BUSINESS SEGMENT AND OTHER OPERATIONS DATA, NET FINANCIAL EARNINGS LOSS RECONCILIATION (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Net Financial Earnings	$ 45,308		$ 29,130
Unrealized (Gain) Loss from Derivative Instruments and Related Transactions Net of Taxes	(17,372)	[1]	34,404	[1]
Effects of Economic Hedging Related to Natural Gas Inventory Net of Taxes	5,323		(29,783)
Net income	57,357		24,509
Transactions Between NJNG and NJRES [Member]
Unrealized (Gain) Loss from Derivative Instruments and Related Transactions Net of Taxes	$ 118		$ 2

[1] Excludes unrealized losses related to an intercompany transaction between NJNG and NJRES that have been eliminated in consolidation of approximately $118,000, and $2,000 for the three months ended December 31, 2011 and 2010, respectively.