Segment Information (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Sep. 30, 2010	Sep. 30, 2009
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 843,615	$ 761,305	$ 3,558,374	$ 3,941,254
Purchased gas cost	576,810	513,639	2,473,177	2,845,141
Gross profit	266,805	247,666	1,085,197	1,096,113
Operating expenses
Operation and maintenance	112,665	111,559	341,317	348,458
Depreciation and amortization	56,932	51,940	167,176	156,201
Taxes, other than income	52,142	51,908	145,868	152,840
Asset impairments	10,988	0	30,270	0
Total operating expenses	232,727	215,407	684,631	657,499
Operating income	34,078	32,259	400,566	438,614
Miscellaneous income (expense)	(1,430)	(798)	24,046	(905)
Interest charges	35,845	37,267	112,615	115,481
Income (loss) before income taxes	(3,197)	(5,806)	311,997	322,228
Income tax expense (benefit)	(1,723)	(1,577)	114,211	124,199
Income (loss) from continuing operations	(1,474)	(4,229)	197,786	198,029
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	908	1,075	7,854	6,273
Net income (loss)	(566)	(3,154)	205,640	204,302
Capital expenditures	143,620	129,720	390,283	362,349
ASSETS
Net property, plant and equipment	4,916,051		4,916,051		4,793,075
Current assets
Cash and cash equivalents	117,429	180,383	117,429	180,383	131,952	111,203
Total current assets	989,748		989,748		875,192
Total Assets	6,993,470		6,993,470		6,763,791
CAPITALIZATION AND LIABILITIES
Shareholders' equity	2,335,824		2,335,824		2,178,348
Long-term debt	2,206,106		2,206,106		1,809,551
Total capitalization	4,541,930		4,541,930		3,987,899
Current liabilities
Current maturities of long-term debt	2,434		2,434		360,131
Short-term debt	0		0		126,100
Total current liabilities	648,282		648,282		1,166,079
Deferred income taxes	967,607		967,607		829,128
Total Equity and Liabilities	6,993,470		6,993,470		6,763,791
Natural Gas Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	406,817	396,097	2,187,256	2,511,350
Intersegment revenues	214	222	651	682
Operating revenues	407,031	396,319	2,187,907	2,512,032
Purchased gas cost	206,839	204,988	1,317,775	1,657,412
Gross profit	200,192	191,331	870,132	854,620
Operating expenses
Operation and maintenance	86,804	87,323	268,299	266,847
Depreciation and amortization	49,099	45,633	145,548	137,580
Taxes, other than income	47,534	47,946	132,070	140,234
Asset impairments	0	0	0	0
Total operating expenses	183,437	180,902	545,917	544,661
Operating income	16,755	10,429	324,215	309,959
Miscellaneous income (expense)	(1,153)	(72)	18,305	1,474
Interest charges	28,042	29,019	87,344	87,877
Income (loss) before income taxes	(12,440)	(18,662)	255,176	223,556
Income tax expense (benefit)	(4,311)	(6,685)	94,323	86,552
Income (loss) from continuing operations	(8,129)	(11,977)	160,853	137,004
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	908	1,075	7,854	6,273
Net income (loss)	(7,221)	(10,902)	168,707	143,277
Capital expenditures	121,452	106,394	340,713	302,621
ASSETS
Net property, plant and equipment	4,085,081		4,085,081		3,959,112
Investment in subsidaries	671,885		671,885		620,863
Current assets
Cash and cash equivalents	39,446		39,446		31,952
Assets from risk management activities	1,972		1,972		2,219
Other current assets	565,265		565,265		528,655
Intercompany receivables	505,709		505,709		546,313
Total current assets	1,112,392		1,112,392		1,109,139
Intangible assets	0		0		0
Goodwill	572,262		572,262		572,262
Noncurrent assets from risk management activities	767		767		47
Deferred charges and other assets	319,019		319,019		324,707
Total Assets	6,761,406		6,761,406		6,586,130
CAPITALIZATION AND LIABILITIES
Shareholders' equity	2,335,824		2,335,824		2,178,348
Long-term debt	2,205,910		2,205,910		1,809,289
Total capitalization	4,541,734		4,541,734		3,987,637
