Discontinued Operations - Narrative (Details) - USD ($) $ in Thousands							1 Months Ended		3 Months Ended		9 Months Ended
	Apr. 30, 2025	Apr. 10, 2025	Apr. 07, 2025	Mar. 14, 2025	Sep. 27, 2024	Apr. 26, 2019	Mar. 28, 2025	Feb. 28, 2025	Jun. 27, 2025	Jun. 28, 2024	Jun. 27, 2025	Jun. 28, 2024
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investment in equity securities					$ 749,468				$ 0		$ 0
Miscellaneous income (expense), net									38,844	$ 1,550	(194,523)	$ (5,118)
Fair value mark-to-market, amount									27,400		227,300
Transition services agreement, expenses incurred									9,800		31,500
Net (loss) earnings attributable to Jacobs from discontinued operations									(1,629)	64,010	(8,180)	177,152
Worley Stock
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ordinary shares included in purchase price						$ 58,200
Net (loss) earnings attributable to Jacobs from discontinued operations									500	500	500	(900)
Worley
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Consideration transferred						3,400,000
Consideration paid in cash						$ 2,800,000
Discontinued Operations, Disposed of by Means Other than Sale, Spinoff | Amentum Holdings, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Discontinued operation, remaining shares held in escrow (in shares)			9,700,000
Discontinued Operations, Disposed of by Means Other than Sale, Spinoff | SpinCo Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Stock issued during period, spinoff transaction ( in shares)					124,084,108
Shares issued during period, spinoff, conversion ratio (in shares)					1
Cash received from SpinCo business					$ 911,000
Discontinued operation, working capital adjustment							$ 70,000
Discontinued operation, working capital adjustment, reduction in recorded receivable							$ 24,000
Discontinued operation, receivable balance collected		$ 70,000
Discontinued operation, remaining shares held in escrow (in shares)	7,300,000		7,300,000
Net (loss) earnings attributable to Jacobs from discontinued operations									(2,085)	$ 63,525	(8,647)	$ 178,010
Discontinued Operations, Disposed of by Means Other than Sale, Spinoff | SpinCo Business | Amentum Holdings, Inc.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity method investment retained, shares owned after disposal (in shares)					29,200,000
Equity method investment retained, shares transferred to escrow (in shares)					10,900,000
Discontinued operation, remaining shares held in escrow (in shares)			2,400,000	9,700,000	18,300,000
Investment in equity securities					$ 749,500
Discontinued operation, entitled to receive shares held in escrow (in shares)								1,200,000
Miscellaneous income (expense), net			$ 21,900
Transition services agreement, expenses incurred									$ 9,800		$ 31,500