Loss contingency minimum number of insurers with possible inability to fulfill insurance obligations.

No definition available.

Name:	jec_LossContingencyMinimumNumberOfInsurersWithPossibleInabilityToFulfillInsuranceObligations
Namespace Prefix:	jec_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of judgment or settlement awarded to (against) the entity in respect of litigation.

+ References

No definition available.

+ Details

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

- Definition

Amount of litigation expense, including but not limited to legal, forensic, accounting, and investigative fees.

+ References

No definition available.

+ Details

Name:	us-gaap_LitigationSettlementExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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

- Definition

The value (monetary amount) of the award the plaintiff seeks in the legal matter.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14557-108349

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14435-108349

+ Details

Name:	us-gaap_LossContingencyDamagesSoughtValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_LitigationCaseAxis=jec_MotivaEnterprisesLLCvBechtelJacobsCEPPortArthurJointVentureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationStatusAxis=us-gaap_PendingLitigationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationCaseAxis=jec_NuiPhaoMiningCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationCaseAxis=jec_JudicialCouncilOfCaliforniaUnpaidFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LitigationCaseAxis=jec_JudicialCouncilOfCaliforniaDisgorgedFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Contractual Guarantees, Litigation, Investigations, and Insurance (Narrative) (Details) $ in Millions					12 Months Ended
	Sep. 30, 2015 USD ($)	Aug. 20, 2015 USD ($)	Aug. 09, 2014 USD ($)	May 02, 2012 USD ($)	Sep. 30, 2016 insurer
Loss Contingencies [Line Items]
Minimum number of insurers with possible inability to fulfill insurance obligations \| insurer					1
Motiva Enterprises LLC v. Bechtel-Jacobs CEP Port Arthur Joint Venture \| Pending Litigation
Loss Contingencies [Line Items]
Damages sought			$ 8,000.0
Nui Phao Mining Company
Loss Contingencies [Line Items]
Damages sought	$ 167.0
Judicial Council Of California - Unpaid Fees
Loss Contingencies [Line Items]
Litigation amount				$ 4.7
Litigation contract amount		$ 4.7
Judicial Council Of California, Disgorged Fees
Loss Contingencies [Line Items]
Litigation amount				$ 18.3
Litigation contract amount		$ 18.3