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Business Combinations - Summary of Unaudited Proforma Operating Results (Details) - CH2M HILL Companies, Ltd. - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 28, 2018	Sep. 29, 2017
Business Acquisition [Line Items]
Revenues	$ 16,012.4	$ 14,612.4
Net earnings	196.3	264.8
Net earnings (loss) attributable to Jacobs	$ 184.5	$ 243.6
Net earnings (loss) attributable to Jacobs per share:
Basic earnings (loss) per share (in dollars per share)	$ 1.28	$ 1.73
Diluted earnings (loss) per share (in dollars per share)	$ 1.27	$ 1.72
Business acquisitions pro forma income tax expense (benefit)	$ 409.7	$ 24.2

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