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DERIVATIVE INSTRUMENTS - CREDIT RISK EXPOSURE (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Credit Risk Exposure [Line Items]    
Gross Credit Exposure $ 291,730us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure  
Derivative, net liability position, aggregate fair value 223us-gaap_DerivativeNetLiabilityPositionAggregateFairValue 39us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
Additional collateral, aggregate fair value 178us-gaap_AdditionalCollateralAggregateFairValue 7us-gaap_AdditionalCollateralAggregateFairValue
Investment Grade [Member]    
Credit Risk Exposure [Line Items]    
Gross Credit Exposure 213,274us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_ExternalCreditRatingInvestmentGradeMember
 
Noninvestment Grade [Member]    
Credit Risk Exposure [Line Items]    
Gross Credit Exposure 27,262us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_ExternalCreditRatingNonInvestmentGradeMember
 
Internally Rated Investment Grade [Member]    
Credit Risk Exposure [Line Items]    
Gross Credit Exposure 36,961us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure
/ us-gaap_InternalCreditAssessmentAxis
= njr_InternallyRatedInvestmentGradeMember
 
Internally Rated Noninvestment Grade [Member]    
Credit Risk Exposure [Line Items]    
Gross Credit Exposure $ 14,233us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure
/ us-gaap_InternalCreditAssessmentAxis
= njr_InternallyRatedNoninvestmentGradeMember