Debt Instrument, Contingent Interest Rate, Increase (Decrease)

No definition available.

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Period Type:	instant

- Definition

Line of credit facility accordion feature potential borrowing capacity.

+ References

No definition available.

+ Details

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

Amount of liabilities incurred by the acquirer as part of consideration transferred in a business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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+ Details

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

- 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

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

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Data Type:	num:percentItemType
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_DebtInstrumentLineItems
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Balance Type:	na
Period Type:	duration

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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+ 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

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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

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

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

- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

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

- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

+ Details

Name:	us-gaap_LongTermDebtNoncurrent
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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 210
-SubTopic 10
-Section S99
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+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	srt_RangeAxis=srt_MinimumMember
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Balance Type:
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- Details

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- Details

Name:	us-gaap_DebtInstrumentAxis=jec_BilateralTermLoanFacilityConvertibleSeniorNotesMember
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Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=jec_TermLoanMember
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Balance Type:
Period Type:

- Details

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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_DebtInstrumentAxis=jec_CommittedAndUncommittedLetterOfCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=jec_KeyWHoldingCorporationMember
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=jec_ConvertibleSeniorNotes2Point5PercentDueJuly152019Member
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Balance Type:
Period Type:

- Details

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Period Type:

- Details

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- Details

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Borrowings - (Narrative) (Details)		9 Months Ended		12 Months Ended
Borrowings - (Narrative) (Details)	Jun. 12, 2019 USD ($)	Jun. 28, 2019 USD ($)	Jun. 29, 2018 tranche	Sep. 28, 2018 USD ($)	Mar. 27, 2019 USD ($)	Feb. 07, 2014 USD ($)
Debt Instrument [Line Items]
Long-term Debt		$ 1,025,198,000		$ 2,144,167,000
Minimum
Debt Instrument [Line Items]
Facility fee percentage		0.08%
Maximum
Debt Instrument [Line Items]
Facility fee percentage		0.20%
Bilateral Term Loan Facility, Convertible Senior Notes
Debt Instrument [Line Items]
Short-term debt		$ 222,700,000
Revolving Credit Facility One Billion Six Hundred Million
Debt Instrument [Line Items]
Available borrowing capacity		2,120,000,000
Term Loan Facility
Debt Instrument [Line Items]
Long-term Debt		400,000,000		1,500,000,000
Senior Notes As Amended Note Purchase Agreement
Debt Instrument [Line Items]
Long-term Debt		500,000,000.0
Long-term debt fair value		523,500,000
Revolving Credit Facility
Debt Instrument [Line Items]
Number of tranches \| tranche			2
Long-term Debt		0		149,129,000
Revolving Credit Facility \| Revolving Credit Facility One Billion Six Hundred Million
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity					$ 2,250,000,000	$ 1,600,000,000
Credit facility, potential borrowing capacity					$ 3,250,000,000
Letter of Credit
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity		400,000,000.0
Letter of Credit \| Revolving Credit Facility One Billion Six Hundred Million
Debt Instrument [Line Items]
Direct borrowings on credit facility		2,300,000
Letter of Credit \| Committed And Uncommitted Letter Of Credit Facility
Debt Instrument [Line Items]
Direct borrowings on credit facility		356,700,000
Sub Facility Of Swing Line Loans
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity		50,000,000.0
KeyW Holding Corporation \| Convertible Senior Notes, 2 Point 5 Percent, Due July 15, 2019 [Member]
Debt Instrument [Line Items]
Equipment financing rate	2.50%
Liabilities assumed in business acquisition	$ 22,600,000
Line of Credit
Debt Instrument [Line Items]
Short-term debt		$ 359,000,000.0		$ 446,600,000
Senior Notes
Debt Instrument [Line Items]
Contingent interest rate increase		0.75%
LIBOR \| Minimum
Debt Instrument [Line Items]
Margin added to variable rate interest rate (percentage)		3.78%		3.71%
LIBOR \| Bilateral Term Loan Facility, Convertible Senior Notes
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity	$ 200,000,000.0
Margin added to variable rate interest rate (percentage)	1.00%
LIBOR \| Revolving Credit Facility \| Minimum
Debt Instrument [Line Items]
Margin added to variable rate interest rate (percentage)		1.38%		1.38%
LIBOR \| Revolving Credit Facility \| Maximum
Debt Instrument [Line Items]
Margin added to variable rate interest rate (percentage)				3.47%