BUSINESS SEGMENT AND OTHER OPERATIONS DATA - NET FINANCIAL EARNINGS LOSS RECONCILIATION (Details) (USD $)	3 Months Ended				6 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Mar. 31, 2013		Mar. 31, 2012
Net financial earnings (loss)	$ 68,664,000		$ 74,126,000		$ 104,157,000		$ 119,434,000
Unrealized loss (gain) from derivative instruments and related transactions, net of taxes (1) (2)	24,534,000	[1],[2]	7,664,000	[1],[2]	12,940,000	[1],[2]	(9,708,000)	[1],[2]
Effects of economic hedging related to natural gas inventory, net of taxes	(1,339,000)	[3]	11,927,000	[3]	(14,458,000)	[3]	17,250,000	[3]
NET INCOME	45,469,000		54,535,000		105,675,000		111,892,000
Taxes related to unrealized loss (gain) from derivative instruments and related transactions	14,700,000		4,400,000		7,900,000		(5,900,000)
Taxes related to effects of economic hedging related to natural gas inventory	(778,000)		6,900,000		(8,400,000)		10,000,000
Transactions Between NJNG and NJRES [Member]
Unrealized loss (gain) from derivative instruments and related transactions, net of taxes (1) (2)	$ (377,000)		$ 79,000		$ (310,000)		$ 197,000

[1] Includes taxes of approximately $14.7 million and $4.4 million, for the three months ended and $7.9 million and $(5.9) million for the six months ended March 31, 2013 and 2012, respectively.

[2] Excludes unrealized (gains) losses related to an intercompany transaction between NJNG and NJRES that have been eliminated in consolidation of approximately $(377,000) and $79,000 for the three months ended and $(310,000) and $197,000 for the six months ended March 31, 2013 and 2012, respectively.

[3] Includes taxes of approximately $(778,000) and $6.9 million for the three months ended and $(8.4) million and $10 million for the six months ended March 31, 2013 and 2012, respectively.