BUSINESS SEGMENT AND OTHER OPERATIONS DATA BUSINESS SEGMENT AND OTHER OPERATIONS DATA, NET FINANCIAL EARNINGS LOSS RECONCILIATION (Details) (USD $)	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2010
Net Financial Earnings									$ 112,417,000		$ 106,533,000		$ 101,764,000
Unrealized (Gain) Loss from Derivative Instruments and Related Transactions Net of Taxes									22,631,000	[1],[2]	23,320,000	[1],[2]	(16,825,000)	[1],[2]
Effects of Economic Hedging Related to Natural Gas Inventory Net of Taxes									(3,093,000)	[3]	(18,086,000)	[3]	1,132,000	[3]
Net income	(8,693,000)	(10,320,000)	54,535,000	57,357,000	(7,511,000)	20,374,000	63,927,000	24,509,000	92,879,000		101,299,000		117,457,000
Taxes Related to Unrealized Loss Gain from Derivative Instruments and Related Transactions									13,000,000		13,400,000		(10,000,000)
Taxes Related to Effects Of Economic Hedging Related To Natural Gas Inventory									(1,800,000)		(10,500,000)		1,300,000
Transactions Between NJNG and NJRES [Member]
Unrealized (Gain) Loss from Derivative Instruments and Related Transactions Net of Taxes									$ 142,000		$ 130,000		$ 228,000

[1] Excludes unrealized losses related to an intercompany transaction between NJNG and NJRES that have been eliminated in consolidation of approximately $142,000, $130,000 and $228,000 for the fiscal years ended September 30, 2012, 2011 and 2010, respectively.

[2] Includes taxes of approximately $13 million, $13.4 million, and $(10) million for the fiscal years ended September 30, 2012, 2011 and 2010, respectively.

[3] Includes taxes of approximately $(1.8) million, $(10.5) million and $1.3 million for the fiscal years ended September 30, 2012, 2011 and 2010, respectively.