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DERIVATIVE INSTRUMENTS - OFFSETTING OF ASSETS AND LIABILITIES (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Sep. 30, 2014
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NJRES [Member] |  |  |  |
Offsetting Assets and Liabilities [Line Items] |  |  |  |
Amounts Presented in Balance Sheets, Derivative assets | $ 149,083 us-gaap_DerivativeFairValueOfDerivativeAsset / dei_LegalEntityAxis = njr_NjresMember | [1] | $ 67,270 us-gaap_DerivativeFairValueOfDerivativeAsset / dei_LegalEntityAxis = njr_NjresMember | [1]
Offsetting Derivative Instruments, Derivative assets | (57,857)us-gaap_DerivativeAssetFairValueGrossLiability / dei_LegalEntityAxis = njr_NjresMember | [2] | (63,375)us-gaap_DerivativeAssetFairValueGrossLiability / dei_LegalEntityAxis = njr_NjresMember | [2]
Financial Collateral Received/Pledged, Derivative assets | (53,821)njr_DerivativeAssetCollateralorEquivalent / dei_LegalEntityAxis = njr_NjresMember | [3] | 0 njr_DerivativeAssetCollateralorEquivalent / dei_LegalEntityAxis = njr_NjresMember | [3]
Net Amounts, Derivative assets | 37,405 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / dei_LegalEntityAxis = njr_NjresMember | [4] | 3,895 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / dei_LegalEntityAxis = njr_NjresMember | [4]
Amounts Presented in Balance Sheets, Derivative liabilities | 78,034 us-gaap_DerivativeFairValueOfDerivativeLiability / dei_LegalEntityAxis = njr_NjresMember | [1] | 84,323 us-gaap_DerivativeFairValueOfDerivativeLiability / dei_LegalEntityAxis = njr_NjresMember | [1]
Offsetting Derivative Instruments, Derivative liabilities | (57,857)us-gaap_DerivativeLiabilityFairValueGrossAsset / dei_LegalEntityAxis = njr_NjresMember | [2] | (63,902)us-gaap_DerivativeLiabilityFairValueGrossAsset / dei_LegalEntityAxis = njr_NjresMember | [2]
Financial Collateral Received/Pledged, Derivative liabilities | (1,200)njr_DerivativeLiabilityCollateralorEquivalent / dei_LegalEntityAxis = njr_NjresMember | [3] | (2,614)njr_DerivativeLiabilityCollateralorEquivalent / dei_LegalEntityAxis = njr_NjresMember | [3]
Net Amounts, Derivative liabilities | 18,977 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral / dei_LegalEntityAxis = njr_NjresMember | [4] | 17,807 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral / dei_LegalEntityAxis = njr_NjresMember | [4]
NJRES [Member] | Physical Forward Commodity Contracts [Member] |  |  |  |
Offsetting Assets and Liabilities [Line Items] |  |  |  |
Amounts Presented in Balance Sheets, Derivative assets | 15,313 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [1] | 15,426 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [1]
Offsetting Derivative Instruments, Derivative assets | (6,054)us-gaap_DerivativeAssetFairValueGrossLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [2] | (11,531)us-gaap_DerivativeAssetFairValueGrossLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [2]
Financial Collateral Received/Pledged, Derivative assets | 0 njr_DerivativeAssetCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [3] | 0 njr_DerivativeAssetCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [3]
Net Amounts, Derivative assets | 9,259 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [4] | 3,895 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [4]
Amounts Presented in Balance Sheets, Derivative liabilities | 26,027 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [1] | 30,910 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [1]
Offsetting Derivative Instruments, Derivative liabilities | (6,054)us-gaap_DerivativeLiabilityFairValueGrossAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [2] | (12,058)us-gaap_DerivativeLiabilityFairValueGrossAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [2]
Financial Collateral Received/Pledged, Derivative liabilities | (1,200)njr_DerivativeLiabilityCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [3] | (1,200)njr_DerivativeLiabilityCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [3]
Net Amounts, Derivative liabilities | 18,773 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [4] | 17,652 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember | [4]
NJRES [Member] | Financial Commodity Contracts [Member] |  |  |  |
Offsetting Assets and Liabilities [Line Items] |  |  |  |
Amounts Presented in Balance Sheets, Derivative assets | 133,770 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [1] | 51,844 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [1]
Offsetting Derivative Instruments, Derivative assets | (51,803)us-gaap_DerivativeAssetFairValueGrossLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [2] | (51,844)us-gaap_DerivativeAssetFairValueGrossLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [2]
Financial Collateral Received/Pledged, Derivative assets | (53,821)njr_DerivativeAssetCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [3] | 0 njr_DerivativeAssetCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [3]
Net Amounts, Derivative assets | 28,146 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [4] | 0 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [4]
