Interest Accrual Period

No definition available.

Name:	njr_InterestAccrualPeriod
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Balance Type:	na
Period Type:	duration

- Definition

NJBPU Dividend Restriction Equity to Capitalization Ratio

+ References

No definition available.

+ Details

Name:	njr_NJBPUDividendRestrictionEquitytoCapitalizationRatio
Namespace Prefix:	njr_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- 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

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ 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(a)(1))

-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 835

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

+ Details

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

- Definition

When presenting a range of interest rates, the highest effective rate for funds borrowed under the debt agreement as of the balance sheet date.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentageRateRangeMaximum
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

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

- Definition

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

+ References

+ Details

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

- Definition

The percentage points added to the reference rate to compute the variable rate on the interest rate derivative.

+ References

No definition available.

+ Details

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

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: 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

Reference 2: 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 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20, 22

-Article 5

+ Details

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

- Definition

Indebtedness divided by net capital.

+ References

No definition available.

+ Details

Name:	us-gaap_RatioOfIndebtednessToNetCapital1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

The cash outflow from the repayment of a long-term debt instrument issued, secured by a first mortgage deed of trust, containing a pledge of real property. The lender has the highest claim on the property in case of default.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 15

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3291-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Financing Activities

-URI http://asc.fasb.org/extlink&oid=6513228

+ Details

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

- Definition

The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

+ References

+ Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=njr_VariableRateDebtMember
Namespace Prefix:
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Balance Type:
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- Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_FirstMortgageMember
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Balance Type:
Period Type:

- Details

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- Details

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Data Type:	na
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- Details

Name:	us-gaap_DebtInstrumentAxis=njr_SeriesQqMember
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- Details

Name:	us-gaap_DebtInstrumentAxis=njr_SeriesRrMember
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- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=njr_SeriesSSMember
Namespace Prefix:
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- Details

Name:	us-gaap_DebtInstrumentAxis=njr_SeriesTTMember
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Data Type:	na
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Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=njr_VariableDemandRateNotesMember
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- Details

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- Details

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DEBT, FIRST MORTGAGE BONDS (Details) $ in Thousands	12 Months Ended
DEBT, FIRST MORTGAGE BONDS (Details) $ in Thousands	Sep. 30, 2015 USD ($) debt_instrument	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)
Debt Instrument [Line Items]
NJBPU dividend restriction, equity to capitalization ratio	0.30
Debt to equity ratio	0.542
Payment of long-term debt	$ 37,039	$ 82,586	$ 8,953
Variable Rate Debt [Member]
Debt Instrument [Line Items]
Number of debt instruments issued \| debt_instrument	3
Long-term debt	$ 97,000
Series HH [Member]
Debt Instrument [Line Items]
Repayment of first mortgage bonds		12,000
First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Maximum amount that can be issued	830,700
NJNG [Member]
Debt Instrument [Line Items]
Payment of long-term debt		60,000
NJNG [Member] \| First Mortgage Bonds [Member] \| Series PP [Member]
Debt Instrument [Line Items]
Long-term debt	$ 50,000	50,000
Stated interest rate	3.15%
NJNG [Member] \| First Mortgage Bonds [Member] \| Series, QQ [Member]
Debt Instrument [Line Items]
Long-term debt	$ 70,000	70,000
Stated interest rate	3.58%
NJNG [Member] \| First Mortgage Bonds [Member] \| Series, RR [Member]
Debt Instrument [Line Items]
Long-term debt	$ 55,000	$ 55,000
Stated interest rate	4.61%
NJNG [Member] \| First Mortgage Bonds [Member] \| Private Placement [Member]
Debt Instrument [Line Items]
Stated interest rate	4.77%
NJNG [Member] \| First Mortgage Bonds [Member] \| Series HH [Member]
Debt Instrument [Line Items]
Stated interest rate		5.00%
NJNG [Member] \| First Mortgage Bonds [Member] \| Series, SS [Member]
Debt Instrument [Line Items]
Long-term debt	$ 50,000	$ 0
Stated interest rate	2.82%
NJNG [Member] \| First Mortgage Bonds [Member] \| Series, TT [Member]
Debt Instrument [Line Items]
Long-term debt	$ 100,000	$ 0
Stated interest rate	3.66%
NJNG [Member] \| Variable Demand Rate Notes [Member]
Debt Instrument [Line Items]
Interest accrual period	5 years
Maximum daily rate	12.00%
Interest rate on EDA Bonds	0.69%
NJNG [Member] \| Variable Demand Rate Notes [Member] \| Base Rate [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.55%
NJNG [Member] \| Variable Demand Rate Notes [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	70.00%