BUSINESS SEGMENT AND OTHER OPERATIONS DATA BUSINESS SEGMENT AND OTHER OPERATIONS DATA, NET FINANCIAL EARNINGS LOSS RECONCILIATION (Details) (USD $) In Thousands	3 Months Ended				9 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Net Financial Earnings	$ 9,699		$ 11,587		$ 105,858		$ 103,011
Unrealized (Gain) Loss from Derivative Instruments and Related Transactions Net of Taxes	(2,875)	[1]	15,886	[1]	33,835	[1]	3,936	[1]
Effects of Economic Hedging Related to Natural Gas Inventory Net of Taxes	(7,800)		5,878		(36,787)		(16,867)
NET INCOME (LOSS)	20,374		(10,177)		108,810		115,942
Transactions Between NJNG and NJRES [Member]
Unrealized (Gain) Loss from Derivative Instruments and Related Transactions Net of Taxes	$ 42		$ 16		$ 74		$ 144

[1] Excludes unrealized losses (gains) related to an intercompany transaction between NJNG and NJRES that have been eliminated in consolidation of $42,000 and $16,000 for the three months ended and $74,000 and $144,000 for the nine months ended June 30, 2011 and 2010, respectively.