Sale of Energy, Chemicals and Resources ("ECR") Business (Narrative) (Details) - USD ($) $ in Thousands, shares in Millions			3 Months Ended	12 Months Ended
	Dec. 27, 2019	Apr. 26, 2019	Sep. 27, 2019	Oct. 02, 2020	Sep. 27, 2019	Sep. 28, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Selling, general and administrative expenses				$ 2,050,695	$ 2,072,177	$ 1,771,107
Interest expense				62,206	83,847	76,760
Loss on investment in equity securities				103,623	78,108	0
Nui Phao Mining Company
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Selling, general and administrative expenses	$ 40,000			$ 40,000
Worley Stock
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity instrument consideration, shares		58.2
Number of shares issued in transaction				6.8
Loss on sale of investments				$ 4,900
Worley Stock | Level 1
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Prepaid expense and other assets estimated fair value			$ 451,100	347,500	451,100
Worley Stock | Restricted Stock
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity instrument consideration, shares		51.4
Dividend income				16,900	5,200
Loss on investment in equity securities				103,600	78,100
Other (Expense) Income, Net
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Other operating expense		$ (35,400)
SG&A Expense
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Transaction costs		15,800
Discontinued Operations | Nui Phao Mining Company
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Interest expense					35,000
Foreign currency revaluations					6,000
Miscellaneous Income					4,400
Other operating expense					9,600
ECR Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration transferred		3,400,000
Business combination consideration paid in cash		2,800,000
ECR Business | Other (Expense) Income, Net
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Miscellaneous Income						8,400
ECR Business | Other (Expense) Income, Net | Guimar Joint Venture
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of businesses						21,000
ECR Business | Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of businesses		1,000,000	$ 17,400	110,236	935,110	0
Equity method investment retained after disposal		33,000
Deferred gain on disposal		34,400
Revenues		$ 95,300		$ 11,235	$ 2,725,699	$ 4,404,873