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DERIVATIVE INSTRUMENTS, BROKER MARGIN DEPOSITS (Details) - USD ($) $ in Thousands | Sep. 30, 2016 | Sep. 30, 2015
--- | --- | ---
Broker margin - Current assets | $ 47,644 | $ 12,990
Broker margin - Current (liabilities) | 0 | (4,103)
NJNG |  |
Broker margin - Current assets | 4,822 | 12,990
NJRES |  |
Broker margin - Current assets | 42,822 | 0
Broker margin - Current (liabilities) | $ 0 | $ (4,103)

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

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: | dei_LegalEntityAxis=njr_NjngMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | dei_LegalEntityAxis=njr_NjresMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |