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Business Combinations (Summary of Unaudited Proforma Operating Results) (Details) - CH2M HILL Companies, Ltd. - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Dec. 29, 2017	Dec. 30, 2016
Business Acquisition [Line Items]
Revenues	$ 3,778.0	$ 3,652.0
Net earnings (loss)	25.8	(47.0)
Net earnings (loss) attributable to Jacobs	$ 23.2	$ (56.6)
Net earnings (loss) attributable to Jacobs per share:
Basic earnings (loss) per share	$ 0.16	$ (0.40)
Diluted earnings (loss) per share	$ 0.16	$ (0.40)

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Business acquisition proforma earnings per share basic and diluted.

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