Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended										12 Months Ended
	Sep. 30, 2014 state regulated_gas_distributions_divisions customers		Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013		Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014 segment state regulated_gas_distributions_divisions customers		Sep. 30, 2013		Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 778,728		$ 942,718	$ 1,964,322	$ 1,255,148	$ 674,374		$ 857,935	$ 1,308,996	$ 1,034,155	$ 4,940,916		$ 3,875,460		$ 3,436,162
Purchased gas cost											3,358,490		2,463,410		2,112,423
Gross profit	337,659		359,533	496,277	388,957	300,440		316,497	432,751	362,362	1,582,426		1,412,050		1,323,739
Operating expenses
Operation and maintenance											505,154		488,020		453,613
Depreciation and amortization											253,987		235,079		237,525
Taxes, other than income											211,936		187,072		181,073
Asset impairments											0		0		5,288
Total operating expenses											971,077		910,171		877,499
Operating income	83,944		106,605	250,080	170,720	50,383		86,396	210,178	154,922	611,349		501,879		446,240
Miscellaneous income (expense), net											(5,235)		(197)		(14,644)
Interest charges											129,295		128,385		141,174
Income (loss) before income taxes											476,819		373,297		290,422
Income tax expense											187,002		142,599		98,226
Income (loss) from continuing operations						7,536		33,474	112,340	77,348	289,817		230,698		192,196
Income from discontinued operations, net of tax						0		0	4,085	3,117	0		7,202		18,172
Gain (loss) on sale of discontinued operations, net of tax						0		5,294	0	0	0		5,294		6,349
Net income	23,713		45,721	133,367	87,016	7,536		38,768	116,425	80,465	289,817		243,194		216,717
Capital expenditures											835,251		845,033		732,858
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues	778,728		942,718	1,964,322	1,255,148	674,374		857,935	1,308,996	1,034,155	4,940,916		3,875,460		3,436,162
ASSETS
Net property, plant and equipment	6,725,906	[1]				6,030,655	[1]				6,725,906	[1]	6,030,655	[1]
Investment in subsidaries	0					0					0		0
Current assets
Cash and cash equivalents	42,258					66,199					42,258		66,199		64,239	131,419
Assets from risk management activities current	45,827					11,966					45,827		11,966
Other current assets	687,755					598,968					687,755		598,968
Intercompany receivables	0					0					0		0
Total current assets	775,840					677,133					775,840		677,133
Goodwill	742,029					741,363					742,029		741,363
Assets from risk management activities noncurrent	13,038					109,354					13,038		109,354
Deferred charges and other assets	337,891					375,763					337,891		375,763
Total Assets	8,594,704					7,934,268					8,594,704		7,934,268
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,086,232					2,580,409					3,086,232		2,580,409		2,359,243	2,255,421
Long-term debt	2,455,986					2,455,671					2,455,986		2,455,671
Total capitalization	5,542,218					5,036,080					5,542,218		5,036,080
Current liabilities
Current maturities of long-term debt	0					0					0		0
Short-term debt	196,695					367,984					196,695		367,984
Liabilities from risk management activities current	1,730					1,543					1,730		1,543
Other current liabilities	712,225					608,959					712,225		608,959
Intercompany payables	0					0					0		0
Total current liabilities	910,650					978,486					910,650		978,486
Deferred income taxes	1,286,616					1,164,053					1,286,616		1,164,053
Liabilities from risk management activities noncurrent	20,126					0					20,126		0
Regulatory cost of removal obligation	445,387					359,299					445,387		359,299
Pension and postretirement liabilities	340,963					358,787					340,963		358,787
Deferred credits and other liabilities	48,744					37,563					48,744		37,563
Total Equity and Liabilities	8,594,704					7,934,268					8,594,704		7,934,268
Number Of Customers, Regulated Distribution	3,000,000										3,000,000
Number Of Divisions, Regulated 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											4,940,916		3,875,460		3,436,162
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											4,940,916		3,875,460		3,436,162
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
Regulated Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											3,056,212		2,394,418		2,144,376
Operating revenues	409,014		517,707	1,290,960	843,865	360,386		467,144	905,176	666,787	3,061,546		2,399,493		2,145,330
Purchased gas cost											1,885,031		1,318,257		1,122,587
Gross profit											1,176,515		1,081,236		1,022,743
Operating expenses
Operation and maintenance											387,228		375,188		353,879
Depreciation and amortization											208,376		195,581		202,026
Taxes, other than income											196,343		167,374		162,377
Asset impairments															0
Total operating expenses											791,947		738,143		718,282
Operating income											384,568		343,093		304,461
