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, 2014		Mar. 31, 2013		Mar. 31, 2014		Mar. 31, 2013
Net financial earnings (loss)	$ 151,920		$ 68,664		$ 191,780		$ 104,157
Unrealized (gain) loss from derivative instruments and related transactions	(13,256)	[1]	38,800	[1]	52,395	[1]	20,466	[1]
Effects of economic hedging related to natural gas inventory	(18,668)		(2,117)		(41,548)		(22,865)
Tax adjustments	10,873		(13,488)		269		881
NET INCOME	172,971		45,469		180,664		105,675
Transactions Between NJNG and NJRES [Member]
Unrealized losses related to intercompany transactions	$ (957)		$ (377)		$ (672)		$ (310)

[1] Excludes unrealized losses related to an intercompany transaction between NJNG and NJRES that have been eliminated in consolidation of approximately $(957,000) and $(377,000) for the three months ended and $(672,000) and $(310,000) for the six months ended March 31, 2014 and 2013, respectively.