DEBT, SCHEDULE OF LONG TERM DEBT (Details) (USD $)	12 Months Ended					12 Months Ended														12 Months Ended	3 Months Ended											12 Months Ended			12 Months Ended		12 Months Ended								12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2012 NJR [Member]	Sep. 30, 2011 NJR [Member]	Sep. 30, 2012 NJNG [Member]	Sep. 30, 2011 NJNG [Member]	Sep. 30, 2010 NJNG [Member]	Sep. 30, 2012 First Mortgage [Member]	Sep. 30, 2012 Unsecured Debt [Member] NJR [Member]	Sep. 30, 2011 Unsecured Debt [Member] NJR [Member]	Sep. 30, 2012 Unsecured Debt [Member] NJNG [Member]	Sep. 30, 2011 Unsecured Debt [Member] NJNG [Member]	Sep. 30, 2012 Unsecured senior note 1.94% [Member] NJR [Member]	Sep. 30, 2011 Unsecured senior note 1.94% [Member] NJR [Member]	Sep. 30, 2012 Unsecured senior note 2.51% [Member] NJR [Member]	Sep. 30, 2011 Unsecured senior note 2.51% [Member] NJR [Member]	Sep. 30, 2012 Unsecured senior note 3.25% [Member] NJR [Member]	Sep. 30, 2011 Unsecured senior note 3.25% [Member] NJR [Member]	Sep. 30, 2010 Debt, Auction Rate Securities [Member]	Dec. 31, 2010 Series CC [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2012 Series HH [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2011 Series HH [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2012 Series II [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2011 Series II [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2012 Series JJ [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2011 Series JJ [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2012 Series KK [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2011 Series KK [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2012 Series LL [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2011 Series LL [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2011 Variable Demand Rate Notes [Member]	Sep. 30, 2012 Series MM [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2011 Series MM [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2012 Series NN [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2011 Series NN [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2012 Series OO [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2011 Series OO [Member] First Mortgage [Member] NJNG [Member]	Sep. 30, 2012 Capital lease obligation-Buildings [Member] Capital Lease Obligations [Member] NJNG [Member]	Sep. 30, 2011 Capital lease obligation-Buildings [Member] Capital Lease Obligations [Member] NJNG [Member]	Sep. 30, 2012 Capital lease obligation-Meters [Member] Capital Lease Obligations [Member] NJNG [Member]	Sep. 30, 2011 Capital lease obligation-Meters [Member] Capital Lease Obligations [Member] NJNG [Member]	Sep. 30, 2012 Capital lease obligation-Equipment [Member] Capital Lease Obligations [Member] NJNG [Member]	Sep. 30, 2011 Capital lease obligation-Equipment [Member] Capital Lease Obligations [Member] NJNG [Member]	Sep. 30, 2012 Interest Rate I [Member] Debt, Auction Rate Securities [Member] NJNG [Member]	Sep. 30, 2012 Interest Rate II [Member] Debt, Auction Rate Securities [Member] NJNG [Member]
Debt Instrument [Line Items]
Long-term Debt										$ 50,000,000	$ 50,000,000	$ 60,000,000	$ 60,000,000	$ 25,000,000	$ 0	$ 25,000,000	$ 0	$ 50,000,000	$ 0			$ 12,000,000	$ 12,000,000	$ 10,300,000	$ 10,300,000	$ 10,500,000	$ 10,500,000	$ 15,000,000	$ 15,000,000	$ 125,000,000	$ 125,000,000		$ 9,545,000	$ 9,545,000	$ 41,000,000	$ 41,000,000	$ 46,500,000	$ 46,500,000	$ 21,907,000	$ 23,314,000	$ 30,887,000	$ 30,683,000	$ 290,000	$ 530,000
Long-term Debt, Current Maturities						(7,760,000)	(7,575,000)
Long-term debt	525,169,000	426,797,000		150,000,000	50,000,000	375,169,000	376,797,000
Debt Instrument, Interest Rate, Stated Percentage										6.05%		4.77%		1.94%		2.51%		3.25%			6.88%	5.00%		4.50%		4.60%		4.90%		5.60%
Debt Instrument, Interest Rate Terms																																	Variable		Variable		Variable
Long-term Debt, by Maturity [Abstract]
2012				0		0
2013				0		60,000,000
2014				25,000,000		0
2015				0		0
2016				50,000,000		0
Thereafter				75,000,000		269,800,000
Amount of Restricted Net Assets for Consolidated and Unconsolidated Subsidiaries									515,900,000
Retained Earnings, Appropriated									313,900,000
Proceeds from Issuance of Long-term Debt	0	97,045,000	0																	97,000,000
Debt Instrument, Multiplier of Variable Rate																																													175.00%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum																																														10.00%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum						12.00%																																								12.00%
Repayments of Long-term Debt	8,025,000	130,091,000	6,749,000																													97,045,000
Debt Weighted Average Interest Rate						0.22%
Redemption of First Mortgage Bond																					20,000,000
Sale Leaseback Transaction, Gross Proceeds						6,500,000	5,900,000	4,900,000
Present Value of Future Minimum Lease Payments, Sale Leaseback Transactions [Abstract]
2012				10,700,000
2013				10,200,000
2014				9,600,000
2015				9,900,000
2016				8,700,000
Thereafter				16,700,000
Subtotal				65,800,000
Less: interest component				(12,700,000)
Total				53,100,000
Sale Leaseback Transaction, Other Payments Required						$ 1,000,000	$ 3,900,000