Current liabilities
Current maturities of long-term debt	2,303		2,303		360,000
Short-term debt	173,845		173,845		258,488
Liabilities from risk management activities	5,207		5,207		48,942
Other current liabilities	419,848		419,848		473,076
Intercompany payables	0		0		0
Total current liabilities	601,203		601,203		1,140,506
Deferred income taxes	798,433		798,433		691,126
Noncurrent liabilities from risk management activities	56		56		2,924
Regulatory cost of removal obligation	396,201		396,201		350,521
Deferred credits and other liabilities	423,779		423,779		413,416
Total Equity and Liabilities	6,761,406		6,761,406		6,586,130
Regulated Transmission and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	19,772	22,796	62,602	64,281
Intersegment revenues	33,798	22,161	94,951	82,717
Operating revenues	53,570	44,957	157,553	146,998
Purchased gas cost	0	0	0	0
Gross profit	53,570	44,957	157,553	146,998
Operating expenses
Operation and maintenance	18,786	16,050	49,591	53,877
Depreciation and amortization	6,790	5,171	18,387	15,395
Taxes, other than income	3,729	3,010	11,395	9,226
Asset impairments	0	0	0	0
Total operating expenses	29,305	24,231	79,373	78,498
Operating income	24,265	20,726	78,180	68,500
Miscellaneous income (expense)	(312)	94	5,267	117
Interest charges	7,653	7,667	23,802	23,589
Income (loss) before income taxes	16,300	13,153	59,645	45,028
Income tax expense (benefit)	5,748	4,688	21,252	16,039
Income (loss) from continuing operations	10,552	8,465	38,393	28,989
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	0	0	0	0
Net income (loss)	10,552	8,465	38,393	28,989
Capital expenditures	20,239	22,964	44,796	56,786
ASSETS
Net property, plant and equipment	771,777		771,777		748,947
Investment in subsidaries	0		0		0
Current assets
Cash and cash equivalents	0		0		0
Assets from risk management activities	0		0		0
Other current assets	15,822		15,822		19,504
Intercompany receivables	0		0		0
Total current assets	15,822		15,822		19,504
Intangible assets	0		0		0
Goodwill	132,341		132,341		132,341
Noncurrent assets from risk management activities	0		0		0
Deferred charges and other assets	16,137		16,137		13,037
Total Assets	936,077		936,077		913,829
CAPITALIZATION AND LIABILITIES
Shareholders' equity	251,080		251,080		212,687
Long-term debt	0		0		0
Total capitalization	251,080		251,080		212,687
Current liabilities
Current maturities of long-term debt	0		0		0
Short-term debt	0		0		0
Liabilities from risk management activities	0		0		0
Other current liabilities	8,862		8,862		10,949
Intercompany payables	503,857		503,857		543,007
Total current liabilities	512,719		512,719		553,956
Deferred income taxes	163,540		163,540		142,337
Noncurrent liabilities from risk management activities	0		0		0
Regulatory cost of removal obligation	0		0		0
Deferred credits and other liabilities	8,738		8,738		4,849
Total Equity and Liabilities	936,077		936,077		913,829
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	417,026	342,412	1,308,516	1,365,623
Intersegment revenues	74,259	84,993	241,940	286,830
Operating revenues	491,285	427,405	1,550,456	1,652,453
Purchased gas cost	477,880	415,634	1,491,815	1,556,746
Gross profit	13,405	11,771	58,641	95,707
Operating expenses
Operation and maintenance	7,437	8,579	24,556	28,946
Depreciation and amortization	1,043	1,136	3,241	3,226
Taxes, other than income	879	952	2,403	3,380
Asset impairments	10,988	0	30,270	0
Total operating expenses	20,347	10,667	60,470	35,552
Operating income	(6,942)	1,104	(1,829)	60,155
Miscellaneous income (expense)	168	511	764	1,524
Interest charges	283	1,912	1,759	8,035
Income (loss) before income taxes	(7,057)	(297)	(2,824)	53,644
Income tax expense (benefit)	(3,160)	420	(1,364)	21,608
