BUSINESS SEGMENT AND OTHER OPERATIONS DATA, RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (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
Segment Reporting Information [Line Items]
Utility									$ 627,713		$ 971,724		$ 937,433
Nonutility									1,621,210		2,037,485		1,701,871
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
Depreciation and amortization									41,643		34,370		32,267
Investment Income, Interest									1,026	[1]	1,216	[1]	2,018	[1]
Interest expense, net of capitalized interest									20,844		19,623		21,251
Income tax provision (benefit)									7,729		37,665		64,692
Equity in earnings of affiliates									10,634		11,839		10,017
Net Financial Earnings (Loss)									112,417		106,533		101,764
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets									207,515		177,531		97,379
Payments to Acquire Equity Method Investments									8,800		0		4,300
Reportable Segment [Member]
Segment Reporting Information [Line Items]
Total operating revenues									2,210,581		3,024,889		2,630,524
Depreciation and amortization									40,992		33,628		31,623
Investment Income, Interest									2,024	[1]	2,043	[1]	2,921	[1]
Interest expense, net of capitalized interest									19,505		19,119		20,477
Income tax provision (benefit)									5,656		36,701		65,558
Net Financial Earnings (Loss)									110,230		103,446		100,907
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets									206,181		173,982		94,376
Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Investment Income, Interest									889	[1]	1,133	[1]	1,973	[1]
Interest expense, net of capitalized interest									14,890		14,875		16,618
Income tax provision (benefit)									38,135		40,322		38,417
Net Financial Earnings (Loss)									73,238		71,322		70,242
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets									116,455		101,993		93,821
Natural Gas Distribution [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Utility									627,713		971,724		937,433
Natural Gas Distribution [Member] | Intercompany [Member]
Segment Reporting Information [Line Items]
Utility									0		0		8,047
Energy Services [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization									59		61		153
Investment Income, Interest									37	[1]	9	[1]	15	[1]
Interest expense, net of capitalized interest									1,096		995		1,439
Income tax provision (benefit)									(4,950)		3,281		23,250
Net Financial Earnings (Loss)									10,791		18,583		24,814
Energy Services [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility									1,577,851		1,996,997		1,671,655
Energy Services [Member] | Intercompany [Member]
Segment Reporting Information [Line Items]
Nonutility									2,760		55,306		13,389
Midstream Assets [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization									6		6		6
Investment Income, Interest									1,098	[1]	901	[1]	933	[1]
Interest expense, net of capitalized interest									2,665		3,165		2,418
Income tax provision (benefit)									4,978		4,702		4,301
Equity in earnings of affiliates									14,308		14,904		12,996
Net Financial Earnings (Loss)									6,749		6,780		6,444
Payments to Acquire Equity Method Investments									0		0		4,300
Clean Energy Ventures [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization									5,680		421		0
Interest expense, net of capitalized interest									854		84		2
Income tax provision (benefit)									(32,507)		(11,604)		(410)
Net Financial Earnings (Loss)									19,452		6,761		(593)
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets									89,726		71,989		555
Payments to Acquire Equity Method Investments									8,800		0		0
Clean Energy Ventures [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility									2,257		862		0
Retail and Other [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization									661		742		644
Investment Income, Interest									3	[1]	43	[1]	4	[1]
Interest expense, net of capitalized interest									1,339		504		774
Income tax provision (benefit)									2,178		1,033		(727)
Net Financial Earnings (Loss)									2,366		3,087		857
Segment Reporting Information, Expenditures for Additions to Long-Lived Assets									1,334		3,549		3,003
Retail and Other [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility									41,102		39,626		30,216
Retail and Other [Member] | Intercompany [Member]
Segment Reporting Information [Line Items]
Nonutility									1,093		334		335
Business Intersegment, Eliminations [Member]
Segment Reporting Information [Line Items]
Nonutility									(3,853)		(55,640)		(21,771)
Depreciation and amortization									(10)		0		0
Investment Income, Interest									(1,001)	[1]	(870)	[1]	(907)	[1]
Income tax provision (benefit)									(105)		(69)		(139)
Equity in earnings of affiliates									(3,674)		(3,065)		(2,979)
Net Financial Earnings (Loss)									(179)		0		0
NJNG [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization									$ 35,247		$ 33,140		$ 31,464

[1] Included in other income on the Consolidated Statement of Operations.