PA Consulting Business Combinations - Summary of Unaudited Proforma Operating Results (Details) - PA Consulting Group Limited $ / shares in Units, $ in Millions	6 Months Ended
	Apr. 02, 2021 USD ($) $ / shares
Business Acquisition [Line Items]
Revenues	$ 7,341.4
Net earnings (loss) of the Group	449.9
Net earnings attributable to Jacobs	$ 337.7
Net earnings (loss) attributable to Jacobs per share:
Basic earnings (loss) per share (in dollars per share) | $ / shares	$ 2.60
Diluted earnings (loss) per share (in dollars per share) | $ / shares	$ 2.58
Business acquisitions pro forma income tax expense (benefit)	$ (122.1)