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Savings and Deferred Compensation Plans Savings and Deferred Compensation Plans (Savings Plans) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 27, 2019	Sep. 28, 2018	Sep. 29, 2017
Savings And Deferred Compensation Plans [Abstract]
Savings plans contributions	$ 103,375	$ 113,135	$ 82,882

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- Definition

Defined Contribution Plan Savings Plans Contributions

+ References

No definition available.

+ Details

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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