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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - GAS IN STORAGE (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Bcf
Sep. 30, 2014
Bcf
Inventory [Line Items]    
Gas in Storage $ 71,402us-gaap_EnergyRelatedInventoryNaturalGasInStorage $ 277,516us-gaap_EnergyRelatedInventoryNaturalGasInStorage
Gas in Storage (Bcf) 23.5njr_GasStoredUndergroundBCF 77.8njr_GasStoredUndergroundBCF
NJRES [Member]    
Inventory [Line Items]    
Gas in Storage 65,030us-gaap_EnergyRelatedInventoryNaturalGasInStorage
/ dei_LegalEntityAxis
= njr_NjresMember		 191,250us-gaap_EnergyRelatedInventoryNaturalGasInStorage
/ dei_LegalEntityAxis
= njr_NjresMember
Gas in Storage (Bcf) 22.0njr_GasStoredUndergroundBCF
/ dei_LegalEntityAxis
= njr_NjresMember		 56.5njr_GasStoredUndergroundBCF
/ dei_LegalEntityAxis
= njr_NjresMember
NJNG [Member]    
Inventory [Line Items]    
Gas in Storage $ 6,372us-gaap_EnergyRelatedInventoryNaturalGasInStorage
/ dei_LegalEntityAxis
= njr_NjngMember		 $ 86,266us-gaap_EnergyRelatedInventoryNaturalGasInStorage
/ dei_LegalEntityAxis
= njr_NjngMember
Gas in Storage (Bcf) 1.5njr_GasStoredUndergroundBCF
/ dei_LegalEntityAxis
= njr_NjngMember		 21.3njr_GasStoredUndergroundBCF
/ dei_LegalEntityAxis
= njr_NjngMember