Goodwill and Intangibles - Schedule of Carrying Value of Goodwill by Reportable Segment Appearing in Accompanying Consolidated Balance Sheets (Details) $ in Thousands	6 Months Ended
	Mar. 30, 2018 USD ($)
Goodwill [Roll Forward]
September 29, 2017	$ 3,009,826
Acquired	2,698,800
Post-Acquisition Adjustments	179,000
Foreign Exchange Impact	29,200
March 30, 2018	5,916,827
Aerospace, Technology, Environmental and Nuclear
Goodwill [Roll Forward]
September 29, 2017	1,038,200
Acquired	945,200
Post-Acquisition Adjustments	106,700
Foreign Exchange Impact	10,000
March 30, 2018	2,100,100
Buildings, Infrastructure and Advanced Facilities
Goodwill [Roll Forward]
September 29, 2017	1,048,800
Acquired	1,417,900
Post-Acquisition Adjustments	40,200
Foreign Exchange Impact	10,200
March 30, 2018	2,517,100
Energy, Chemicals and Resources
Goodwill [Roll Forward]
September 29, 2017	922,800
Acquired	335,700
Post-Acquisition Adjustments	32,100
Foreign Exchange Impact	9,000
March 30, 2018	$ 1,299,600