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PA Consulting Business Combinations - Summary of Unaudited Proforma Operating Results (Details) - PA Consulting Group Limited - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Oct. 01, 2021	Oct. 02, 2020
Business Acquisition [Line Items]
Revenues	$ 14,504.3	$ 14,348.1
Net earnings of the Group	695.1	83.5
Net earnings attributable to Jacobs	$ 548.0	$ 138.6
Net earnings attributable to Jacobs per share:
Basic earnings (loss) per share (in dollars per share)	$ 4.21	$ 0.62
Diluted earnings (loss) per share (in dollars per share)	$ 4.17	$ 0.61
Business acquisitions pro forma income tax expense (benefit)	$ (330.7)	$ (13.5)

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