Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended										12 Months Ended
	Sep. 30, 2013 state customers regulated_gas_distributions_divisions		Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012		Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2013 segment state customers regulated_gas_distributions_divisions		Sep. 30, 2012		Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 685,171		$ 857,935	$ 1,308,996	$ 1,034,155	$ 552,566		$ 576,414	$ 1,225,509	$ 1,083,994	$ 3,886,257		$ 3,438,483		$ 4,286,435
Purchased gas cost											2,474,207		2,114,744		2,985,615
Gross profit	300,440		316,497	432,751	362,362	249,389		293,171	425,787	355,392	1,412,050		1,323,739		1,300,820
Operating expenses
Operation and maintenance											488,020		453,613		442,965
Depreciation and amortization											235,079		237,525		223,832
Taxes, other than income											187,072		181,073		177,767
Asset impairments											0		5,288		30,270
Total operating expenses											910,171		877,499		874,834
Operating income	50,383		86,396	210,178	154,922	22,791		81,546	202,432	139,471	501,879		446,240		425,986
Miscellaneous income (expense), net											(197)		(14,644)		21,184
Interest charges											128,385		141,174		150,763
Income (loss) before income taxes											373,297		290,422		296,407
Income tax expense											142,599		98,226		106,819
Income (loss) from continuing operations	7,536		33,474	112,340	77,348	(286)		28,014	102,084	62,384	230,698		192,196		189,588
Income from discontinued operations, net of tax	0		0	4,085	3,117	1,904		3,118	7,027	6,123	7,202		18,172		18,013
Gain (loss) on sale of discontinued operations, net of tax	0		5,294	0	0	6,349		0	0	0	5,294		6,349		0
Net income	7,536		38,768	116,425	80,465	7,967		31,132	109,111	68,507	243,194		216,717		207,601
Capital expenditures											845,033		732,858		622,965
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues	685,171		857,935	1,308,996	1,034,155	552,566		576,414	1,225,509	1,083,994	3,886,257		3,438,483		4,286,435
ASSETS
Net property, plant and equipment	6,030,655					5,475,604					6,030,655		5,475,604
Investment in subsidaries	0					0					0		0
Current assets
Cash and cash equivalents	66,199					64,239					66,199		64,239		131,419	131,952
Assets from risk management activities current	18,099					24,707					18,099		24,707
Other current assets	598,968					739,016					598,968		739,016
Intercompany receivables	0					0					0		0
Total current assets	683,266					827,962					683,266		827,962
Intangible assets	121					164					121		164
Goodwill	741,363					740,683					741,363		740,683
Assets from risk management activities noncurrent	109,354					2,283					109,354		2,283
Deferred charges and other assets	375,642					448,979					375,642		448,979
Total Assets	7,940,401					7,495,675					7,940,401		7,495,675
CAPITALIZATION AND LIABILITIES
Shareholders' equity	2,580,409					2,359,243					2,580,409		2,359,243		2,255,421	2,178,348
Long-term debt	2,455,671					1,956,305					2,455,671		1,956,305
Total capitalization	5,036,080					4,315,548					5,036,080		4,315,548
Current liabilities
Current maturities of long-term debt	0					131					0		131
Short-term debt	367,984					570,929					367,984		570,929
Liabilities from risk management activities current	1,543					85,381					1,543		85,381
Other current liabilities	608,959					619,513					608,959		619,513
Intercompany payables	0					0					0		0
Total current liabilities	978,486					1,275,954					978,486		1,275,954
Deferred income taxes	1,164,053					1,015,083					1,164,053		1,015,083
Liabilities from risk management activities noncurrent	6,133					9,206					6,133		9,206
Regulatory cost of removal obligation	359,299					381,164					359,299		381,164
Pension and postretirement liabilities	358,787					457,196					358,787		457,196
Deferred credits and other liabilities	37,563					41,524					37,563		41,524
Total Equity and Liabilities	7,940,401					7,495,675					7,940,401		7,495,675
Number Of Customers, Natural Gas Distribution	3,000,000										3,000,000
Number Of Divisions, Natural Gas Distribution	6										6
Number of States in which Entity Operates	8										8
Number of Operating Segments											3
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											3,886,257		3,438,483		4,286,435
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											3,886,257		3,438,483		4,286,435
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											0		0		0
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											0		0		0
Natural Gas Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											2,394,418		2,144,376		2,469,781
Operating revenues	360,386		467,144	905,176	666,787	282,516		315,634	871,067	676,113	2,399,493		2,145,330		2,470,664
Purchased gas cost											1,318,257		1,122,587		1,452,721
Gross profit											1,081,236		1,022,743		1,017,943
Operating expenses
Operation and maintenance											375,188		353,879		341,758
Depreciation and amortization											195,581		202,026		193,642
Taxes, other than income											167,374		162,377		160,455
Asset impairments													0		0
Total operating expenses											738,143		718,282		695,855
Operating income											343,093		304,461		322,088
Miscellaneous income (expense), net											2,535		(12,657)		16,242
Interest charges											98,296		110,642		115,740
