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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - SALE OF ASSET (Details) (USD $) In Thousands, unless otherwise specified | 6 Months Ended | 0 Months Ended
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Oct. 22, 2013
Long Lived Assets Held-for-sale [Line Items] |  |  |
Proceeds from sale of assets | $ 0us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment | $ 6,000us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment |
CR&R [Member] |  |  |
Long Lived Assets Held-for-sale [Line Items] |  |  |
Number of acres of undeveloped land |  |  | 25.4us-gaap_AreaOfLand / dei_LegalEntityAxis = njr_CommercialRealtyResourcesCorp.Member
Proceeds from sale of assets |  |  | 6,000us-gaap_ProceedsFromSaleOfPropertyPlantAndEquipment / dei_LegalEntityAxis = njr_CommercialRealtyResourcesCorp.Member
Pre-tax gain after closing costs |  |  | $ 313us-gaap_GainLossOnSaleOfPropertyPlantEquipment / dei_LegalEntityAxis = njr_CommercialRealtyResourcesCorp.Member

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- Definition Area of land held. + References No definition available. + Details
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Balance Type: | na
Period Type: | instant

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- Definition Amount of gain (loss) on sale or disposal of property, plant and equipment assets, including oil and gas property and timber property. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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- Details
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- Definition The cash inflow from the sale of long-lived, physical assets that are used in the normal conduct of business to produce goods and services and not intended for resale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3179-108585 + Details
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