Line of credit facility accordion feature potential borrowing capacity.

No definition available.

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

- Definition

Number of Tranches in Revolving Credit Facility

+ References

No definition available.

+ Details

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

- Definition

Percentage of unused commitments ticking fee payment.

+ References

No definition available.

+ Details

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

- Definition

Carrying value as of the balance sheet date of payments made in excess of existing cash balances, which will be honored by the bank but reflected as a loan to the entity. Overdrafts generally have a very short time frame for correction or repayment and are therefore more similar to short-term bank financing than trade financing.

+ References

+ Details

Name:	us-gaap_BankOverdrafts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Period remaining on line of credit facility before it terminates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
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_LineOfCreditFacilityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_ShortTermBorrowings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Weighted average interest rate of short-term debt outstanding calculated at point in time.

+ References

+ Details

Name:	us-gaap_ShortTermDebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=jec_EurocurrencyInterestRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=jec_BaseInterestRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=jec_SubFacilityOfSwingLineLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=jec_RevolvingCreditFacilityOneBillionSixHundredMillionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=jec_CommittedAndUncommittedLetterOfCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=jec_TermLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=jec_OtherLoanAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_BankOverdraftsMember
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Borrowings (Narrative) (Details)	12 Months Ended
Borrowings (Narrative) (Details)	Sep. 29, 2017 USD ($) tranche	Sep. 28, 2017 USD ($)	Sep. 30, 2016 USD ($)	Feb. 07, 2014 USD ($)
Line of Credit Facility [Line Items]
Total issued letters of credit	$ 262,100,000
Number of tranches in revolving credit facility (in tranches) \| tranche	2
Minimum
Line of Credit Facility [Line Items]
Facility fee percentage	0.10%
Maximum
Line of Credit Facility [Line Items]
Facility fee percentage	0.25%
Revolving Credit Facility
Line of Credit Facility [Line Items]
Direct borrowings on credit facility	$ 235,000,000
Available borrowing capacity	1,363,000,000
Credit facility, maximum borrowing capacity	2,100,000,000
Letter of Credit
Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity	300,000,000
Direct borrowings on credit facility	$ 2,500,000
Eurocurrency Interest Rate \| Minimum
Line of Credit Facility [Line Items]
Margin added to variable rate interest rate (percentage)	1.00%
Eurocurrency Interest Rate \| Maximum
Line of Credit Facility [Line Items]
Margin added to variable rate interest rate (percentage)	1.50%
Base Interest Rate \| Minimum
Line of Credit Facility [Line Items]
Margin added to variable rate interest rate (percentage)	0.00%
Base Interest Rate \| Maximum
Line of Credit Facility [Line Items]
Margin added to variable rate interest rate (percentage)	0.50%
Sub Facility Of Swing Line Loans
Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity	$ 50,000,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Credit facility, maximum borrowing capacity				$ 1,600,000,000
Revolving Credit Facility \| Letter of Credit
Line of Credit Facility [Line Items]
Direct borrowings on credit facility	2,500,000
Committed And Uncommitted Letter Of Credit Facility \| Letter of Credit
Line of Credit Facility [Line Items]
Direct borrowings on credit facility	$ 259,600,000
Term Loan Facility
Line of Credit Facility [Line Items]
Direct borrowings on credit facility		$ 1,500,000,000
Line of credit facility, expiration period	3 years
Term Loan Facility \| Minimum
Line of Credit Facility [Line Items]
Percentage of unused commitments, ticking fee payment	0.10%
Term Loan Facility \| Maximum
Line of Credit Facility [Line Items]
Percentage of unused commitments, ticking fee payment	0.25%
Term Loan Facility \| Eurocurrency Interest Rate \| Minimum
Line of Credit Facility [Line Items]
Margin added to variable rate interest rate (percentage)	1.00%
Term Loan Facility \| Eurocurrency Interest Rate \| Maximum
Line of Credit Facility [Line Items]
Margin added to variable rate interest rate (percentage)	1.50%
Term Loan Facility \| Base Interest Rate \| Minimum
Line of Credit Facility [Line Items]
Margin added to variable rate interest rate (percentage)	0.00%
Term Loan Facility \| Base Interest Rate \| Maximum
Line of Credit Facility [Line Items]
Margin added to variable rate interest rate (percentage)	0.50%
Other Loan Agreements
Line of Credit Facility [Line Items]
Overdrafts outstanding	$ 3,100,000
Short-term Debt			$ 2,400,000
Bank Overdrafts
Line of Credit Facility [Line Items]
Short-term debt interest rate (percent)	4.33%