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Business Combinations - Summary of Unaudited Proforma Operating Results (Details) - CH2M HILL Companies, Ltd. $ / shares in Units, $ in Millions	3 Months Ended
	Dec. 29, 2017 USD ($) $ / shares
Business Acquisition [Line Items]
Revenues	$ 3,778.6
Net earnings of the Group	33.6
Net earnings (loss) attributable to Jacobs	$ 31.1
Net earnings (loss) attributable to Jacobs per share:
Basic earnings (loss) per share (in dollars per share) | $ / shares	$ 0.22
Diluted earnings (loss) per share (in dollars per share) | $ / shares	$ 0.21
Business acquisitions pro forma income tax expense (benefit)	$ 70.5

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