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DERIVATIVE INSTRUMENTS - CREDIT RISK EXPOSURE (Details) (USD $) In Thousands, unless otherwise specified | 6 Months Ended |
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Mar. 31, 2015 | Sep. 30, 2014
Credit Risk Exposure [Line Items] |  |
Gross Credit Exposure | $ 209,487us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure |
Derivative, net liability position, aggregate fair value | 76us-gaap_DerivativeNetLiabilityPositionAggregateFairValue | 39us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
Additional collateral, aggregate fair value | 75us-gaap_AdditionalCollateralAggregateFairValue | 7us-gaap_AdditionalCollateralAggregateFairValue
Investment Grade [Member] |  |
Credit Risk Exposure [Line Items] |  |
Gross Credit Exposure | 144,536us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_InternalCreditAssessmentAxis = us-gaap_ExternalCreditRatingInvestmentGradeMember |
Noninvestment Grade [Member] |  |
Credit Risk Exposure [Line Items] |  |
Gross Credit Exposure | 23,734us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_InternalCreditAssessmentAxis = us-gaap_ExternalCreditRatingNonInvestmentGradeMember |
Internally Rated Investment Grade [Member] |  |
Credit Risk Exposure [Line Items] |  |
Gross Credit Exposure | 26,404us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_InternalCreditAssessmentAxis = njr_InternallyRatedInvestmentGradeMember |
Internally Rated Noninvestment Grade [Member] |  |
Credit Risk Exposure [Line Items] |  |
Gross Credit Exposure | $ 14,813us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure / us-gaap_InternalCreditAssessmentAxis = njr_InternallyRatedNoninvestmentGradeMember |

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- Definition The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959 + Details
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- Definition Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 21 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13537-108611 + Details
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- Definition The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624258-113959 + Details
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