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, 2014		Mar. 31, 2013		Mar. 31, 2014		Mar. 31, 2013
Segment Reporting Information [Line Items]
Utility	$ 394,528		$ 351,750		$ 627,997		$ 570,599
Nonutility	1,185,041		609,135		1,829,977		1,126,305
Total operating revenues	1,579,569		960,885		2,457,974		1,696,904
Depreciation and amortization	12,828		11,721		25,394		23,024
Interest income	216	[1]	192	[1]	512	[1]	399	[1]
Interest expense, net of capitalized interest	6,306		5,746		12,601		11,571
Income tax provision (benefit)	71,680		12,065		73,185		36,045
Equity in earnings of affiliates	3,233		3,530		5,375		6,085
Net financial earnings (loss)	151,920		68,664		191,780		104,157
Capital expenditures	56,242		41,243		116,765		90,862
Canada
Segment Reporting Information [Line Items]
Percentage to total operating revenues					4.00%		6.90%
Intercompany [Member]
Segment Reporting Information [Line Items]
Nonutility	297		186		499		449
Energy Services [Member] | Intercompany [Member]
Segment Reporting Information [Line Items]
Nonutility	67,749		4,444		71,767		4,551
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total operating revenues	1,638,791		956,996		2,512,142		1,682,766
Depreciation and amortization	12,622		11,515		24,981		22,636
Interest income	451	[1]	409	[1]	980	[1]	847	[1]
Interest expense, net of capitalized interest	6,175		5,487		12,359		11,013
Income tax provision (benefit)	72,255		12,753		74,117		36,898
Equity in earnings of affiliates	3,233		3,530		5,375		6,085
Net financial earnings (loss)	153,511		69,713		193,572		105,309
Capital expenditures	55,970		41,099		116,288		90,564
Operating Segments [Member] | Natural Gas Distribution [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	9,972		9,399		19,807		18,676
Interest income	181	[1]	146	[1]	442	[1]	313	[1]
Interest expense, net of capitalized interest	3,888		3,549		7,872		7,133
Income tax provision (benefit)	23,725		22,794		37,908		37,301
Net financial earnings (loss)	47,043		45,917		74,682		71,409
Capital expenditures	35,484		31,554		70,885		65,699
Operating Segments [Member] | Natural Gas Distribution [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Utility	394,528		351,750		627,997		570,599
Operating Segments [Member] | Energy Services [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	13		11		25		22
Interest expense, net of capitalized interest	661		640		1,277		1,208
Income tax provision (benefit)	64,345		(5,203)		53,072		10,961
Net financial earnings (loss)	91,407		16,368		98,781		19,382
Operating Segments [Member] | Energy Services [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility	1,173,835	[2]	599,362	[2]	1,807,526	[2]	1,102,997	[2]
Operating Segments [Member] | Clean Energy Ventures [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	2,635		2,104		5,146		3,935
Interest expense, net of capitalized interest	1,249		822		2,435		1,605
Income tax provision (benefit)	(17,388)		(6,457)		(19,432)		(14,226)
Net financial earnings (loss)	12,807		5,154		16,421		10,459
Capital expenditures	20,486		9,545		45,403		24,865
Operating Segments [Member] | Clean Energy Ventures [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility	2,679		1,440		4,852		4,619
Operating Segments [Member] | Midstream Investments [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	2		1		3		3
Interest income	270	[1]	263	[1]	538	[1]	534	[1]
Interest expense, net of capitalized interest	377		476		775		1,067
Income tax provision (benefit)	1,573		1,619		2,569		2,862
Equity in earnings of affiliates	4,141		4,469		7,083		7,960
Net financial earnings (loss)	2,254		2,274		3,688		4,059
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization	208		199		415		390
Interest income	0	[1]	1	[1]	0	[1]	2	[1]
Interest expense, net of capitalized interest	131		259		242		558
Income tax provision (benefit)	(1,129)		(906)		(1,320)		(1,029)
Net financial earnings (loss)	(1,576)		(1,037)		(1,777)		(1,131)
Capital expenditures	272		144		477		298
Corporate, Non-Segment [Member] | External Customers [Member]
Segment Reporting Information [Line Items]
Nonutility	8,527		8,334		17,599		18,689
Consolidation, Eliminations [Member]
Segment Reporting Information [Line Items]
Nonutility	(68,046)		(4,631)		(72,266)		(5,000)
Depreciation and amortization	(2)		7		(2)		(2)
Interest income	(235)	[1]	(218)	[1]	(468)	[1]	(450)	[1]
Income tax provision (benefit)	554		218		388		176
Equity in earnings of affiliates	(908)		(939)		(1,708)		(1,875)
Net financial earnings (loss)	$ (15)		$ (12)		$ (15)		$ (21)

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

[2] Includes sales to Canada, which accounted for 4 percent and 6.9 percent of total operating revenues during the six months ended March 31, 2014 and 2013, respectively.