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DERIVATIVE INSTRUMENTS - VOLUME (Details) | Mar. 31, 2015 Bcf | Sep. 30, 2014 Bcf
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NJNG [Member] | Futures [Member] | Long [Member] |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount | 27.2invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_FutureMember / dei_LegalEntityAxis = njr_NjngMember / us-gaap_PositionAxis = us-gaap_LongMember | 17.3invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_FutureMember / dei_LegalEntityAxis = njr_NjngMember / us-gaap_PositionAxis = us-gaap_LongMember
NJRES [Member] | Futures [Member] | Short [Member] |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount | (37.9)invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_FutureMember / dei_LegalEntityAxis = njr_NjresMember / us-gaap_PositionAxis = us-gaap_ShortMember | (62.1)invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_FutureMember / dei_LegalEntityAxis = njr_NjresMember / us-gaap_PositionAxis = us-gaap_ShortMember
NJRES [Member] | Options [Member] | Long [Member] |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount | 1.2invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_OptionMember / dei_LegalEntityAxis = njr_NjresMember / us-gaap_PositionAxis = us-gaap_LongMember | 1.2invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_OptionMember / dei_LegalEntityAxis = njr_NjresMember / us-gaap_PositionAxis = us-gaap_LongMember
NJRES [Member] | Physical [Member] | Long [Member] |  |
Derivative [Line Items] |  |
Derivative, nonmonetary notional amount | 74.7invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember / us-gaap_PositionAxis = us-gaap_LongMember | 28.6invest_DerivativeNonmonetaryNotionalAmount / us-gaap_DerivativeByNatureAxis = us-gaap_ForwardContractsMember / dei_LegalEntityAxis = njr_NjresMember / us-gaap_PositionAxis = us-gaap_LongMember

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- Definition Aggregate notional amount of derivative expressed in nonmonetary units. For example, the number of barrels specified in a fuel oil forward purchase contract. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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