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Segment Information - Revenues Earned Directly or Indirectly from the U.S. Federal Government and its Agencies (Details)	12 Months Ended
	Sep. 29, 2017	Sep. 30, 2016	Oct. 02, 2015
Segment Reporting [Abstract]
Percentage of revenue earned from the U.S. federal government and its agencies	19.20%	21.40%	21.70%

X

- Definition

Percentage of revenue earned from the U.S. federal government and their agencies

+ References

No definition available.

+ Details

Name:	jec_PercentageOfRevenueEarnedFromUSFederalGovernmentAndTheirAgencies
Namespace Prefix:	jec_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- References

No definition available.

+ Details

Name:	us-gaap_SegmentReportingAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration