EMPLOYEE BENEFIT PLANS - AMOUNTS EXPECTED TO BE RECOGNIZED IN NET PERIODIC BENEFIT COST (Details) $ in Thousands	12 Months Ended
Sep. 30, 2017 USD ($)
Pension
Regulatory Assets
Net actuarial loss	$ 6,177
Prior service cost (credit)	105
Total	6,282
Accumulated Other Comprehensive Income (Loss)
Net actuarial loss	1,360
Prior service cost (credit)	1
Total	1,361
OPEB
Regulatory Assets
Net actuarial loss	4,464
Prior service cost (credit)	(311)
Total	4,153
Accumulated Other Comprehensive Income (Loss)
Net actuarial loss	196
Prior service cost (credit)	(53)
Total	$ 143

Defined Benefit Plan Amortization Of Net Gains Losses Regulated Assets

No definition available.

Name:	njr_DefinedBenefitPlanAmortizationOfNetGainsLossesRegulatedAssets
Namespace Prefix:	njr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Defined Benefit Plan Amortization Of Net Prior Service Cost Credit Regulatory Assets

+ References

No definition available.

+ Details

Name:	njr_DefinedBenefitPlanAmortizationOfNetPriorServiceCostCreditRegulatoryAssets
Namespace Prefix:	njr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Defined Benefit Plan Amounts That Will Be Amortized From Regulatory Assets In Next Fiscal Year

+ References

No definition available.

+ Details

Name:	njr_DefinedBenefitPlanAmountsThatWillBeAmortizedFromRegulatoryAssetsInNextFiscalYear
Namespace Prefix:	njr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Defined Benefit Plan, Amount to be Amortized from Regulatory Assets [Abstract]

+ References

No definition available.

+ Details

Name:	njr_DefinedBenefitPlanAmounttobeAmortizedfromRegulatoryAssetsAbstract
Namespace Prefix:	njr_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Amount of cost (credit) included in accumulated other comprehensive (income) loss expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (s)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedAmortizationNextFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount included in accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan expected to be recognized in net periodic benefit (cost) credit for fiscal year following most recent annual statement of financial position.

+ References

+ Details

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

- Definition

Amount included in accumulated other comprehensive (income) loss for prior service cost (credit) expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position.

+ References

+ Details

Name:	us-gaap_DefinedBenefitPlanExpectedAmortizationOfPriorServiceCostCreditNextFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: