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Business Combinations (Summary of Unaudited Proforma Operating Results) (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 29, 2017	Dec. 30, 2016
Business Acquisition Pro Forma Information [Abstract]
Business acquisitions pro forma income tax expense (benefit)	$ 67.4	$ (78.6)

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- Definition

Business acquisitions pro forma income tax expense (benefit).

+ References

No definition available.

+ Details

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Namespace Prefix:	jec_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
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