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COMMITMENTS AND CONTINGENT LIABILITIES - CAPITAL EXPENDITURES (Details) $ in Thousands | Sep. 30, 2020 USD ($)
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Commitments and Contingencies Disclosure [Abstract] |
Operating leases, future minimum payments due, next five years (not more than) | $ 1,400
Operating leases, future minimum payments due, thereafter | $ 73

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- Definition Lessee, Operating Lease, Liability, Payments, Due Next Five Years + References No definition available. + Details
Name: | njr_LesseeOperatingLeaseLiabilityPaymentsDueNextFiveYears
Namespace Prefix: | njr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Lessee, Operating Lease, Liability, Payments, Due Thereafter + References No definition available. + Details
Name: | njr_LesseeOperatingLeaseLiabilityPaymentsDueThereafter
Namespace Prefix: | njr_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration