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Segment Information - Revenues Earned Directly or Indirectly from the U.S. Federal Government and its Agencies (Details) | 12 Months Ended
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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