Miscellaneous income (expense), net											(381)		2,535		(12,657)
Interest charges											94,918		98,296		110,642
Income (loss) before income taxes											289,269		247,332		181,162
Income tax expense											117,684		96,476		57,314
Income (loss) from continuing operations													150,856		123,848
Income from discontinued operations, net of tax													7,202		18,172
Gain (loss) on sale of discontinued operations, net of tax													5,649		6,349
Net income											171,585		163,707		148,369
Capital expenditures											584,065		528,599		546,818
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											3,056,212		2,394,418		2,144,376
Operating revenues	409,014		517,707	1,290,960	843,865	360,386		467,144	905,176	666,787	3,061,546		2,399,493		2,145,330
ASSETS
Net property, plant and equipment	5,202,761					4,719,873					5,202,761		4,719,873
Investment in subsidaries	952,171					831,136					952,171		831,136
Current assets
Cash and cash equivalents	33,303					4,237					33,303		4,237
Assets from risk management activities current	23,102					1,837					23,102		1,837
Other current assets	490,408					428,366					490,408		428,366
Intercompany receivables	790,442					783,738					790,442		783,738
Total current assets	1,337,255					1,218,178					1,337,255		1,218,178
Goodwill	574,816					574,190					574,816		574,190
Assets from risk management activities noncurrent	13,038					109,354					13,038		109,354
Deferred charges and other assets	309,965					347,687					309,965		347,687
Total Assets	8,390,006					7,800,418					8,390,006		7,800,418
CAPITALIZATION AND LIABILITIES
Shareholders' equity	3,086,232					2,580,409					3,086,232		2,580,409
Long-term debt	2,455,986					2,455,671					2,455,986		2,455,671
Total capitalization	5,542,218					5,036,080					5,542,218		5,036,080
Current liabilities
Short-term debt	522,695					645,984					522,695		645,984
Liabilities from risk management activities current	1,730					1,543					1,730		1,543
Other current liabilities	559,765					491,681					559,765		491,681
Intercompany payables	0					0					0		0
Total current liabilities	1,084,190					1,139,208					1,084,190		1,139,208
Deferred income taxes	913,260					871,360					913,260		871,360
Liabilities from risk management activities noncurrent	20,126					0					20,126		0
Regulatory cost of removal obligation	445,387					359,299					445,387		359,299
Pension and postretirement liabilities	340,963					358,787					340,963		358,787
Deferred credits and other liabilities	43,862					35,684					43,862		35,684
Total Equity and Liabilities	8,390,006					7,800,418					8,390,006		7,800,418
Regulated Distribution Segment [Member] | Gas Sales Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											2,964,456		2,316,137		2,065,424
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											2,964,456		2,316,137		2,065,424
Regulated Distribution Segment [Member] | Gas Sales Revenues [Member] | Residential Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											1,933,099		1,512,495		1,351,479
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											1,933,099		1,512,495		1,351,479
Regulated Distribution Segment [Member] | Gas Sales Revenues [Member] | Commercial Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											876,042		661,930		587,651
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											876,042		661,930		587,651
Regulated Distribution Segment [Member] | Gas Sales Revenues [Member] | Industrial Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											90,536		81,155		71,960
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											90,536		81,155		71,960
Regulated Distribution Segment [Member] | Gas Sales Revenues [Member] | Public Authority and Other Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											64,779		60,557		54,334
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											64,779		60,557		54,334
Regulated Distribution Segment [Member] | Gas Transportation Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											64,049		55,938		53,924
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											64,049		55,938		53,924
Regulated Distribution Segment [Member] | Other Gas Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											27,707		22,343		25,028
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											27,707		22,343		25,028
Regulated Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											3,056,212		2,394,418		2,144,376
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											3,056,212		2,394,418		2,144,376
Regulated Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											5,334		5,075		954
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											5,334		5,075		954
Regulated Pipeline Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											92,166		89,011		92,604
