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INVESTMENTS IN EQUITY INVESTEES - SCHEDULE OF EQUITY METHOD INVESTMENTS (Details) (USD $) | 6 Months Ended |
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Mar. 31, 2015 mi | Sep. 30, 2014
Schedule of Equity Method Investments [Line Items] |  |  |  |
Investments in equity investees | $ 151,136,000us-gaap_EquityMethodInvestments |  | $ 153,010,000us-gaap_EquityMethodInvestments |
Steckman Ridge [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Investments in equity investees | 127,384,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = njr_SteckmanRidgeMember | [1] | 128,413,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = njr_SteckmanRidgeMember | [1]
Steckman Ridge [Member] | Equity Method Investee [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Outstanding principal balance | 70,400,000us-gaap_DueFromRelatedPartiesNoncurrent / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_EquityMethodInvesteeMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = njr_SteckmanRidgeMember |  | 70,400,000us-gaap_DueFromRelatedPartiesNoncurrent / us-gaap_RelatedPartyTransactionsByRelatedPartyAxis = us-gaap_EquityMethodInvesteeMember / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = njr_SteckmanRidgeMember |
Iroquois [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Investments in equity investees | 21,928,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = njr_IroquoisMember |  | 24,042,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = njr_IroquoisMember |
PennEast [Member] |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |
Investments in equity investees | $ 1,824,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = njr_PennEastMember |  | $ 555,000us-gaap_EquityMethodInvestments / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = njr_PennEastMember |
Construction plan, project area | 115njr_ConstructionPlanProjectArea / us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis = njr_PennEastMember |  |  |
[1] | Includes loans with a total outstanding principal balance of $70.4 million for both March 31, 2015 and September 30, 2014. The loans accrue interest at a variable rate that resets quarterly and are due October 1, 2023.

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