BUSINESS SEGMENT AND OTHER OPERATIONS DATA - RECONCILIATION OF SEGMENT INCOME TO CONSOLIDATED (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Mar. 31, 2013		Mar. 31, 2012
Segment Reporting Information [Line Items]
Utility	$ 351,750		$ 226,023		$ 570,599		$ 417,397
Nonutility	609,135		386,898		1,126,305		837,935
Total operating revenues	960,885		612,921		1,696,904		1,255,332
Depreciation and amortization	11,721		10,439		23,024		20,039
Investment income, interest	192		227		399		517
Interest expense, net of capitalized interest	5,746		5,427		11,571		10,432
Income tax provision (benefit)	12,065		11,094		36,045		27,131
Equity in earnings of affiliates	3,530		3,018		6,085		5,672
Net financial earnings (loss)	68,664		74,126		104,157		119,434
Capital expenditures	41,243		41,972		90,862		111,390
Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	9,399		8,749		18,676		17,381
Investment income, interest	146	[1]	214	[1]	313	[1]	460	[1]
Interest expense, net of capitalized interest	3,549		3,713		7,133		7,450
Income tax provision (benefit)	22,794		27,016		37,301		42,612
Net financial earnings (loss)	45,917		44,936		71,409		70,910
Capital expenditures	31,554		27,427		65,699		49,145
Natural Gas Distribution [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Utility	351,750		226,023		570,599		417,397
Clean Energy Ventures [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	2,104		1,511		3,935		2,321
Interest expense, net of capitalized interest	822		198		1,605		340
Income tax provision (benefit)	(6,457)		(14,826)		(14,226)		(26,997)
Net financial earnings (loss)	5,154		11,862		10,459		21,959
Capital expenditures	9,545		14,175		24,865		61,786
Clean Energy Ventures [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility	1,440		527		4,619		907
Energy Services [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	11		16		22		32
Investment income, interest	0	[1]	(5)	[1]	0	[1]	26	[1]
Interest expense, net of capitalized interest	640		245		1,208		515
Income tax provision (benefit)	(5,203)		(2,105)		10,961		9,403
Net financial earnings (loss)	16,368		15,871		19,382		23,486
Energy Services [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility	599,362		378,356		1,102,997		820,162
Energy Services [Member] | Business Intersegment, Eliminations [Member]
Segment Reporting Information [Line Items]
Nonutility	4,444		2,654		4,551		2,848
Energy Holdings [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	1		2		3		3
Investment income, interest	263	[1]	285	[1]	534	[1]	533	[1]
Interest expense, net of capitalized interest	476		683		1,067		1,397
Income tax provision (benefit)	1,619		1,393		2,862		2,624
Equity in earnings of affiliates	4,469		3,967		7,960		7,582
Net financial earnings (loss)	2,274		2,021		4,059		3,804
Reportable Segment [Member]
Segment Reporting Information [Line Items]
Total operating revenues	956,996		607,560		1,682,766		1,241,314
Depreciation and amortization	11,515		10,278		22,636		19,737
Investment income, interest	409		494		847		1,019
Interest expense, net of capitalized interest	5,487		4,839		11,013		9,702
Income tax provision (benefit)	12,753		11,478		36,898		27,642
Net financial earnings (loss)	69,713		74,690		105,309		120,159
Capital expenditures	41,099		41,602		90,564		110,931
Retail and Other [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	199		161		390		302
Investment income, interest	1		1		2		1
Interest expense, net of capitalized interest	259		588		558		730
Income tax provision (benefit)	(906)		(312)		(1,029)		(371)
Net financial earnings (loss)	(1,037)		(543)		(1,131)		(689)
Capital expenditures	144		370		298		459
Retail and Other [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility	8,334		8,014		18,689		16,866
Retail and Other [Member] | Business Intersegment, Eliminations [Member]
Segment Reporting Information [Line Items]
Nonutility	186		263		449		442
Business Intersegment, Eliminations [Member]
Segment Reporting Information [Line Items]
Nonutility	(4,631)		(2,916)		(5,000)		(3,290)
Depreciation and amortization	7		0		(2)		0
Investment income, interest	(218)		(268)		(450)		(503)
Income tax provision (benefit)	218		(72)		176		(140)
Equity in earnings of affiliates	(939)		(949)		(1,875)		(1,910)
Net financial earnings (loss)	$ (12)		$ (21)		$ (21)		$ (36)

[1] Included in other income on the Unaudited Condensed Consolidated Statements of Operations.