Income (loss) from continuing operations	(3,897)	(717)	(1,460)	32,036
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	0	0	0	0
Net income (loss)	(3,897)	(717)	(1,460)	32,036
Capital expenditures	1,929	362	4,774	2,942
ASSETS
Net property, plant and equipment	59,193		59,193		85,016
Investment in subsidaries	(2,096)		(2,096)		(2,096)
Current assets
Cash and cash equivalents	77,983		77,983		100,000
Assets from risk management activities	13,041		13,041		18,356
Other current assets	469,576		469,576		325,348
Intercompany receivables	0		0		0
Total current assets	560,600		560,600		443,704
Intangible assets	363		363		834
Goodwill	34,711		34,711		34,711
Noncurrent assets from risk management activities	16		16		890
Deferred charges and other assets	12,055		12,055		16,695
Total Assets	664,842		664,842		579,754
CAPITALIZATION AND LIABILITIES
Shareholders' equity	420,805		420,805		408,176
Long-term debt	196		196		262
Total capitalization	421,001		421,001		408,438
Current liabilities
Current maturities of long-term debt	131		131		131
Short-term debt	0		0		0
Liabilities from risk management activities	2,995		2,995		731
Other current liabilities	226,352		226,352		162,508
Intercompany payables	1,852		1,852		3,306
Total current liabilities	231,330		231,330		166,676
Deferred income taxes	5,634		5,634		(4,335)
Noncurrent liabilities from risk management activities	6,089		6,089		6,000
Regulatory cost of removal obligation	0		0		0
Deferred credits and other liabilities	788		788		2,975
Total Equity and Liabilities	664,842		664,842		579,754
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	0	0	0	0
Intersegment revenues	(108,271)	(107,376)	(337,542)	(370,229)
Operating revenues	(108,271)	(107,376)	(337,542)	(370,229)
Purchased gas cost	(107,909)	(106,983)	(336,413)	(369,017)
Gross profit	(362)	(393)	(1,129)	(1,212)
Operating expenses
Operation and maintenance	(362)	(393)	(1,129)	(1,212)
Depreciation and amortization	0	0	0	0
Taxes, other than income	0	0	0	0
Asset impairments	0	0	0	0
Total operating expenses	(362)	(393)	(1,129)	(1,212)
Operating income	0	0	0	0
Miscellaneous income (expense)	(133)	(1,331)	(290)	(4,020)
Interest charges	(133)	(1,331)	(290)	(4,020)
Income (loss) before income taxes	0	0	0	0
Income tax expense (benefit)	0	0	0	0
Income (loss) from continuing operations	0	0	0	0
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	0	0	0	0
Net income (loss)	0	0	0	0
Capital expenditures	0	0	0	0
ASSETS
Net property, plant and equipment	0		0		0
Investment in subsidaries	(669,789)		(669,789)		(618,767)
Current assets
Cash and cash equivalents	0		0		0
Assets from risk management activities	0		0		0
Other current assets	(193,357)		(193,357)		(150,842)
Intercompany receivables	(505,709)		(505,709)		(546,313)
Total current assets	(699,066)		(699,066)		(697,155)
Intangible assets	0		0		0
Goodwill	0		0		0
Noncurrent assets from risk management activities	0		0		0
Deferred charges and other assets	0		0		0
Total Assets	(1,368,855)		(1,368,855)		(1,315,922)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(671,885)		(671,885)		(620,863)
Long-term debt	0		0		0
Total capitalization	(671,885)		(671,885)		(620,863)
Current liabilities
Current maturities of long-term debt	0		0		0
Short-term debt	(173,845)		(173,845)		(132,388)
Liabilities from risk management activities	0		0		0
Other current liabilities	(17,416)		(17,416)		(16,358)
Intercompany payables	(505,709)		(505,709)		(546,313)
Total current liabilities	(696,970)		(696,970)		(695,059)
Deferred income taxes	0		0		0
Noncurrent liabilities from risk management activities	0		0		0
Regulatory cost of removal obligation	0		0		0
Deferred credits and other liabilities	0		0		0
Total Equity and Liabilities	$ (1,368,855)		$ (1,368,855)		$ (1,315,922)