Operating revenues	86,314		87,189	73,615	71,341	72,330		74,041	61,848	60,681	318,459		268,900		247,351
Purchased gas cost											0		0		0
Gross profit											318,459		268,900		247,351
Operating expenses
Operation and maintenance											91,466		76,686		71,521
Depreciation and amortization											41,031		35,302		31,438
Taxes, other than income											13,143		17,059		15,568
Asset impairments															0
Total operating expenses											145,640		129,047		118,527
Operating income											172,819		139,853		128,824
Miscellaneous income (expense), net											(3,181)		(2,285)		(1,051)
Interest charges											36,280		30,678		29,414
Income (loss) before income taxes											133,358		106,890		98,359
Income tax expense											47,167		38,630		35,300
Income (loss) from continuing operations													68,260		63,059
Income from discontinued operations, net of tax													0		0
Gain (loss) on sale of discontinued operations, net of tax													0
Net income											86,191		68,260		63,059
Capital expenditures											249,347		313,230		175,768
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											92,166		89,011		92,604
Operating revenues	86,314		87,189	73,615	71,341	72,330		74,041	61,848	60,681	318,459		268,900		247,351
ASSETS
Net property, plant and equipment	1,464,572					1,249,767					1,464,572		1,249,767
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	14,009					11,709					14,009		11,709
Intercompany receivables	0					0					0		0
Total current assets	14,009					11,709					14,009		11,709
Goodwill	132,502					132,462					132,502		132,462
Assets from risk management activities noncurrent	0					0					0		0
Deferred charges and other assets	21,826					19,227					21,826		19,227
Total Assets	1,632,909					1,413,165					1,632,909		1,413,165
CAPITALIZATION AND LIABILITIES
Shareholders' equity	482,612					396,421					482,612		396,421
Long-term debt	0					0					0		0
Total capitalization	482,612					396,421					482,612		396,421
Current liabilities
Short-term debt	0					0					0		0
Liabilities from risk management activities current	0					0					0		0
Other current liabilities	24,790					20,288					24,790		20,288
Intercompany payables	763,635					712,768					763,635		712,768
Total current liabilities	788,425					733,056					788,425		733,056
Deferred income taxes	361,688					283,554					361,688		283,554
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	184					134					184		134
Total Equity and Liabilities	1,632,909					1,413,165					1,632,909		1,413,165
Regulated Pipeline Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											92,166		89,011		92,604
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											92,166		89,011		92,604
Regulated Pipeline Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											226,293		179,889		154,747
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											226,293		179,889		154,747
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											1,792,538		1,392,031		1,199,182
Operating revenues	396,855		465,033	757,683	447,721	337,264		421,808	428,948	399,894	2,067,292		1,587,914		1,348,982
Purchased gas cost											1,979,337		1,524,583		1,293,858
Gross profit											87,955		63,331		55,124
Operating expenses
Operation and maintenance											26,963		37,569		29,697
Depreciation and amortization											4,580		4,196		4,061
Taxes, other than income											2,450		2,639		3,128
Asset impairments															5,288
Total operating expenses											33,993		44,404		42,174
Operating income											53,962		18,927		12,950
Miscellaneous income (expense), net											2,216		2,316		1,035
Interest charges											1,986		2,168		3,084
Income (loss) before income taxes											54,192		19,075		10,901
Income tax expense											22,151		7,493		5,612
Income (loss) from continuing operations													11,582		5,289
Income from discontinued operations, net of tax													0		0
Gain (loss) on sale of discontinued operations, net of tax													(355)
Net income											32,041		11,227		5,289
Capital expenditures											1,839		3,204		10,272
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											1,792,538		1,392,031		1,199,182
Operating revenues	396,855		465,033	757,683	447,721	337,264		421,808	428,948	399,894	2,067,292		1,587,914		1,348,982
ASSETS
Net property, plant and equipment	58,573					61,015					58,573		61,015
Investment in subsidaries	(2,096)					(2,096)					(2,096)		(2,096)
Current assets
Cash and cash equivalents	8,955					61,962					8,955		61,962
Assets from risk management activities current	22,725	[2]				10,129	[3]				22,725	[2]	10,129	[3]
Other current assets	526,161					452,126					526,161		452,126
Intercompany receivables	0					0					0		0
Total current assets	557,841					524,217					557,841		524,217
Goodwill	34,711					34,711					34,711		34,711
