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DEBT - NJNG SHORT-TERM DEBT (Details) - Natural Gas Distribution	12 Months Ended
	Sep. 30, 2017 USD ($) debt_instrument	Sep. 30, 2016 USD ($)
Letters of credit
Line of Credit Facility [Line Items]
Number of debt instruments (in debt instruments) | debt_instrument	2
Letters of credit outstanding, amount	$ 731,000	$ 731,000
Committed Credit Facilities Due May 2019 | Bank revolving credit facilities
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 250,000,000	$ 250,000,000
Bank revolving credit facilities
Line of Credit Facility [Line Items]
Debt instrument, term	5 years
Line of credit facility, maximum borrowing capacity, incremental increase	$ 15,000,000
Line of credit facility, maximum borrowing capacity, maximum increase	$ 50,000,000

X

- Definition

Line of Credit Facility, Maximum Borrowing Capacity, Incremental Increase

+ References

No definition available.

+ Details

Name:	njr_LineOfCreditFacilityMaximumBorrowingCapacityIncrementalIncrease
Namespace Prefix:	njr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Line of Credit Facility, Maximum Borrowing Capacity, Maximum Increase

+ References

No definition available.

+ Details

Name:	njr_LineOfCreditFacilityMaximumBorrowingCapacityMaximumIncrease
Namespace Prefix:	njr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Number of Debt Instruments

+ References

No definition available.

+ Details

Name:	njr_NumberofDebtInstruments
Namespace Prefix:	njr_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Period of time between issuance and maturity of debt instrument, 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_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- 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

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

X

- 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

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(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

X

- Details

Name:	dei_LegalEntityAxis=njr_NjngMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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

X

- Details

Name:	us-gaap_DebtInstrumentAxis=njr_CommittedCreditFacilitiesDueMay2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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

X

- Details

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