Amounts Presented in Balance Sheets, Derivative liabilities | 51,803 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [1] | 53,258 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [1]
Offsetting Derivative Instruments, Derivative liabilities | (51,803)us-gaap_DerivativeLiabilityFairValueGrossAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [2] | (51,844)us-gaap_DerivativeLiabilityFairValueGrossAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [2]
Financial Collateral Received/Pledged, Derivative liabilities | 0 njr_DerivativeLiabilityCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [3] | (1,414)njr_DerivativeLiabilityCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [3]
Net Amounts, Derivative liabilities | 0 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [4] | 0 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjresMember | [4]
NJRES [Member] | Foreign Currency Contract [Member] |  |  |  |
Offsetting Assets and Liabilities [Line Items] |  |  |  |
Amounts Presented in Balance Sheets, Derivative liabilities | 204 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = njr_NjresMember | [1] | 155 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = njr_NjresMember | [1]
Offsetting Derivative Instruments, Derivative liabilities | 0 us-gaap_DerivativeLiabilityFairValueGrossAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = njr_NjresMember | [2] | 0 us-gaap_DerivativeLiabilityFairValueGrossAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = njr_NjresMember | [2]
Financial Collateral Received/Pledged, Derivative liabilities | 0 njr_DerivativeLiabilityCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = njr_NjresMember | [3] | 0 njr_DerivativeLiabilityCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = njr_NjresMember | [3]
Net Amounts, Derivative liabilities | 204 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = njr_NjresMember | [4] | 155 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForeignExchangeContractMember / dei_LegalEntityAxis = njr_NjresMember | [4]
NJNG [Member] | Financial Commodity Contracts [Member] |  |  |  |
Offsetting Assets and Liabilities [Line Items] |  |  |  |
Amounts Presented in Balance Sheets, Derivative assets | 62 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [1] | 2,607 us-gaap_DerivativeFairValueOfDerivativeAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [1]
Offsetting Derivative Instruments, Derivative assets | (62)us-gaap_DerivativeAssetFairValueGrossLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [2] | (2,230)us-gaap_DerivativeAssetFairValueGrossLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [2]
Financial Collateral Received/Pledged, Derivative assets | 0 njr_DerivativeAssetCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [3] | (377)njr_DerivativeAssetCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [3]
Net Amounts, Derivative assets | 0 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [4] | 0 us-gaap_DerivativeFairValueOfDerivativeAssetAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [4]
Amounts Presented in Balance Sheets, Derivative liabilities | 18,914 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [1] | 2,230 us-gaap_DerivativeFairValueOfDerivativeLiability / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [1]
Offsetting Derivative Instruments, Derivative liabilities | (62)us-gaap_DerivativeLiabilityFairValueGrossAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [2] | (2,230)us-gaap_DerivativeLiabilityFairValueGrossAsset / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [2]
Financial Collateral Received/Pledged, Derivative liabilities | (18,852)njr_DerivativeLiabilityCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [3] | 0 njr_DerivativeLiabilityCollateralorEquivalent / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [3]
Net Amounts, Derivative liabilities | $ 0 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [4] | $ 0 us-gaap_DerivativeFairValueOfDerivativeLiabilityAmountOffsetAgainstCollateral / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember / dei_LegalEntityAxis = njr_NjngMember | [4]
[1] | Derivative assets and liabilities are presented on a gross basis in the balance sheet as the Company does not elect balance sheet offsetting under ASC 210-20.
[2] | Offsetting derivative instruments include: transactions with NAESB netting election, transactions held by FCM's with net margining and transactions with ISDA netting.
[3] | Financial collateral includes cash balances at FCMs as well as cash received from or pledged to other counterparties.
[4] | Net amounts represent presentation of derivative assets and liabilities if the Company were to elect balance sheet offsetting under ASC 210-20.

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- Definition Collateral or equivalent pledged or received against derivative liabilities. + References No definition available. + Details
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- Definition Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226003-175313 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959 + Details
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