DERIVATIVE INSTRUMENTS, INCOME STATEMENT RELATED DISCLOSURES (Details) (USD $)	12 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2009
Derivative Instruments, Gain (Loss) [Line Items]
Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months	$ 144,000
Maximum Length of Time Hedged in Foreign Currency Cash Flow Hedge	November 2013
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	41,538,000		40,392,000		8,762,000
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	6,474,000		(3,608,000)		20,907,000
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	(7,008,000)		89,987,000		33,529,000
NJRES [Member] | Nondesignated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	41,004,000		126,771,000		63,198,000
NJNG [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Gain Loss Recognized in Regulatory Assets and Liabilities Net	(15,100,000)		(34,500,000)		(32,800,000)
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		(509,000)		(9,899,000)
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	241,000	[1],[2]	(144,000)	[1],[2]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	119,000		5,000	[1]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0		0	[1]
Nondesignated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	$ 41,004,000		$ 126,262,000		$ 53,299,000

[1] NJRES began hedging its foreign currency exposure in May 2010, therefore, amounts for fiscal 2010 only include gains and losses for May 2010 through September 2010 and is not comparative to fiscal 2011.

[2] The settlement of foreign currency transactions over the next twelve months is expected to result in the reclassification of $144,000 from OCI into earnings. The maximum tenor is November 2013.