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NATURE OF THE BUSINESS (Details)
 3 Months Ended
Dec. 31, 2014
customer
Steckman Ridge [Member]  
Nature of Business [Line Items]  
Equity method investment, ownership percentage 50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= njr_SteckmanRidgeMember
Iroquois [Member]  
Nature of Business [Line Items]  
Equity method investment, ownership percentage 5.53%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= njr_IroquoisMember
PennEast [Member]  
Nature of Business [Line Items]  
Equity method investment, ownership percentage 20.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= njr_PennEastMember
NJNG [Member]  
Nature of Business [Line Items]  
Total retail customers 509,600njr_TotalRetailCustomers
/ dei_LegalEntityAxis
= njr_NjngMember