Segment Information (Details) $ in Thousands, customers in Millions		3 Months Ended										12 Months Ended
		Sep. 30, 2015 USD ($) customers regulated_gas_distributions_divisions state		Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)		Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2015 USD ($) customers regulated_gas_distributions_divisions state segment		Sep. 30, 2014 USD ($)		Sep. 30, 2013 USD ($)	Sep. 30, 2012 USD ($)
Segment Reporting Information Profit Loss [Abstract]
Operating revenues		$ 656,902		$ 686,401	$ 1,540,068	$ 1,258,765	$ 789,034		$ 943,170	$ 1,964,854	$ 1,243,858	$ 4,142,136		$ 4,940,916		$ 3,875,460
Purchased gas cost												2,462,119		3,358,490		2,463,410
Gross profit		354,321		381,673	520,738	423,285	337,659		359,533	496,277	388,957	1,680,017		1,582,426		1,412,050
Operating expenses
Operation and maintenance												541,868		505,154		488,020
Depreciation and amortization												274,796		253,987		235,079
Taxes, other than income												231,958		211,936		187,072
Total operating expenses												1,048,622		971,077		910,171
Operating income		75,853		117,607	250,210	187,725	83,944		106,605	250,080	170,720	631,395		611,349		501,879
Miscellaneous income (expense), net												(4,389)		(5,235)		(197)
Interest charges												116,241		129,295		128,385
Income (loss) before income taxes												510,765		476,819		373,297
Income tax expense												195,690		187,002		142,599
Income (loss) from continuing operations												315,075		289,817		230,698
Income from discontinued operations, net of tax												0		0		7,202
Gain (loss) on sale of discontinued operations, net of tax												0		0		5,294
Net income		23,515		56,281	137,684	97,595	23,713		45,721	133,367	87,016	315,075		289,817		243,194
Capital expenditures												975,132		835,251		845,033
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues		656,902		686,401	1,540,068	1,258,765	789,034		943,170	1,964,854	1,243,858	4,142,136		4,940,916		3,875,460
ASSETS
Net property, plant and equipment	[1]	7,430,580					6,725,906					7,430,580		6,725,906
Investment in subsidaries		0					0					0		0
Current assets
Cash and cash equivalents		28,653					42,258					28,653		42,258		66,199	$ 64,239
Assets from risk management activities current		9,232					45,827					9,232		45,827
Other current assets		593,100					687,755					593,100		687,755
Intercompany receivables		0					0					0		0
Total current assets		630,985					775,840					630,985		775,840
Goodwill		742,702					742,029					742,702		742,029
Assets from risk management activities noncurrent		368					13,038					368		13,038
Deferred charges and other assets		288,310					337,891					288,310		337,891
Total assets		9,092,945					8,594,704					9,092,945		8,594,704
CAPITALIZATION AND LIABILITIES
Shareholders' equity		3,194,797					3,086,232					3,194,797		3,086,232		2,580,409	$ 2,359,243
Long-term debt		2,455,388					2,455,986					2,455,388		2,455,986
Total capitalization		5,650,185					5,542,218					5,650,185		5,542,218
Current liabilities
Short-term debt		457,927					196,695					457,927		196,695
Liabilities from risk management activities current		9,568					1,730					9,568		1,730
Other current liabilities		687,328					712,225					687,328		712,225
Intercompany payables		0					0					0		0
Total current liabilities		1,154,823					910,650					1,154,823		910,650
Deferred income taxes		1,411,315					1,286,616					1,411,315		1,286,616
Liabilities from risk management activities noncurrent		110,539					20,126					110,539		20,126
Regulatory cost of removal obligation		427,553					445,387					427,553		445,387
Pension and postretirement liabilities		287,373					340,963					287,373		340,963
Deferred credits and other liabilities		51,157					48,744					51,157		48,744
Total shareholders' equity and liabilities		$ 9,092,945					8,594,704					$ 9,092,945		8,594,704
Number Of Customers, Regulated Distribution | customers		3										3
Number Of Divisions, Regulated Distribution | regulated_gas_distributions_divisions		6										6
Number of States in which Entity Operates | state		8										8
Number of Operating Segments | segment												3
Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												$ 4,142,136		4,940,916		3,875,460
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												4,142,136		4,940,916		3,875,460
Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												0		0		0
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												0		0		0
Regulated Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												2,757,585		3,056,212		2,394,418
Operating revenues		$ 369,656		416,794	1,130,613	846,772	409,014		517,707	1,290,960	843,865	2,763,835		3,061,546		2,399,493
Purchased gas cost												1,526,258		1,885,031		1,318,257
Gross profit												1,237,577		1,176,515		1,081,236
Operating expenses
Operation and maintenance												388,486		387,228		375,188
Depreciation and amortization												223,048		208,376		195,581
Taxes, other than income												205,894		196,343		167,374
