DERIVATIVE INSTRUMENTS, INCOME STATEMENT RELATED DISCLOSURES (Details) (USD $)	12 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months	$ 18,000
Maximum Length of Time Hedged in Foreign Currency Cash Flow Hedge	April 2015
Nondesignated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	87,652,000		41,004,000		126,262,000
Foreign Exchange Contract [Member] | Gas Purchases [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income, Effective Portion, Net	(90,000)	[1]	241,000	[1]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(102,000)		119,000
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0		0
NJRES [Member] | Nondesignated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	87,652,000		41,004,000		126,771,000
NJRES [Member] | Physical Commodity Contracts [Member] | Operating Revenues [Member] | Nondesignated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(7,187,000)		41,538,000		40,392,000
NJRES [Member] | Physical Commodity Contracts [Member] | Gas Purchases [Member] | Nondesignated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	12,967,000		6,474,000		(3,608,000)
NJRES [Member] | Financial Commodity Contracts [Member] | Gas Purchases [Member] | Nondesignated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	81,872,000		(7,008,000)		89,987,000
NJR Energy [Member] | Financial Commodity Contracts [Member] | Operating Revenues [Member] | Nondesignated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	0		0		(509,000)
NJNG [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Gain Loss Recognized in Regulatory Assets and Liabilities Net	$ (25,300,000)		$ (15,100,000)		$ (34,500,000)

[1] The settlement of foreign currency transactions over the next twelve months is expected to result in the reclassification of $18,000 from OCI into earnings. The maximum tenor is April 2015.