Income (loss) before income taxes											247,332		181,162		222,590
Income tax expense											96,476		57,314		77,885
Income (loss) from continuing operations											150,856		123,848		144,705
Income from discontinued operations, net of tax											7,202		18,172		18,013
Gain (loss) on sale of discontinued operations, net of tax											5,649		6,349
Net income											163,707		148,369		162,718
Capital expenditures											528,599		546,818		496,899
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											2,394,418		2,144,376		2,469,781
Operating revenues	360,386		467,144	905,176	666,787	282,516		315,634	871,067	676,113	2,399,493		2,145,330		2,470,664
ASSETS
Net property, plant and equipment	4,719,873					4,432,017					4,719,873		4,432,017
Investment in subsidaries	831,136					747,496					831,136		747,496
Current assets
Cash and cash equivalents	4,237					12,787					4,237		12,787
Assets from risk management activities current	1,837					6,934	[1]				1,837		6,934	[1]
Other current assets	428,366					546,187					428,366		546,187
Intercompany receivables	783,738					636,557					783,738		636,557
Total current assets	1,218,178					1,202,465					1,218,178		1,202,465
Intangible assets	0					0					0		0
Goodwill	574,190					573,550					574,190		573,550
Assets from risk management activities noncurrent	109,354					2,283					109,354		2,283
Deferred charges and other assets	347,687					417,893					347,687		417,893
Total Assets	7,800,418					7,375,704					7,800,418		7,375,704
CAPITALIZATION AND LIABILITIES
Shareholders' equity	2,580,409					2,359,243					2,580,409		2,359,243
Long-term debt	2,455,671					1,956,305					2,455,671		1,956,305
Total capitalization	5,036,080					4,315,548					5,036,080		4,315,548
Current liabilities
Current maturities of long-term debt	0					0					0		0
Short-term debt	645,984					782,719					645,984		782,719
Liabilities from risk management activities current	1,543					85,366	[1]				1,543		85,366	[1]
Other current liabilities	491,681					526,089					491,681		526,089
Intercompany payables	0					0					0		0
Total current liabilities	1,139,208					1,394,174					1,139,208		1,394,174
Deferred income taxes	871,360					789,288					871,360		789,288
Liabilities from risk management activities noncurrent	0					0					0		0
Regulatory cost of removal obligation	359,299					381,164					359,299		381,164
Pension and postretirement liabilities	358,787					457,196					358,787		457,196
Deferred credits and other liabilities	35,684					38,334					35,684		38,334
Total Equity and Liabilities	7,800,418					7,375,704					7,800,418		7,375,704
Natural Gas Distribution Segment [Member] | Gas Sales Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											2,316,137		2,065,424		2,386,579
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											2,316,137		2,065,424		2,386,579
Natural Gas Distribution Segment [Member] | Gas Sales Revenues [Member] | Residential Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											1,512,495		1,351,479		1,535,887
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											1,512,495		1,351,479		1,535,887
Natural Gas Distribution Segment [Member] | Gas Sales Revenues [Member] | Commercial Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											661,930		587,651		685,380
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											661,930		587,651		685,380
Natural Gas Distribution Segment [Member] | Gas Sales Revenues [Member] | Industrial Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											81,155		71,960		96,636
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											81,155		71,960		96,636
Natural Gas Distribution Segment [Member] | Gas Sales Revenues [Member] | Public Authority and Other Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											60,557		54,334		68,676
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											60,557		54,334		68,676
Natural Gas Distribution Segment [Member] | Gas Transportation Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											55,938		53,924		57,331
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											55,938		53,924		57,331
Natural Gas Distribution Segment [Member] | Other Gas Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											22,343		25,028		25,871
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											22,343		25,028		25,871
Natural Gas Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											2,394,418		2,144,376		2,469,781
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											2,394,418		2,144,376		2,469,781
Natural Gas Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											5,075		954		883
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											5,075		954		883
Regulated Transmission and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											89,011		92,604		87,141
Operating revenues	72,330		74,041	61,848	60,681	65,482		67,073	58,037	56,759	268,900		247,351		219,373
Purchased gas cost											0		0		0
Gross profit											268,900		247,351		219,373
Operating expenses
Operation and maintenance											76,686		71,521		70,401
Depreciation and amortization											35,302		31,438		25,997
Taxes, other than income											17,059		15,568		14,700
Asset impairments													0		0
Total operating expenses											129,047		118,527		111,098