Total operating expenses												817,428		791,947		738,143
Operating income												420,149		384,568		343,093
Miscellaneous income (expense), net												(377)		(381)		2,535
Interest charges												84,132		94,918		98,296
Income (loss) before income taxes												335,640		289,269		247,332
Income tax expense												130,827		117,684		96,476
Income (loss) from continuing operations																150,856
Income from discontinued operations, net of tax																7,202
Gain (loss) on sale of discontinued operations, net of tax																5,649
Net income												204,813		171,585		163,707
Capital expenditures												680,961		584,065		528,599
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												2,757,585		3,056,212		2,394,418
Operating revenues		369,656		416,794	1,130,613	846,772	409,014		517,707	1,290,960	843,865	2,763,835		3,061,546		2,399,493
ASSETS
Net property, plant and equipment		5,670,306					5,202,761					5,670,306		5,202,761
Investment in subsidaries		1,038,670					952,171					1,038,670		952,171
Current assets
Cash and cash equivalents		23,863					33,303					23,863		33,303
Assets from risk management activities current		378					23,102					378		23,102
Other current assets		426,270					490,408					426,270		490,408
Intercompany receivables		887,713					790,442					887,713		790,442
Total current assets		1,338,224					1,337,255					1,338,224		1,337,255
Goodwill		575,449					574,816					575,449		574,816
Assets from risk management activities noncurrent		368					13,038					368		13,038
Deferred charges and other assets		265,693					309,965					265,693		309,965
Total assets		8,888,710					8,390,006					8,888,710		8,390,006
CAPITALIZATION AND LIABILITIES
Shareholders' equity		3,194,797					3,086,232					3,194,797		3,086,232
Long-term debt		2,455,388					2,455,986					2,455,388		2,455,986
Total capitalization		5,650,185					5,542,218					5,650,185		5,542,218
Current liabilities
Short-term debt		782,927					522,695					782,927		522,695
Liabilities from risk management activities current		9,568					1,730					9,568		1,730
Other current liabilities		569,273					559,765					569,273		559,765
Intercompany payables		0					0					0		0
Total current liabilities		1,361,768					1,084,190					1,361,768		1,084,190
Deferred income taxes		1,008,091					913,260					1,008,091		913,260
Liabilities from risk management activities noncurrent		110,539					20,126					110,539		20,126
Regulatory cost of removal obligation		427,553					445,387					427,553		445,387
Pension and postretirement liabilities		287,373					340,963					287,373		340,963
Deferred credits and other liabilities		43,201					43,862					43,201		43,862
Total shareholders' equity and liabilities		8,888,710					8,390,006					8,888,710		8,390,006
Regulated Distribution Segment [Member] | Gas Sales Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												2,662,258		2,964,456		2,316,137
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												2,662,258		2,964,456		2,316,137
Regulated Distribution Segment [Member] | Gas Sales Revenues [Member] | Residential Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												1,761,689		1,933,099		1,512,495
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												1,761,689		1,933,099		1,512,495
Regulated Distribution Segment [Member] | Gas Sales Revenues [Member] | Commercial Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												772,187		876,042		661,930
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												772,187		876,042		661,930
Regulated Distribution Segment [Member] | Gas Sales Revenues [Member] | Industrial Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												74,981		90,536		81,155
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												74,981		90,536		81,155
Regulated Distribution Segment [Member] | Gas Sales Revenues [Member] | Public Authority and Other Customers [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												53,401		64,779		60,557
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												53,401		64,779		60,557
Regulated Distribution Segment [Member] | Gas Transportation Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												67,475		64,049		55,938
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												67,475		64,049		55,938
Regulated Distribution Segment [Member] | Other Gas Revenues [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												27,852		27,707		22,343
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												27,852		27,707		22,343
Regulated Distribution Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												2,757,585		3,056,212		2,394,418
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												2,757,585		3,056,212		2,394,418
Regulated Distribution Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												6,250		5,334		5,075
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												6,250		5,334		5,075
Regulated Pipeline Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												97,662		92,166		89,011
Operating revenues		97,807		97,008	91,730	83,567	86,314		87,189	73,615	71,341	370,112		318,459		268,900
Purchased gas cost												0		0		0
Gross profit												370,112		318,459		268,900
Operating expenses
Operation and maintenance												118,866		91,466		76,686
Depreciation and amortization												47,236		41,031		35,302
Taxes, other than income												22,743		13,143		17,059
Total operating expenses												188,845		145,640		129,047
Operating income												181,267		172,819		139,853
Miscellaneous income (expense), net												(1,243)		(3,181)		(2,285)
Interest charges												33,151		36,280		30,678
Income (loss) before income taxes												146,873		133,358		106,890
Income tax expense												52,211		47,167		38,630
Income (loss) from continuing operations																68,260
Income from discontinued operations, net of tax																0
Gain (loss) on sale of discontinued operations, net of tax																0
Net income												94,662		86,191		68,260
Capital expenditures												292,728		249,347		313,230
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												97,662		92,166		89,011
Operating revenues		97,807		97,008	91,730	83,567	86,314		87,189	73,615	71,341	370,112		318,459		268,900
ASSETS
Net property, plant and equipment		1,706,449					1,464,572					1,706,449		1,464,572
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		24,628					14,009					24,628		14,009
Intercompany receivables		0					0					0		0
Total current assets		24,628					14,009					24,628		14,009
Goodwill		132,542					132,502					132,542		132,502
Assets from risk management activities noncurrent		0					0					0		0
Deferred charges and other assets		17,288					21,826					17,288		21,826
Total assets		1,880,907					1,632,909					1,880,907		1,632,909
CAPITALIZATION AND LIABILITIES
Shareholders' equity		577,275					482,612					577,275		482,612
Long-term debt		0					0					0		0
Total capitalization		577,275					482,612					577,275		482,612
Current liabilities
Short-term debt		0					0					0		0
Liabilities from risk management activities current		0					0					0		0
Other current liabilities		29,780					24,790					29,780		24,790
Intercompany payables		867,409					763,635					867,409		763,635
Total current liabilities		897,189					788,425					897,189		788,425
Deferred income taxes		406,254					361,688					406,254		361,688
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		189					184					189		184
Total shareholders' equity and liabilities		1,880,907					1,632,909					1,880,907		1,632,909
Regulated Pipeline Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												97,662		92,166		89,011
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												97,662		92,166		89,011
Regulated Pipeline Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												272,450		226,293		179,889
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												272,450		226,293		179,889
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												1,286,889		1,792,538		1,392,031
Operating revenues		292,830		278,769	438,322	462,288	407,161		465,485	758,215	436,431	1,472,209		2,067,292		1,587,914
Purchased gas cost												1,399,349		1,979,337		1,524,583
Gross profit												72,860		87,955		63,331
Operating expenses
Operation and maintenance												35,048		26,963		37,569
Depreciation and amortization												4,512		4,580		4,196
Taxes, other than income												3,321		2,450		2,639
Total operating expenses												42,881		33,993		44,404
Operating income												29,979		53,962		18,927
Miscellaneous income (expense), net												(760)		2,216		2,316
Interest charges												967		1,986		2,168
Income (loss) before income taxes												28,252		54,192		19,075
Income tax expense												12,652		22,151		7,493
Income (loss) from continuing operations																11,582
Income from discontinued operations, net of tax																0
Gain (loss) on sale of discontinued operations, net of tax																(355)
Net income												15,600		32,041		11,227
Capital expenditures												1,443		1,839		3,204
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												1,286,889		1,792,538		1,392,031
Operating revenues		292,830		278,769	438,322	462,288	407,161		465,485	758,215	436,431	1,472,209		2,067,292		1,587,914
ASSETS
Net property, plant and equipment		53,825					58,573					53,825		58,573
Investment in subsidaries		(2,096)					(2,096)					(2,096)		(2,096)
Current assets
Cash and cash equivalents		4,790					8,955					4,790		8,955
Assets from risk management activities current		8,854	[2]				22,725	[3]				8,854	[2]	22,725	[3]
Other current assets		480,503					526,161					480,503		526,161
Intercompany receivables		0					0					0		0
Total current assets		494,147					557,841					494,147		557,841
Goodwill		34,711					34,711					34,711		34,711
Assets from risk management activities noncurrent		0					0					0		0
Deferred charges and other assets		5,329					6,100					5,329		6,100
Total assets		585,916					655,129					585,916		655,129
CAPITALIZATION AND LIABILITIES
Shareholders' equity		461,395					469,559					461,395		469,559
Long-term debt		0					0					0		0
Total capitalization		461,395					469,559					461,395		469,559
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		99,480					142,397					99,480		142,397
Intercompany payables		20,304					26,807					20,304		26,807
Total current liabilities		119,784					169,204					119,784		169,204
Deferred income taxes		(3,030)					11,668					(3,030)		11,668
Liabilities from risk management activities noncurrent		0	[2]				0	[3]				0	[2]	0	[3]
Regulatory cost of removal obligation		0					0					0		0
Pension and postretirement liabilities		0					0					0		0
Deferred credits and other liabilities		7,767					4,698					7,767		4,698
Total shareholders' equity and liabilities		585,916					655,129					585,916		655,129
Nonregulated Segment [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												1,286,889		1,792,538		1,392,031
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												1,286,889		1,792,538		1,392,031
Nonregulated Segment [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												185,320		274,754		195,883
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												185,320		274,754		195,883
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues		(103,391)		(106,170)	(120,597)	(133,862)	(113,455)		(127,211)	(157,936)	(107,779)	(464,020)		(506,381)		(380,847)
Purchased gas cost												(463,488)		(505,878)		(379,430)
Gross profit												(532)		(503)		(1,417)
Operating expenses
Operation and maintenance												(532)		(503)		(1,423)
Depreciation and amortization												0		0		0
Taxes, other than income												0		0		0
Total operating expenses												(532)		(503)		(1,423)
Operating income												0		0		6
Miscellaneous income (expense), net												(2,009)		(3,889)		(2,763)
Interest charges												(2,009)		(3,889)		(2,757)
Income (loss) before income taxes												0		0		0
Income tax expense												0		0		0
Income (loss) from continuing operations																0
Income from discontinued operations, net of tax																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		(103,391)		$ (106,170)	$ (120,597)	$ (133,862)	(113,455)		$ (127,211)	$ (157,936)	$ (107,779)	(464,020)		(506,381)		(380,847)
ASSETS
Net property, plant and equipment		0					0					0		0
Investment in subsidaries		(1,036,574)					(950,075)					(1,036,574)		(950,075)
Current assets
Cash and cash equivalents		0					0					0		0
Assets from risk management activities current		0					0					0		0
Other current assets		(338,301)					(342,823)					(338,301)		(342,823)
Intercompany receivables		(887,713)					(790,442)					(887,713)		(790,442)
Total current assets		(1,226,014)					(1,133,265)					(1,226,014)		(1,133,265)
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		(2,262,588)					(2,083,340)					(2,262,588)		(2,083,340)
CAPITALIZATION AND LIABILITIES
Shareholders' equity		(1,038,670)					(952,171)					(1,038,670)		(952,171)
Long-term debt		0					0					0		0
Total capitalization		(1,038,670)					(952,171)					(1,038,670)		(952,171)
Current liabilities
Short-term debt		(325,000)					(326,000)					(325,000)		(326,000)
Liabilities from risk management activities current		0					0					0		0
Other current liabilities		(11,205)					(14,727)					(11,205)		(14,727)
Intercompany payables		(887,713)					(790,442)					(887,713)		(790,442)
Total current liabilities		(1,223,918)					(1,131,169)					(1,223,918)		(1,131,169)
Deferred income taxes		0					0					0		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 shareholders' equity and liabilities		$ (2,262,588)					$ (2,083,340)					(2,262,588)		(2,083,340)
Intersegment Elimination [Member] | Reportable Subsegments [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												0		0		0
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												0		0		0
Intersegment Elimination [Member] | Intersubsegment Eliminations [Member]
Segment Reporting Information Profit Loss [Abstract]
Operating revenues from external parties												(464,020)		(506,381)		(380,847)
Revenue from External Customers by Products and Services Table [Abstract]
Operating revenues from external parties												$ (464,020)		$ (506,381)		$ (380,847)

[1] Net property, plant and equipment includes plant acquisition adjustments of $(68.1) million and $(76.4) million at September 30, 2015 and 2014.

[2] Includes $43.5 million of cash held on deposit to collateralize certain financial instruments. Of this amount, $34.6 million was used to offset current and noncurent risk management liabilities under master netting arrangements and the remaining $8.9 million is classified as current risk management assets.

[3] 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 noncurrent risk management liabilities under master netting arrangements and the remaining $22.7 million is classified as current risk management assets.