BUSINESS SEGMENT AND OTHER OPERATIONS DATA, NET FINANCIAL EARNINGS LOSS RECONCILIATION (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Mar. 31, 2012		Mar. 31, 2011
Net Financial Earnings	$ 74,126		$ 67,029		$ 119,434		$ 96,159
Unrealized (Gain) Loss from Derivative Instruments and Related Transactions Net of Taxes	7,664	[1]	2,306	[1]	(9,708)	[1]	36,710	[1]
Effects of Economic Hedging Related to Natural Gas Inventory Net of Taxes	11,927		796		17,250		(28,987)
Net income	54,535		63,927		111,892		88,436
Transactions Between NJNG and NJRES [Member]
Unrealized (Gain) Loss from Derivative Instruments and Related Transactions Net of Taxes	$ 79		$ 30		$ 197		$ 32

[1] Excludes unrealized losses related to an intercompany transaction between NJNG and NJRES that have been eliminated in consolidation of approximately $79,000 and $30,000, for the three months ended March 31, 2012 and March 31, 2011, respectively, and $197,000, and $32,000 for the six months ended March 31, 2012 and 2011, respectively.