DERIVATIVE INSTRUMENTS, INCOME STATEMENT RELATED DISCLOSURES (Details) (USD $)	3 Months Ended				6 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Mar. 31, 2012		Mar. 31, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Foreign Currency Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months	$ 105,000				$ 105,000
Maximum Length of Time Hedged in Foreign Currency Cash Flow Hedge					April 2015
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	(25,000)	[1]	42,000	[1]	(75,000)	[1]	161,000	[1]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	(23,000)		(50,000)		(35,000)		(59,000)
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0		0		0		0
NJRES [Member] | Nondesignated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Net	49,372,000		32,354,000		104,193,000		(8,677,000)
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	8,745,000		41,306,000		(3,163,000)		24,837,000
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	116,000		(9,448,000)		4,391,000		(5,739,000)
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	40,511,000		496,000		102,965,000		(27,775,000)
NJNG [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments Gain Loss Recognized in Regulatory Assets and Liabilities Net	$ (15,400,000)		$ (2,200,000)		$ (35,300,000)		$ (4,900,000)

[1] )The settlement of foreign currency transactions over the next twelve months is expected to result in the reclassification of $105,000 from OCI into earnings. The maximum tenor is April 2015