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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
Inventory [Line Items]    
Gas in Storage $ 269,420us-gaap_EnergyRelatedInventoryNaturalGasInStorage $ 277,516us-gaap_EnergyRelatedInventoryNaturalGasInStorage
Gas in Storage (mmcf) 74,900njr_GasStoredUndergroundBCF 77,800njr_GasStoredUndergroundBCF
NJRES [Member]    
Inventory [Line Items]    
Gas in Storage 201,722us-gaap_EnergyRelatedInventoryNaturalGasInStorage
/ dei_LegalEntityAxis
= njr_NjresMember		 191,250us-gaap_EnergyRelatedInventoryNaturalGasInStorage
/ dei_LegalEntityAxis
= njr_NjresMember
Gas in Storage (mmcf) 57,800njr_GasStoredUndergroundBCF
/ dei_LegalEntityAxis
= njr_NjresMember		 56,500njr_GasStoredUndergroundBCF
/ dei_LegalEntityAxis
= njr_NjresMember
NJNG [Member]    
Inventory [Line Items]    
Gas in Storage $ 67,698us-gaap_EnergyRelatedInventoryNaturalGasInStorage
/ dei_LegalEntityAxis
= njr_NjngMember		 $ 86,266us-gaap_EnergyRelatedInventoryNaturalGasInStorage
/ dei_LegalEntityAxis
= njr_NjngMember
Gas in Storage (mmcf) 17,100njr_GasStoredUndergroundBCF
/ dei_LegalEntityAxis
= njr_NjngMember		 21,300njr_GasStoredUndergroundBCF
/ dei_LegalEntityAxis
= njr_NjngMember