Assets from risk management activities noncurrent	0					0					0		0
Deferred charges and other assets	6,100					8,849					6,100		8,849
Total Assets	655,129					626,696					655,129		626,696
CAPITALIZATION AND LIABILITIES
Shareholders' equity	469,559					434,715					469,559		434,715
Long-term debt	0					0					0		0
Total capitalization	469,559					434,715					469,559		434,715
Current liabilities
Short-term debt	0					0					0		0
Liabilities from risk management activities current	0	[2]				0	[3]				0	[2]	0	[3]
Other current liabilities	142,397					110,306					142,397		110,306
Intercompany payables	26,807					70,970					26,807		70,970
Total current liabilities	169,204					181,276					169,204		181,276
Deferred income taxes	11,668					8,960					11,668		8,960
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	4,698					1,745					4,698		1,745
Total Equity and Liabilities	655,129					626,696					655,129		626,696
Nonregulated Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											1,792,538		1,392,031		1,199,182
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											1,792,538		1,392,031		1,199,182
Nonregulated Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues											274,754		195,883		149,800
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											274,754		195,883		149,800
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(113,455)		(127,211)	(157,936)	(107,779)	(95,606)		(105,058)	(86,976)	(93,207)	(506,381)		(380,847)		(305,501)
Purchased gas cost											(505,878)		(379,430)		(304,022)
Gross profit											(503)		(1,417)		(1,479)
Operating expenses
Operation and maintenance											(503)		(1,423)		(1,484)
Depreciation and amortization											0		0		0
Taxes, other than income											0		0		0
Asset impairments															0
Total operating expenses											(503)		(1,423)		(1,484)
Operating income											0		6		5
Miscellaneous income (expense), net											(3,889)		(2,763)		(1,971)
Interest charges											(3,889)		(2,757)		(1,966)
Income (loss) before income taxes											0		0		0
Income tax expense											0		0		0
Income (loss) from continuing operations													0		0
Income from discontinued operations, net of tax													0		0
Gain (loss) on sale of discontinued operations, net of tax													0
Net income											0		0		0
Capital expenditures											0		0		0
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues	(113,455)		(127,211)	(157,936)	(107,779)	(95,606)		(105,058)	(86,976)	(93,207)	(506,381)		(380,847)		(305,501)
ASSETS
Net property, plant and equipment	0					0					0		0
Investment in subsidaries	(950,075)					(829,040)					(950,075)		(829,040)
Current assets
Cash and cash equivalents	0					0					0		0
Assets from risk management activities current	0					0					0		0
Other current assets	(342,823)					(293,233)					(342,823)		(293,233)
Intercompany receivables	(790,442)					(783,738)					(790,442)		(783,738)
Total current assets	(1,133,265)					(1,076,971)					(1,133,265)		(1,076,971)
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	(2,083,340)					(1,906,011)					(2,083,340)		(1,906,011)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(952,171)					(831,136)					(952,171)		(831,136)
Long-term debt	0					0					0		0
Total capitalization	(952,171)					(831,136)					(952,171)		(831,136)
Current liabilities
Short-term debt	(326,000)					(278,000)					(326,000)		(278,000)
Liabilities from risk management activities current	0					0					0		0
Other current liabilities	(14,727)					(13,316)					(14,727)		(13,316)
Intercompany payables	(790,442)					(783,738)					(790,442)		(783,738)
Total current liabilities	(1,131,169)					(1,075,054)					(1,131,169)		(1,075,054)
Deferred income taxes	0					179					0		179
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	(2,083,340)					(1,906,011)					(2,083,340)		(1,906,011)
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											(506,381)		(380,847)		(305,501)
Revenue from External Customers by Products and Services Table [Abstract]
Revenues											$ (506,381)		$ (380,847)		$ (305,501)

[1] Net property, plant and equipment includes plant acquisition adjustments of $(76.4) million and $(83.8) million at September 30, 2014 and 2013.

[2] Includes $25.8 million of cash held on deposit to collateralize certain financial instruments. Of this amount, $3.1 million was used to offset current and noncurent risk management liabilities under master netting arrangements and the remaining $22.7 million is classified as current risk management assets.

[3] Includes $24.8 million of cash held on deposit to collateralize certain financial instruments. Of this amount, $14.7 million was used to offset current and noncurrent risk management liabilities under master netting arrangements and the remaining $10.1 million is classified as current risk management assets.