Business Combinations - Summary of Unaudited Proforma Operating Results (Details) - CH2M HILL Companies, Ltd. - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
Mar. 30, 2018	Mar. 31, 2017
Business Acquisition [Line Items]
Revenues	$ 7,713.1	$ 7,043.4
Net earnings	77.7	10.6
Net earnings (loss) attributable to Jacobs	$ 71.8	$ (2.8)
Net earnings (loss) attributable to Jacobs per share:
Basic earnings (loss) per share (in dollars per share)	$ 0.50	$ (0.02)
Diluted earnings (loss) per share (in dollars per share)	$ 0.50	$ (0.02)
Business acquisitions pro forma income tax expense (benefit)	$ 137.7	$ (61.9)

Business acquisition proforma earnings per share basic and diluted.

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Business acquisitions pro forma income tax expense (benefit).

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Business acquisitions proforma net income loss from acquiree.

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The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(2)-(3)
-URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463

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The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

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The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

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