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Significant Accounting Policies (Basis to Determine Earned Relative TSR Based on PSU) (Details) - Earned Relative TSR Based PSUs	12 Months Ended
	Sep. 29, 2017
Below 30th percentile
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
TSR Performance Multiplier	0.00%
30th percentile
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
TSR Performance Multiplier	50.00%
50th percentile
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
TSR Performance Multiplier	100.00%
70th percentile or above
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
TSR Performance Multiplier	150.00%

X

- Definition

Share-based compensation arrangement by share-based payment award, equity instruments other than options, TSR performance multiplier.

+ References

No definition available.

+ Details

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

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_AwardTypeAxis=jec_EarnedRelativeTSRRestrictedStockUnitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	jec_CompanyTSRPercentileRankAxis=jec_Below30ThPercentileMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	jec_CompanyTSRPercentileRankAxis=jec_ThirtiethPercentileMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	jec_CompanyTSRPercentileRankAxis=jec_FiftiethPercentileMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	jec_CompanyTSRPercentileRankAxis=jec_SeventiethPercentileOrAboveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: