DERIVATIVE INSTRUMENTS - INCOME STATEMENT RELATED DISCLOSURES (Details) (USD $)	3 Months Ended				9 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Expected reclassification from OCI into earnings	$ 33,000				$ 33,000
Maximum tenor					April 2015
Foreign Exchange Contract [Member] | Gas Purchases [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Foreign currency contracts, Amount of Gain or (Loss) Recognized in OCI on Derivatives (Effective Portion)	(14,000)	[1]	12,000	[1]	(85,000)	[1]	(63,000)	[1]
Foreign currency contracts, Amount of Gain or (Loss) Reclassified from OCI into Income (Effective Portion)	(21,000)		(35,000)		23,000		(70,000)
Foreign currency contracts, Amount of Gain or (Loss) Recognized on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)	0		0		0		0
NJRES [Member] | Nondesignated [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments, gain (loss) recognized in income, net	34,387,000		(4,422,000)		27,617,000		99,771,000
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	3,595,000		(2,860,000)		(4,264,000)		(6,023,000)
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	(8,809,000)		8,155,000		(6,253,000)		12,546,000
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	39,601,000		(9,717,000)		38,134,000		93,248,000
NJNG [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative instruments, gain (loss) recognized in regulatory assets and liabilities net	$ (4,700,000)		$ 3,700,000		$ 1,400,000		$ (31,600,000)

[1] The settlement of foreign currency transactions over the next twelve months is expected to result in the reclassification of $33,000 from OCI into earnings. The maximum tenor is April 2015