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DERIVATIVE INSTRUMENTS - BROKER MARGIN DEPOSITS (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Sep. 30, 2017
--- | --- | ---
Natural Gas Distribution |  |
Broker margin - Current assets | $ 2,038 | $ 2,661
Energy Services |  |
Broker margin - Current assets | $ 51,681 | $ 23,166

X
- Definition Amount of other receivables due from broker-dealers or clearing organizations. + References No definition available. + Details
Name: | us-gaap_OtherReceivablesFromBrokerDealersAndClearingOrganizations
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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