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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - GAS IN STORAGE (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 MMcf | Sep. 30, 2014 MMcf
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Inventory [Line Items] |  |
Gas in Storage | $ 269,420 us-gaap_EnergyRelatedInventoryNaturalGasInStorage | $ 277,516 us-gaap_EnergyRelatedInventoryNaturalGasInStorage
Gas in Storage (mmcf) | 74,900 njr_GasStoredUndergroundBCF | 77,800 njr_GasStoredUndergroundBCF
NJRES [Member] |  |
Inventory [Line Items] |  |
Gas in Storage | 201,722 us-gaap_EnergyRelatedInventoryNaturalGasInStorage / dei_LegalEntityAxis = njr_NjresMember | 191,250 us-gaap_EnergyRelatedInventoryNaturalGasInStorage / dei_LegalEntityAxis = njr_NjresMember
Gas in Storage (mmcf) | 57,800 njr_GasStoredUndergroundBCF / dei_LegalEntityAxis = njr_NjresMember | 56,500 njr_GasStoredUndergroundBCF / dei_LegalEntityAxis = njr_NjresMember
NJNG [Member] |  |
Inventory [Line Items] |  |
Gas in Storage | $ 67,698 us-gaap_EnergyRelatedInventoryNaturalGasInStorage / dei_LegalEntityAxis = njr_NjngMember | $ 86,266 us-gaap_EnergyRelatedInventoryNaturalGasInStorage / dei_LegalEntityAxis = njr_NjngMember
Gas in Storage (mmcf) | 17,100 njr_GasStoredUndergroundBCF / dei_LegalEntityAxis = njr_NjngMember | 21,300 njr_GasStoredUndergroundBCF / dei_LegalEntityAxis = njr_NjngMember

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- Definition Gas Stored Underground (BCF) + References No definition available. + Details
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- Definition Carrying amount as of the balance sheet date of natural gas in storage, which is a mixture of gases (liquefied or otherwise), used for fuel and manufacturing purposes, which is ready for sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 6 -Subparagraph a -Article 5 + Details
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