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DERIVATIVE INSTRUMENTS - BROKER MARGIN DEPOSITS (Details) (USD $)
In Thousands, unless otherwise specified
Dec. 31, 2014
Sep. 30, 2014
Derivative [Line Items]    
Broker margin - Current (liabilities) assets $ 19,761us-gaap_OtherReceivablesFromBrokerDealersAndClearingOrganizations $ 27,339us-gaap_OtherReceivablesFromBrokerDealersAndClearingOrganizations
Broker margin - Current (liabilities) assets (38,972)us-gaap_OtherPayablesToBrokerDealersAndClearingOrganizations 0us-gaap_OtherPayablesToBrokerDealersAndClearingOrganizations
NJNG [Member]    
Derivative [Line Items]    
Broker margin - Current (liabilities) assets 19,761us-gaap_OtherReceivablesFromBrokerDealersAndClearingOrganizations
/ dei_LegalEntityAxis
= njr_NjngMember		 1,057us-gaap_OtherReceivablesFromBrokerDealersAndClearingOrganizations
/ dei_LegalEntityAxis
= njr_NjngMember
NJRES [Member]    
Derivative [Line Items]    
Broker margin - Current (liabilities) assets   26,282us-gaap_OtherReceivablesFromBrokerDealersAndClearingOrganizations
/ dei_LegalEntityAxis
= njr_NjresMember
Broker margin - Current (liabilities) assets $ (38,972)us-gaap_OtherPayablesToBrokerDealersAndClearingOrganizations
/ dei_LegalEntityAxis
= njr_NjresMember