REVENUE - DISAGGREGATED REVENUE - TYPE (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 155,392	$ 157,024	$ 720,495	$ 712,554
Revenues out of scope	212,201	141,950	903,590	841,070
Total operating revenues	367,593	298,974	1,624,085	1,553,624
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	94,835	103,527	487,085	485,564
Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	44,615	37,754	165,667	164,540
Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	15,052	14,331	64,346	58,043
Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	890	1,412	3,397	4,407
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	354,250	287,334	1,592,068	1,520,648
Home Services and Other
Disaggregation of Revenue [Line Items]
Revenues out of scope	0	0	0	0
Total operating revenues	13,071	12,163	38,002	36,694
Home Services and Other \| Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12,877	11,845	37,315	35,955
Home Services and Other \| Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	194	318	687	739
Home Services and Other \| Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Home Services and Other \| Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Natural Gas Distribution \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	118,367	124,888	604,619	611,650
Revenues out of scope	9,259	3,644	28,903	33,725
Total operating revenues	127,626	128,532	633,522	645,375
Natural Gas Distribution \| Operating Segments \| Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	79,054	89,095	441,452	442,093
Natural Gas Distribution \| Operating Segments \| Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	23,371	20,050	95,424	107,107
Natural Gas Distribution \| Operating Segments \| Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	15,052	14,331	64,346	58,043
Natural Gas Distribution \| Operating Segments \| Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	890	1,412	3,397	4,407
Clean Energy Ventures \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,914	5,294	19,691	13,770
Revenues out of scope	4,467	8,102	6,536	11,833
Total operating revenues	13,381	13,396	26,227	25,603
Clean Energy Ventures \| Operating Segments \| Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,904	2,587	8,318	7,516
Clean Energy Ventures \| Operating Segments \| Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	6,010	2,707	11,373	6,254
Clean Energy Ventures \| Operating Segments \| Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Clean Energy Ventures \| Operating Segments \| Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Energy Services \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,041	3,536	21,480	20,491
Revenues out of scope	198,475	130,204	868,151	795,512
Total operating revenues	202,516	133,740	889,631	816,003
Energy Services \| Operating Segments \| Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Energy Services \| Operating Segments \| Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,041	3,536	21,480	20,491
Energy Services \| Operating Segments \| Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Energy Services \| Operating Segments \| Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Storage and Transportation \| Operating Segments
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	10,999	11,143	36,703	29,949
Revenues out of scope	0	0	0	0
Total operating revenues	10,999	11,143	36,703	29,949
Storage and Transportation \| Operating Segments \| Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Storage and Transportation \| Operating Segments \| Commercial and industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	10,999	11,143	36,703	29,949
Storage and Transportation \| Operating Segments \| Firm transportation
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Storage and Transportation \| Operating Segments \| Interruptible and off-tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 0	$ 0	$ 0	$ 0

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.

No definition available.

+ Details

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

- Definition

The total amount of operating revenues recognized during the period.

+ References

No definition available.

+ Details

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

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8906-108599

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 40
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9031-108599

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 41
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9038-108599

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8933-108599

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130543-203045

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8736-108599

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599

Reference 10: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=121604090&loc=SL49130545-203045

Reference 11: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 924
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 11.L)
-URI http://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856

+ Details

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

- Definition

Amount of revenue that is not accounted for under Topic 606.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03(1))
-URI http://asc.fasb.org/extlink&oid=120395209&loc=SL114868664-224227

+ Details

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

- Details

Name:	srt_MajorCustomersAxis=njr_ResidentialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=njr_CommercialandIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=njr_FirmTransportationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_MajorCustomersAxis=njr_InterruptibleandOfftariffMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_OperatingSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_CorporateNonSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=njr_NaturalGasDistributionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=njr_CleanEnergyVenturesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=njr_EnergyServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=njr_StorageAndTransportationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: