BUSINESS SEGMENT AND OTHER OPERATIONS DATA BUSINESS SEGMENT AND OTHER OPERATIONS DATA, NET FINANCIAL EARNINGS LOSS RECONCILIATION (Details) (USD $) In Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2009
Net Financial Earnings									$ 106,533		$ 101,764		$ 100,970
Unrealized (Gain) Loss from Derivative Instruments and Related Transactions Net of Taxes									23,320	[1]	(16,825)	[1]	39,254	[1]
Effects of Economic Hedging Related to Natural Gas Inventory Net of Taxes									(18,086)		1,132		34,474
Net income	(7,511)	20,374	63,927	24,509	1,515	(10,177)	74,217	51,902	101,299		117,457		27,242
Transactions Between NJNG and NJRES [Member]
Unrealized (Gain) Loss from Derivative Instruments and Related Transactions Net of Taxes									$ 130		$ 228

[1] Excludes unrealized losses related to an intercompany transaction between NJNG and NJRES that have been eliminated in consolidation of approximately $130,000, and $228,000 for the fiscal years ended September 30, 2011 and 2010, respectively. There was no related intercompany transaction between NJNG and NJRES for fiscal 2009.