Months Ended
	Oct. 21, 2018	Dec. 15, 2017	Mar. 29, 2019	Mar. 30, 2018	Mar. 29, 2019	Mar. 30, 2018
ECR Business
Business Acquisition [Line Items]
Consideration transferred	$ 3,300.0
Business combination consideration paid in cash	2,600.0
Business combination consideration equity issued	58.2
CH2M HILL Companies, Ltd.
Business Acquisition [Line Items]
Integration related activities	$ 3,300.0		$ (12.3)	$ 14.1	$ 13.5	$ 28.0
Business combination consideration paid in cash		$ 1,800.0
Business combination, cash acquired		315.2
Business combination consideration equity issued		$ 1,400.0
Percentage of outstanding shares of common and preferred stock acquired		100.00%
Business combination consideration equity issued, shares		20.7
Long-term debt		$ 706.0
Revolving Credit Facility and Second Lien Notes | CH2M HILL Companies, Ltd.
Business Acquisition [Line Items]
Business combination prepayment penalty		20.0
Long-term debt		$ 700.0