CONDENSED FINANCIAL INFORMATION OF PARENT COMPANY (Details) (USD $)	3 Months Ended								12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2009	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009
Income Statement [Abstract]
Total operating revenues	$ 670,901,000	$ 648,169,000	$ 976,987,000	$ 713,152,000	$ 631,518,000	$ 479,894,000	$ 918,346,000	$ 609,546,000	$ 3,009,209,000	$ 2,639,304,000	$ 2,592,460,000
Operating expenses									2,866,208,000	2,451,179,000	2,544,390,000
Operating income (loss)	17,646,000	(27,182,000)	(93,202,000)	(40,263,000)	486,000	18,786,000	(122,012,000)	(85,385,000)	(143,001,000)	(188,125,000)	(48,070,000)
Other income									3,747,000	5,258,000	4,409,000
Interest expense, net of capitalized interest									19,623,000	21,251,000	21,014,000
INCOME (LOSS) BEFORE INCOME TAXES AND EQUITY IN EARNINGS OF AFFILIATES									127,125,000	172,132,000	31,465,000
Income tax provision (benefit)									37,665,000	64,692,000	11,376,000
Equity in earnings of subsidiaries									11,839,000	10,017,000	7,153,000
NET INCOME (LOSS)	(7,511,000)	20,374,000	63,927,000	24,509,000	1,515,000	(10,177,000)	74,217,000	51,902,000	101,299,000	117,457,000	27,242,000
Statement of Cash Flows [Abstract]
Net cash provided by operating activities									250,101,000	139,415,000	267,241,000
Net cash (used in) investing activities									(175,077,000)	(101,403,000)	(121,277,000)
Cash flows from (used in) financing activities:
Payments from long-term debt									(130,091,000)	(6,749,000)	(60,362,000)
Tax benefit from stock options exercised									2,007,000	669,000	1,686,000
Proceeds from common stock									13,704,000	6,487,000	16,441,000
Purchases of treasury stock									(10,193,000)	(29,650,000)	(30,670,000)
Payments of common stock dividends									(58,650,000)	(53,137,000)	(50,967,000)
Net (payments) proceeds of short-term debt									11,750,000	4,200,000	(34,800,000)
Cash flows (used in) financing activities									(68,527,000)	(73,255,000)	(152,404,000)
Change in cash and temporary investments									6,497,000	(35,243,000)	(6,440,000)
Cash and temporary investments at beginning of period				943,000				36,186,000	943,000	36,186,000	42,626,000
Cash and temporary investments at end of period	7,440,000				943,000				7,440,000	943,000	36,186,000
ASSETS
Current assets	732,367,000				785,008,000				732,367,000	785,008,000
Total assets	2,649,444,000				2,563,133,000				2,649,444,000	2,563,133,000	2,321,030,000
CAPITALIZATION AND LIABILITIES
Current liabilities	703,384,000				705,798,000				703,384,000	705,798,000
Common stock equity	110,258,000				109,713,000				110,258,000	109,713,000
Total capitalization and liabilities	2,649,444,000				2,563,133,000				2,649,444,000	2,563,133,000
NJR [Member]
Income Statement [Abstract]
Total operating revenues									0	0	0
Operating expenses									10,138,000	9,213,000	9,159,000
Operating income (loss)									10,138,000	9,213,000	9,159,000
Other income									10,580,000	9,960,000	9,980,000
Interest expense, net of capitalized interest									442,000	748,000	276,000
INCOME (LOSS) BEFORE INCOME TAXES AND EQUITY IN EARNINGS OF AFFILIATES									0	(1,000)	545,000
Income tax provision (benefit)									(11,000)	57,000	230,000
Equity in earnings of subsidiaries									101,288,000	117,515,000	26,927,000
NET INCOME (LOSS)									101,299,000	117,457,000	27,242,000
Statement of Cash Flows [Abstract]
Net cash provided by operating activities									60,937,000	40,370,000	52,971,000
Net cash (used in) investing activities									(54,568,000)	(41,397,000)	(80,269,000)
Cash flows from (used in) financing activities:
Payments from long-term debt									(77,000)	0	(25,000,000)
Tax benefit from stock options exercised									2,007,000	669,000	1,686,000
Proceeds from common stock									13,704,000	6,487,000	16,441,000
Net payments from associated companies									54,538,000	79,361,000	5,187,000
Purchases of treasury stock									(10,193,000)	(29,650,000)	(30,670,000)
Payments of common stock dividends									(58,650,000)	(53,137,000)	(50,967,000)
Net (payments) proceeds of short-term debt									(7,750,000)	(2,800,000)	110,700,000
Cash flows (used in) financing activities									(6,421,000)	930,000	27,377,000
Change in cash and temporary investments									(52,000)	(97,000)	79,000
Cash and temporary investments at beginning of period				57,000				154,000	57,000	154,000	75,000
Cash and temporary investments at end of period	5,000				57,000				5,000	57,000	154,000
ASSETS
Current assets	3,018,000				14,879,000				3,018,000	14,879,000
Investments	814,732,000				770,980,000				814,732,000	770,980,000
Intercompany receivable, net	139,193,000				124,807,000				139,193,000	124,807,000
Deferred charges and other assets	1,320,000				1,781,000				1,320,000	1,781,000
Total assets	958,263,000				912,447,000				958,263,000	912,447,000
CAPITALIZATION AND LIABILITIES
Current liabilities	131,117,000				134,662,000				131,117,000	134,662,000
Long-term debt	50,000,000				50,000,000				50,000,000	50,000,000
Deferred credits and other liabilities	889,000				2,302,000				889,000	2,302,000
Common stock equity	776,257,000				725,483,000				776,257,000	725,483,000
Total capitalization and liabilities	958,263,000				912,447,000				958,263,000	912,447,000
Cash Dividends Paid to Parent Company by Unconsolidated Subsidiaries									$ 58,700,000	$ 53,100,000	$ 51,000,000