CONDENSED FINANCIAL INFORMATION OF PARENT COMPANY (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2012 NJR [Member]	Sep. 30, 2011 NJR [Member]	Sep. 30, 2010 NJR [Member]	Sep. 30, 2008 NJR [Member]
Income Statement [Abstract]
Total operating revenues	$ 568,470	$ 425,121	$ 612,921	$ 642,411	$ 670,901	$ 648,169	$ 976,987	$ 713,152	$ 2,248,923	$ 3,009,209	$ 2,639,304	$ 0	$ 0	$ 0
Operating expenses									2,140,233	2,866,208	2,451,179	9,957	10,138	9,213
Operating loss	14,306	19,911	(67,689)	(75,218)	17,646	(27,182)	(93,202)	(40,263)	(108,690)	(143,001)	(188,125)	9,957	10,138	9,213
Other income									2,128	3,747	5,258	11,254	10,580	9,960
Interest expense, net of capitalized interest									20,844	19,623	21,251	1,297	442	748
(LOSS) INCOME BEFORE INCOME TAXES AND EQUITY IN EARNINGS OF AFFILIATES									89,974	127,125	172,132	0	0	(1)
Income tax provision									7,729	37,665	64,692	95	(11)	57
Equity in earnings of subsidiaries									10,634	11,839	10,017	92,974	101,288	117,515
NET INCOME	(8,693)	(10,320)	54,535	57,357	(7,511)	20,374	63,927	24,509	92,879	101,299	117,457	92,879	101,299	117,457
Comprehensive Income (Loss) Statement [Abstract]
Adjustment to postemployment benefit obligation, net of tax of $348, $(205) and $323, respectively									(436)	1,219	(3,402)	(505)	296	(467)
Other comprehensive (loss) income									(167)	1,403	(1,955)	(505)	296	(467)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent									92,712	102,702	115,502	92,374	101,595	116,990
Statement of Cash Flows [Abstract]
Net cash provided from operating activities									51,081	250,101	139,415	47,491	60,937	40,370
Net cash (used in) investing activities									(217,117)	(175,077)	(101,403)	(115,236)	(54,568)	(41,397)
Cash flows from (used in) financing activities:
Payments from long-term debt									(8,025)	(130,091)	(6,749)	(1,678)	(77)	0
Tax benefit from stock options exercised									780	2,007	669	780	2,007	669
Proceeds from common stock									13,834	13,704	6,487	13,834	13,704	6,487
Proceeds from long-term debt									100,000	0	0	100,000	0	0
Net proceeds from associated companies												13,315	54,538	79,361
Purchases of treasury stock									(8,768)	(10,193)	(29,650)	(8,768)	(10,193)	(29,650)
Payments of common stock dividends									(61,688)	(58,650)	(53,137)	(61,688)	(58,650)	(53,137)
Net proceeds (payments) of short-term debt									120,450	11,750	4,200	11,950	(7,750)	(2,800)
Cash flows from (used in) financing activities									163,105	(68,527)	(73,255)	67,745	(6,421)	930
Change in cash and temporary investments									(2,931)	6,497	(35,243)	0	(52)	(97)
Cash and cash equivalents at beginning of period				7,440				943	7,440	943	36,186	5	57		154
Cash and cash equivalents at end of period	4,509				7,440				4,509	7,440	943	5	5	57	154
ASSETS
Current assets	647,344				732,367				647,344	732,367		2,778	3,018
Investments												845,023	814,732
Intercompany receivable, net												241,021	139,193
Deferred charges and other assets												4,945	1,320
Total assets	2,770,005				2,649,444				2,770,005	2,649,444	2,563,133	1,093,767	958,263
CAPITALIZATION AND LIABILITIES
Current liabilities	653,139				703,384				653,139	703,384		127,525	131,117
Long-term debt												150,000	50,000
Deferred credits and other liabilities												2,377	889
Common stock equity	110,956				110,258				110,956	110,258		813,865	776,257
Total capitalization and liabilities	$ 2,770,005				$ 2,649,444				$ 2,770,005	$ 2,649,444		$ 1,093,767	$ 958,263