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Savings and Deferred Compensation Plans Savings and Deferred Compensation Plans (Savings Plans) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 28, 2018 | Sep. 29, 2017 | Sep. 30, 2016
Savings And Deferred Compensation Plans [Abstract] |  |  |
Savings plans contributions | $ 113,135 | $ 82,882 | $ 89,966

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- Definition Defined Contribution Plan Savings Plans Contributions + 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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- Definition Savings And Deferred Compensation Plans [Abstract] + References No definition available. + Details
Name: | jec_SavingsAndDeferredCompensationPlansAbstract
Namespace Prefix: | jec_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration