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DERIVATIVE INSTRUMENTS, BROKER MARGIN DEPOSITS (Details) - USD ($) $ in Thousands | Sep. 30, 2015 | Sep. 30, 2014
--- | --- | ---
Broker margin - Current assets | $ 12,990 | $ 27,339
Broker margin - Current (liabilities) | (4,103) | 0
NJNG [Member] |  |
Broker margin - Current assets | 12,990 | 1,057
NJRES [Member] |  |
Broker margin - Current assets |  | $ 26,282
Broker margin - Current (liabilities) | $ (4,103) |

X
- Definition Amount of other payables due to broker-dealers or clearing organizations. + References No definition available. + Details
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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: |