Operating income											139,853		128,824		108,275
Miscellaneous income (expense), net											(2,285)		(1,051)		4,715
Interest charges											30,678		29,414		31,432
Income (loss) before income taxes											106,890		98,359		81,558
Income tax expense											38,630		35,300		29,143
Income (loss) from continuing operations											68,260		63,059		52,415
Income from discontinued operations, net of tax											0		0		0
Gain (loss) on sale of discontinued operations, net of tax											0		0
Net income											68,260		63,059		52,415
Capital expenditures											313,230		175,768		118,452
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											89,011		92,604		87,141
Operating revenues	72,330		74,041	61,848	60,681	65,482		67,073	58,037	56,759	268,900		247,351		219,373
ASSETS
Net property, plant and equipment	1,249,767					979,443					1,249,767		979,443
Investment in subsidaries	0					0					0		0
Current assets
Cash and cash equivalents	0					0					0		0
Assets from risk management activities current	0					0					0		0
Other current assets	11,709					11,788					11,709		11,788
Intercompany receivables	0					0					0		0
Total current assets	11,709					11,788					11,709		11,788
Intangible assets	0					0					0		0
Goodwill	132,462					132,422					132,462		132,422
Assets from risk management activities noncurrent	0					0					0		0
Deferred charges and other assets	19,227					24,353					19,227		24,353
Total Assets	1,413,165					1,148,006					1,413,165		1,148,006
CAPITALIZATION AND LIABILITIES
Shareholders' equity	396,421					328,161					396,421		328,161
Long-term debt	0					0					0		0
Total capitalization	396,421					328,161					396,421		328,161
Current liabilities
Current maturities of long-term debt	0					0					0		0
Short-term debt	0					0					0		0
Liabilities from risk management activities current	0					0					0		0
Other current liabilities	20,288					12,478					20,288		12,478
Intercompany payables	712,768					584,578					712,768		584,578
Total current liabilities	733,056					597,056					733,056		597,056
Deferred income taxes	283,554					220,647					283,554		220,647
Liabilities from risk management activities noncurrent	0					0					0		0
Regulatory cost of removal obligation	0					0					0		0
Pension and postretirement liabilities	0					0					0		0
Deferred credits and other liabilities	134					2,142					134		2,142
Total Equity and Liabilities	1,413,165					1,148,006					1,413,165		1,148,006
Regulated Transmission and Storage Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											89,011		92,604		87,141
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											89,011		92,604		87,141
Regulated Transmission and Storage Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											179,889		154,747		132,232
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											179,889		154,747		132,232
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											1,402,828		1,201,503		1,729,513
Operating revenues	348,061		421,808	428,948	399,894	280,114		256,250	370,763	444,176	1,598,711		1,351,303		2,024,893
Purchased gas cost											1,535,380		1,296,179		1,959,893
Gross profit											63,331		55,124		65,000
Operating expenses
Operation and maintenance											37,569		29,697		32,308
Depreciation and amortization											4,196		4,061		4,193
Taxes, other than income											2,639		3,128		2,612
Asset impairments													5,288		30,270
Total operating expenses											44,404		42,174		69,383
Operating income											18,927		12,950		(4,383)
Miscellaneous income (expense), net											2,316		1,035		657
Interest charges											2,168		3,084		4,015
Income (loss) before income taxes											19,075		10,901		(7,741)
Income tax expense											7,493		5,612		(209)
Income (loss) from continuing operations											11,582		5,289		(7,532)
Income from discontinued operations, net of tax											0		0		0
Gain (loss) on sale of discontinued operations, net of tax											(355)		0
Net income											11,227		5,289		(7,532)
Capital expenditures											3,204		10,272		7,614
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											1,402,828		1,201,503		1,729,513
Operating revenues	348,061		421,808	428,948	399,894	280,114		256,250	370,763	444,176	1,598,711		1,351,303		2,024,893
ASSETS
Net property, plant and equipment	61,015					64,144					61,015		64,144
Investment in subsidaries	(2,096)					(2,096)					(2,096)		(2,096)
Current assets
Cash and cash equivalents	61,962					51,452					61,962		51,452
Assets from risk management activities current	16,262	[2]				17,773	[3]				16,262	[2]	17,773	[3]
Other current assets	452,126					404,097					452,126		404,097
Intercompany receivables	0					0					0		0
Total current assets	530,350					473,322					530,350		473,322
Intangible assets	121					164					121		164
Goodwill	34,711					34,711					34,711		34,711
Assets from risk management activities noncurrent	0					0					0		0
Deferred charges and other assets	8,728					6,733					8,728		6,733
Total Assets	632,829					576,978					632,829		576,978
CAPITALIZATION AND LIABILITIES
Shareholders' equity	434,715					419,335					434,715		419,335
Long-term debt	0					0					0		0
Total capitalization	434,715					419,335					434,715		419,335
Current liabilities
Current maturities of long-term debt	0					131					0		131
Short-term debt	0					0					0		0
