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Business Combinations - Summary of Unaudited Proforma Operating Results (Details) - CH2M HILL Companies, Ltd. - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Jun. 29, 2018	Jun. 30, 2017
Business Acquisition [Line Items]
Revenues	$ 11,869.8	$ 10,709.9
Net earnings	226.1	130.8
Net earnings (loss) attributable to Jacobs	$ 222.1	$ 112.7
Net earnings (loss) attributable to Jacobs per share:
Basic earnings (loss) per share (in dollars per share)	$ 1.56	$ 0.80
Diluted earnings (loss) per share (in dollars per share)	$ 1.55	$ 0.80
Business acquisitions pro forma income tax expense (benefit)	$ 180.4	$ (14.9)

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