Discontinued Operations - Narrative (Details) - USD ($) $ in Thousands								1 Months Ended		12 Months Ended
	Sep. 26, 2025	Apr. 30, 2025	Apr. 10, 2025	Apr. 07, 2025	Mar. 14, 2025	Sep. 27, 2024	Apr. 26, 2019	Mar. 31, 2025	Feb. 28, 2025	Sep. 26, 2025	Sep. 27, 2024	Sep. 29, 2023
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Final settlement of the escrow shares at a recorded fair value	$ 0					$ 749,468				$ 0	$ 749,468
Charge to miscellaneous expense										189,663	(219,454)	$ 12,399
Equity method investment retained after disposal, mark-to market (losses) gains										(227,300)	186,900
Transition services agreement, income										40,500
Net (Loss) Earnings Attributable to Jacobs from Discontinued Operations										(23,966)	193,289	286,652
Worley Stock
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ordinary shares included in purchase price (in shares)							58,200,000
Net (Loss) Earnings Attributable to Jacobs from Discontinued Operations										2,700	(600)	(800)
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
Discontinued Operations, 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, Spinoff | SpinCo Business
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Distribution to shareholders, pro rata basis (in shares)						124,084,108
Distribution to shareholders, conversion ratio (in shares)	1					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										(26,646)	$ 193,877	$ 287,493
Discontinued Operations, 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					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
Final settlement of the escrow shares at a recorded fair value						$ 749,500					$ 749,500
Discontinued operation, entitled to receive shares held in escrow (in shares)									1,200,000
Charge to miscellaneous expense				$ 21,900
Equity method investment retained after disposal, mark-to market (losses) gains										(227,300)	$ 186,900
Transition services agreement, income										$ 40,500