Liabilities from risk management activities current	0	[2]				15	[3]				0	[2]	15	[3]
Other current liabilities	110,306					90,116					110,306		90,116
Intercompany payables	70,970					51,979					70,970		51,979
Total current liabilities	181,276					142,241					181,276		142,241
Deferred income taxes	8,960					5,148					8,960		5,148
Liabilities from risk management activities noncurrent	6,133					9,206					6,133		9,206
Regulatory cost of removal obligation	0					0					0		0
Pension and postretirement liabilities	0					0					0		0
Deferred credits and other liabilities	1,745					1,048					1,745		1,048
Total Equity and Liabilities	632,829					576,978					632,829		576,978
Nonregulated Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											1,402,828		1,201,503		1,729,513
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											1,402,828		1,201,503		1,729,513
Nonregulated Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											195,883		149,800		295,380
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											195,883		149,800		295,380
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(95,606)		(105,058)	(86,976)	(93,207)	(75,546)		(62,543)	(74,358)	(93,054)	(380,847)		(305,501)		(428,495)
Purchased gas cost											(379,430)		(304,022)		(426,999)
Gross profit											(1,417)		(1,479)		(1,496)
Operating expenses
Operation and maintenance											(1,423)		(1,484)		(1,502)
Depreciation and amortization											0		0		0
Taxes, other than income											0		0		0
Asset impairments													0		0
Total operating expenses											(1,423)		(1,484)		(1,502)
Operating income											6		5		6
Miscellaneous income (expense), net											(2,763)		(1,971)		(430)
Interest charges											(2,757)		(1,966)		(424)
Income (loss) before income taxes											0		0		0
Income tax expense											0		0		0
Income (loss) from continuing operations											0		0		0
Income from discontinued operations, net of tax											0		0		0
Gain (loss) on sale of discontinued operations, net of tax											0		0
Net income											0		0		0
Capital expenditures											0		0		0
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues	(95,606)		(105,058)	(86,976)	(93,207)	(75,546)		(62,543)	(74,358)	(93,054)	(380,847)		(305,501)		(428,495)
ASSETS
Net property, plant and equipment	0					0					0		0
Investment in subsidaries	(829,040)					(745,400)					(829,040)		(745,400)
Current assets
Cash and cash equivalents	0					0					0		0
Assets from risk management activities current	0					0					0		0
Other current assets	(293,233)					(223,056)					(293,233)		(223,056)
Intercompany receivables	(783,738)					(636,557)					(783,738)		(636,557)
Total current assets	(1,076,971)					(859,613)					(1,076,971)		(859,613)
Intangible assets	0					0					0		0
Goodwill	0					0					0		0
Assets from risk management activities noncurrent	0					0					0		0
Deferred charges and other assets	0					0					0		0
Total Assets	(1,906,011)					(1,605,013)					(1,906,011)		(1,605,013)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(831,136)					(747,496)					(831,136)		(747,496)
Long-term debt	0					0					0		0
Total capitalization	(831,136)					(747,496)					(831,136)		(747,496)
Current liabilities
Current maturities of long-term debt	0					0					0		0
Short-term debt	(278,000)					(211,790)					(278,000)		(211,790)
Liabilities from risk management activities current	0					0					0		0
Other current liabilities	(13,316)					(9,170)					(13,316)		(9,170)
Intercompany payables	(783,738)					(636,557)					(783,738)		(636,557)
Total current liabilities	(1,075,054)					(857,517)					(1,075,054)		(857,517)
Deferred income taxes	179					0					179		0
Liabilities from risk management activities noncurrent	0					0					0		0
Regulatory cost of removal obligation	0					0					0		0
Pension and postretirement liabilities	0					0					0		0
Deferred credits and other liabilities	0					0					0		0
Total Equity and Liabilities	(1,906,011)					(1,605,013)					(1,906,011)		(1,605,013)
Intersegment Elimination [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											0		0		0
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											0		0		0
Intersegment Elimination [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											(380,847)		(305,501)		(428,495)
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											$ (380,847)		$ (305,501)		$ (428,495)

[1] The September 30, 2012 amounts are presented net of assets and liabilities held for sale in conjunction with the sale of our Georgia operations. At September 30, 2012, assets and liabilities held for sale included $0.1 million of current assets from risk management activities and $0.3 million of current liabilities from risk management activities.

[2] Includes $24.8 million of cash held on deposit to collateralize certain financial instruments. Of this amount, $8.6 million was used to offset current risk management liabilities under master netting arrangements and the remaining $16.2 million is classified as current risk management assets.

[3] Includes $23.7 million of cash held on deposit to collateralize certain financial instruments. Of this amount, $5.9 million was used to offset current risk management liabilities under master netting arrangements and the remaining $17.8 million is classified as current risk management assets.