Defined Benefit Plan, Fair Value of Plan Liabilities

No definition available.

Name:	jec_DefinedBenefitPlanFairValueofPlanLiabilities
Namespace Prefix:	jec_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Number of noncontributory defined benefit pension plans.

+ References

No definition available.

+ Details

Name:	jec_NumberOfNoncontributoryDefinedBenefitPensionPlans
Namespace Prefix:	jec_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

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

- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(7)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanRecognizedNetGainLossDueToSettlements1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of payment for pension benefits. Excludes other postretirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585

+ Details

Name:	us-gaap_PensionContributions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_BusinessAcquisitionAxis=jec_CH2MHILLCompaniesLimitedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Defined Pension Benefit Obligations - Narrative (Details) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Jul. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 29, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 29, 2018 USD ($)	Jun. 30, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Payment for pension benefits					$ 18,862
Settlement loss (gain)		$ 3,800	$ 0	$ 51	3,819	$ 135
CH2M HILL Companies, Ltd.
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plan assets			1,100,000		1,100,000
Defined benefit pension plan liabilities			$ 1,200,000		$ 1,200,000
Number of noncontributory defined benefit pension plans			3		3
Subsequent Event [Member] \| CH2M HILL Companies, Ltd.
Defined Benefit Plan Disclosure [Line Items]
Payment for pension benefits	$ 55,000