COMMITMENTS AND CONTINGENT LIABILITIES - SCHEDULE OF FUTURE COMMITTED EXPENSES (Details) $ in Thousands	12 Months Ended
Sep. 30, 2020 USD ($)
Long-term Purchase Commitment [Line Items]
Current charges recoverable through BGSS	$ 124,700
Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	378,626
2021	231,779
2022	166,285
2023	124,321
2024	106,037
Thereafter	$ 593,464
Minimum
Long-term Purchase Commitment [Line Items]
Storage and pipeline capacity, fixed period	1 year
Maximum
Long-term Purchase Commitment [Line Items]
Storage and pipeline capacity, fixed period	10 years
Energy Services
Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	$ 249,589
2021	68,079
2022	38,368
2023	26,435
2024	19,598
Thereafter	37,469
Energy Services \| Natural gas purchases
Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	151,270
2021	1,600
2022	0
2023	0
2024	0
Thereafter	0
Energy Services \| Storage demand fees
Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	21,857
2021	13,028
2022	8,632
2023	3,748
2024	2,488
Thereafter	942
Energy Services \| Pipeline demand fees
Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	76,462
2021	53,451
2022	29,736
2023	22,687
2024	17,110
Thereafter	36,527
Natural Gas Distribution
Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	129,037
2021	163,700
2022	127,917
2023	97,886
2024	86,439
Thereafter	555,995
Natural Gas Distribution \| Natural gas purchases
Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	4,377
2021	0
2022	0
2023	0
2024	0
Thereafter	0
Natural Gas Distribution \| Storage demand fees
Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	36,096
2021	32,122
2022	20,303
2023	12,768
2024	6,830
Thereafter	3,530
Natural Gas Distribution \| Pipeline demand fees
Purchase Obligation, Fiscal Year Maturity [Abstract]
2020	88,564
2021	131,578
2022	107,614
2023	85,118
2024	79,609
Thereafter	$ 552,465

Current Charges Recoverable Through Basic Gas Supply Service as shown for the reporting period

No definition available.

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

- Definition

Storage and Pipeline Capacity, Term of Contract

+ References

No definition available.

+ Details

Name:	njr_StorageandPipelineCapacityTermofContract
Namespace Prefix:	njr_
Data Type:	xbrli:durationItemType
Balance Type:	na
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_LongTermPurchaseCommitmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of purchase arrangement to be paid after fifth fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of purchase arrangement to be paid in fifth fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of purchase arrangement to be paid in fourth fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of purchase arrangement to be paid in next fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of purchase arrangement to be paid in second fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- Definition

Amount of purchase arrangement to be paid in third fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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

- 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:

- Details

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

- Details

Name:	us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=us-gaap_InventoriesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=njr_StorageDemandFeesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis=njr_PipelineDemandFeesMember
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: