DERIVATIVE INSTRUMENTS, BALANCE SHEET RELATED DISCLOSURES (Details) (USD $)	Jun. 30, 2011	Sep. 30, 2010
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	$ 59,018,000	$ 135,186,000
Derivative Liabilities, Current	45,138,000	78,447,000
Derivative Assets, Noncurrent	5,081,000	7,957,000
Derivative Liabilities, Noncurrent	4,029,000	5,640,000
Derivative Assets	64,099,000	143,143,000
Derivative Liabilities	49,167,000	84,087,000
Notional Amount of Foreign Currency Derivatives	2,600,000
NJRES [Member] | Physical Commodity Contracts [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	19,439,000	18,566,000
Derivative Liabilities, Current	5,219,000	5,879,000
Derivative Assets, Noncurrent	3,734,000	5,482,000
Derivative Liabilities, Noncurrent	989,000	179,000
NJNG [Member] | Financial Commodity Contracts [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	5,338,000	9,952,000
Derivative Liabilities, Current	13,020,000	24,724,000
Derivative Assets, Noncurrent	0	0
Derivative Liabilities, Noncurrent	0	1,725,000
NJRES [Member] | Financial Commodity Contracts [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	34,134,000	106,653,000
Derivative Liabilities, Current	26,899,000	47,844,000
Derivative Assets, Noncurrent	1,265,000	2,465,000
Derivative Liabilities, Noncurrent	3,040,000	3,736,000
NJRES [Member] | Foreign Exchange Contract [Member] | Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets, Current	107,000	15,000
Derivative Liabilities, Current	0	0
Derivative Assets, Noncurrent	82,000	10,000
Derivative Liabilities, Noncurrent	0	0
Derivative Assets	189,000	25,000
Derivative Liabilities	0	0
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Assets	63,910,000	143,118,000
Derivative Liabilities	$ 49,167,000	$ 84,087,000