EMPLOYEE BENEFIT PLANS - DEFINED CONTRIBUTION (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined contribution, company match of employee contribution	65.00%
Defined contribution plan, maximum employer contribution by percentage of employee salary	6.00%
Defined contribution plan, cost recognized	$ 3,900	$ 2,900	$ 2,800
Deferred compensation arrangement with individual, employer contribution	$ 959	$ 781	$ 571
NJRHS \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer contribution for employees not qualifying for the defined benefit plan	3.00%
NJRHS \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined contribution plan, employer contribution for employees not qualifying for the defined benefit plan	4.00%

Defined Contribution Plan, Company Match of Employee Contribution

No definition available.

Name:	njr_DefinedContributionPlanCompanyMatchOfEmployeeContribution
Namespace Prefix:	njr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Contribution Plan, Employer Contribution For Employees Not Qualify For The Defined Benefit Plan

+ References

No definition available.

+ Details

Name:	njr_DefinedContributionPlanEmployerContributionForEmployeesNotQualifyForDefinedBenefitPlan
Namespace Prefix:	njr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Defined Contribution Plan, Maximum Employer Contribution by Percentage of Employee Salary

+ References

No definition available.

+ Details

Name:	njr_DefinedContributionPlanMaximumEmployerContributionByPercentageOfEmployeeSalary
Namespace Prefix:	njr_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of contributions made by employer into fund established for purposes of making future disbursement to individual in accordance with deferred compensation arrangement.

+ References

No definition available.

+ Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cost for defined contribution plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

+ Details

Name:	us-gaap_DefinedContributionPlanCostRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

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

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: