Segment Reporting (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2009
Gas Sales Revenue [Abstract]
Residential									$ 1,351,479	$ 1,535,887	$ 1,751,186
Commercial									587,651	685,380	775,714
Industrial									71,960	96,636	101,814
Public authority and other									54,334	68,676	69,944
Total gas sales revenue									2,065,424	2,386,579	2,698,658
Transportation revenues									53,924	57,331	56,539
Other gas revenues									25,028	25,871	27,796
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers									3,438,483	4,286,435	4,661,060
Intersegment revenues									0	0	0
Operating revenues	552,566	576,414	1,225,509	1,083,994	779,667	833,168	1,556,374	1,117,226	3,438,483	4,286,435	4,661,060
Purchased gas cost									2,114,744	2,985,615	3,346,924
Gross profit	249,389	293,171	425,787	355,392	237,160	261,612	444,466	357,582	1,323,739	1,300,820	1,314,136
Operating expenses
Operation and maintenance									453,613	442,965	454,621
Depreciation and amortization									237,525	223,832	208,539
Taxes, other than income									181,073	177,767	187,143
Asset impairments									5,288	30,270	0
Total operating expenses									877,499	874,834	850,303
Operating income	22,791	81,546	202,432	139,471	39,195	31,394	204,624	150,773	446,240	425,986	463,833
Miscellaneous income (expense)									(14,644)	21,184	(591)
Interest charges									141,174	150,763	154,188
Income (loss) before income taxes									290,422	296,407	309,054
Income tax expense (benefit)									98,226	106,819	119,203
Income (loss) from continuing operations	(286)	28,014	102,084	62,384	237	(3,150)	124,293	68,208	192,196	189,588	189,851
Income (loss) from discontinued operations	1,904	3,118	7,027	6,123	1,724	2,584	7,916	5,789	18,172	18,013	15,988
Gain on sale of discountinued operations	6,349	0	0	0					6,349	0	0
Net income (loss)	7,967	31,132	109,111	68,507	1,961	(566)	132,209	73,997	216,717	207,601	205,839
Capital expenditures									732,858	622,965	542,636
ASSETS
Net property, plant and equipment	5,475,604				5,147,918				5,475,604	5,147,918
Investment in subsidaries	0				0				0	0
Current assets
Cash and cash equivalents	64,239				131,419				64,239	131,419	131,952	111,203
Assets from risk management activities current	24,707				18,344				24,707	18,344
Other current assets	739,016				861,190				739,016	861,190
Intercompany receivables	0				0				0	0
Total current assets	827,962				1,010,953				827,962	1,010,953
Intangible assets	164				207				164	207
Goodwill	740,683				740,000				740,683	740,000
Assets from risk management activities noncurrent	2,283				998				2,283	998
Deferred charges and other assets	448,979				382,795				448,979	382,795
Total Assets	7,495,675				7,282,871				7,495,675	7,282,871
CAPITALIZATION AND LIABILITIES
Shareholders' equity	2,359,243				2,255,421				2,359,243	2,255,421	2,178,348	2,176,761
Long-term debt	1,956,305				2,206,117				1,956,305	2,206,117
Total capitalization	4,315,548				4,461,538				4,315,548	4,461,538
Current liabilities
Current maturities of long-term debt	131				2,434				131	2,434
Short-term debt	570,929				206,396				570,929	206,396
Liabilitied from risk management activities current	85,381				15,453				85,381	15,453
Other current liabilities	619,513				643,315				619,513	643,315
Intercompany payables	0				0				0	0
Total current liabilities	1,275,954				867,598				1,275,954	867,598
Deferred income taxes	1,015,083				960,093				1,015,083	960,093
Liabilities from risk management activities noncurrent	9,206				78,089				9,206	78,089
Deferred credits and other liabilities	498,720				486,606				498,720	486,606
Total Equity and Liabilities	7,495,675				7,282,871				7,495,675	7,282,871
Natural Gas Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers									2,144,376	2,469,781	2,782,993
Intersegment revenues									954	883	870
Operating revenues	282,516	315,634	871,067	676,113	334,363	396,584	1,052,291	687,426	2,145,330	2,470,664	2,783,863
Purchased gas cost									1,122,587	1,452,721	1,785,221
Gross profit									1,022,743	1,017,943	998,642
Operating expenses
Operation and maintenance									353,879	341,758	349,465
Depreciation and amortization									202,026	193,642	182,097
Taxes, other than income									162,377	160,455	170,229
Asset impairments									0	0
Total operating expenses									718,282	695,855	701,791
Operating income									304,461	322,088	296,851
Miscellaneous income (expense)									(12,657)	16,242	1,132
Interest charges									110,642	115,740	118,147
Income (loss) before income taxes									181,162	222,590	179,836
Income tax expense (benefit)									57,314	77,885	69,875
Income (loss) from continuing operations									123,848	144,705	109,961
Income (loss) from discontinued operations									18,172	18,013	15,988
Gain on sale of discountinued operations									6,349
Net income (loss)									148,369	162,718	125,949
Capital expenditures									546,818	496,899	437,815
ASSETS
Net property, plant and equipment	4,432,017				4,248,198				4,432,017	4,248,198
Investment in subsidaries	747,496				670,993				747,496	670,993
Current assets
Cash and cash equivalents	12,787				24,646				12,787	24,646
Assets from risk management activities current	6,934				843				6,934	843
Other current assets	546,187				655,716				546,187	655,716
Intercompany receivables	636,557				569,898				636,557	569,898
Total current assets	1,202,465				1,251,103				1,202,465	1,251,103
Intangible assets	0				0				0	0
Goodwill	573,550				572,908				573,550	572,908
Assets from risk management activities noncurrent	2,283				998				2,283	998
Deferred charges and other assets	417,893				353,960				417,893	353,960
Total Assets	7,375,704				7,098,160				7,375,704	7,098,160
CAPITALIZATION AND LIABILITIES
Shareholders' equity	2,359,243				2,255,421				2,359,243	2,255,421
Long-term debt	1,956,305				2,205,986				1,956,305	2,205,986
Total capitalization	4,315,548				4,461,407				4,315,548	4,461,407
Current liabilities
Current maturities of long-term debt	0				2,303				0	2,303
Short-term debt	782,719				387,691				782,719	387,691
Liabilitied from risk management activities current	85,366				11,916				85,366	11,916
Other current liabilities	526,089				474,783				526,089	474,783
Intercompany payables	0				0				0	0
Total current liabilities	1,394,174				876,693				1,394,174	876,693
Deferred income taxes	789,288				789,649				789,288	789,649
Liabilities from risk management activities noncurrent	0				67,862				0	67,862
Regulatory cost of removal obligation	381,164				428,947				381,164	428,947
Deferred credits and other liabilities	495,530				473,602				495,530	473,602
Total Equity and Liabilities	7,375,704				7,098,160				7,375,704	7,098,160
Regulated Transmission and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers									92,604	87,141	97,023
Intersegment revenues									154,747	132,232	105,990
Operating revenues	65,482	67,073	58,037	56,759	61,820	53,570	54,976	49,007	247,351	219,373	203,013
Purchased gas cost									0	0	0
Gross profit									247,351	219,373	203,013
Operating expenses
Operation and maintenance									71,521	70,401	72,249
Depreciation and amortization									31,438	25,997	21,368
Taxes, other than income									15,568	14,700	12,358
Asset impairments									0	0
Total operating expenses									118,527	111,098	105,975
Operating income									128,824	108,275	97,038
Miscellaneous income (expense)									(1,051)	4,715	135
Interest charges									29,414	31,432	31,174
Income (loss) before income taxes									98,359	81,558	65,999
Income tax expense (benefit)									35,300	29,143	24,513
Income (loss) from continuing operations									63,059	52,415	41,486
Income (loss) from discontinued operations									0	0	0
Gain on sale of discountinued operations									0
Net income (loss)									63,059	52,415	41,486
Capital expenditures									175,768	118,452	95,835
ASSETS
Net property, plant and equipment	979,443				838,302				979,443	838,302
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	11,788				15,413				11,788	15,413
Intercompany receivables	0				0				0	0
Total current assets	11,788				15,413				11,788	15,413
Intangible assets	0				0				0	0
Goodwill	132,422				132,381				132,422	132,381
Assets from risk management activities noncurrent	0				0				0	0
Deferred charges and other assets	24,353				18,028				24,353	18,028
Total Assets	1,148,006				1,004,124				1,148,006	1,004,124
CAPITALIZATION AND LIABILITIES
Shareholders' equity	328,161				265,102				328,161	265,102
Long-term debt	0				0				0	0
Total capitalization	328,161				265,102				328,161	265,102
Current liabilities
Current maturities of long-term debt	0				0				0	0
Short-term debt	0				0				0	0
Liabilitied from risk management activities current	0				0				0	0
Other current liabilities	12,478				10,369				12,478	10,369
Intercompany payables	584,578				543,084				584,578	543,084
Total current liabilities	597,056				553,453				597,056	553,453
Deferred income taxes	220,647				173,351				220,647	173,351
Liabilities from risk management activities noncurrent	0				0				0	0
Regulatory cost of removal obligation	0				0				0	0
Deferred credits and other liabilities	2,142				12,218				2,142	12,218
Total Equity and Liabilities	1,148,006				1,004,124				1,148,006	1,004,124
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers									1,201,503	1,729,513	1,781,044
Intersegment revenues									149,800	295,380	365,614
Operating revenues	280,114	256,250	370,763	444,176	474,437	491,285	583,531	475,640	1,351,303	2,024,893	2,146,658
Purchased gas cost									1,296,179	1,959,893	2,032,567
Gross profit									55,124	65,000	114,091
Operating expenses
Operation and maintenance									29,697	32,308	34,517
Depreciation and amortization									4,061	4,193	5,074
Taxes, other than income									3,128	2,612	4,556
Asset impairments									5,288	30,270
Total operating expenses									42,174	69,383	44,147
Operating income									12,950	(4,383)	69,944
Miscellaneous income (expense)									1,035	657	3,859
Interest charges									3,084	4,015	10,584
Income (loss) before income taxes									10,901	(7,741)	63,219
Income tax expense (benefit)									5,612	(209)	24,815
Income (loss) from continuing operations									5,289	(7,532)	38,404
Income (loss) from discontinued operations									0	0	0
Gain on sale of discountinued operations									0
Net income (loss)									5,289	(7,532)	38,404
Capital expenditures									10,272	7,614	8,986
ASSETS
Net property, plant and equipment	64,144				61,418				64,144	61,418
Investment in subsidaries	(2,096)				(2,096)				(2,096)	(2,096)
Current assets
Cash and cash equivalents	51,452				106,773				51,452	106,773
Assets from risk management activities current	17,773				17,501				17,773	17,501
Other current assets	404,097				386,215				404,097	386,215
Intercompany receivables	0				0				0	0
Total current assets	473,322				510,489				473,322	510,489
Intangible assets	164				207				164	207
Goodwill	34,711				34,711				34,711	34,711
Assets from risk management activities noncurrent	0				0				0	0
Deferred charges and other assets	6,733				10,807				6,733	10,807
Total Assets	576,978				615,536				576,978	615,536
CAPITALIZATION AND LIABILITIES
Shareholders' equity	419,335				405,891				419,335	405,891
Long-term debt	0				131				0	131
Total capitalization	419,335				406,022				419,335	406,022
Current liabilities
Current maturities of long-term debt	131				131				131	131
Short-term debt	0				0				0	0
Liabilitied from risk management activities current	15				3,537				15	3,537
Other current liabilities	90,116				170,926				90,116	170,926
Intercompany payables	51,979				26,814				51,979	26,814
Total current liabilities	142,241				201,408				142,241	201,408
Deferred income taxes	5,148				(2,907)				5,148	(2,907)
Liabilities from risk management activities noncurrent	9,206				10,227				9,206	10,227
Regulatory cost of removal obligation	0				0				0	0
Deferred credits and other liabilities	1,048				786				1,048	786
Total Equity and Liabilities	576,978				615,536				576,978	615,536
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers									0	0	0
Intersegment revenues									(305,501)	(428,495)	(472,474)
Operating revenues	(75,546)	(62,543)	(74,358)	(93,054)	(90,953)	(108,271)	(134,424)	(94,847)	(305,501)	(428,495)	(472,474)
Purchased gas cost									(304,022)	(426,999)	(470,864)
Gross profit									(1,479)	(1,496)	(1,610)
Operating expenses
Operation and maintenance									(1,484)	(1,502)	(1,610)
Depreciation and amortization									0	0	0
Taxes, other than income									0	0	0
Asset impairments									0	0
Total operating expenses									(1,484)	(1,502)	(1,610)
Operating income									5	6	0
Miscellaneous income (expense)									(1,971)	(430)	(5,717)
Interest charges									(1,966)	(424)	(5,717)
Income (loss) before income taxes									0	0	0
Income tax expense (benefit)									0	0	0
Income (loss) from continuing operations									0	0	0
Income (loss) from discontinued operations									0	0	0
Gain on sale of discountinued operations									0
Net income (loss)									0	0	0
Capital expenditures									0	0	0
ASSETS
Net property, plant and equipment	0				0				0	0
Investment in subsidaries	(745,400)				(668,897)				(745,400)	(668,897)
Current assets
Cash and cash equivalents	0				0				0	0
Assets from risk management activities current	0				0				0	0
Other current assets	(223,056)				(196,154)				(223,056)	(196,154)
Intercompany receivables	(636,557)				(569,898)				(636,557)	(569,898)
Total current assets	(859,613)				(766,052)				(859,613)	(766,052)
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	(1,605,013)				(1,434,949)				(1,605,013)	(1,434,949)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(747,496)				(670,993)				(747,496)	(670,993)
Long-term debt	0				0				0	0
Total capitalization	(747,496)				(670,993)				(747,496)	(670,993)
Current liabilities
Current maturities of long-term debt	0				0				0	0
Short-term debt	(211,790)				(181,295)				(211,790)	(181,295)
Liabilitied from risk management activities current	0				0				0	0
Other current liabilities	(9,170)				(12,763)				(9,170)	(12,763)
Intercompany payables	(636,557)				(569,898)				(636,557)	(569,898)
Total current liabilities	(857,517)				(763,956)				(857,517)	(763,956)
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
Deferred credits and other liabilities	0				0				0	0
Total Equity and Liabilities	$ (1,605,013)				$ (1,434,949)				$ (1,605,013)	$ (1,434,949)