Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	$ 1,308,996	$ 1,225,509	$ 2,343,151	$ 2,309,503
Intersegment revenues	0	0	0	0
Total operating revenues	1,308,996	1,225,509	2,343,151	2,309,503
Purchased gas cost	876,245	799,722	1,548,038	1,528,324
Gross profit	432,751	425,787	795,113	781,179
Operating expenses
Operation and maintenance	111,086	109,300	217,613	223,944
Depreciation and amortization	57,180	59,420	116,759	117,786
Taxes, other than income	54,307	54,635	95,641	97,546
Total operating expenses	222,573	223,355	430,013	439,276
Operating income	210,178	202,432	365,100	341,903
Miscellaneous income (expense)	1,712	506	2,410	(1,510)
Interest charges	33,331	36,643	63,853	72,369
Income (loss) before income taxes	178,559	166,295	303,657	268,024
Income tax expense (benefit)	66,219	64,211	113,969	103,556
Income (loss) from continuing operations	112,340	102,084	189,688	164,468
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	4,085	7,027	7,202	13,150
Net income (loss)	116,425	109,111	196,890	177,618
Capital expenditures	199,090	156,729	389,117	311,123
ASSETS
Net property, plant and equipment	5,710,273		5,710,273		5,475,604
Investment in subsidaries	0		0		0
Current assets
Cash and cash equivalents	65,547	47,040	65,547	47,040	64,239	131,419
Assets from risk management activities	7,080		7,080		24,707
Other current assets	929,793		929,793		739,016
Intercompany receivables	0		0		0
Total current assets	1,002,420		1,002,420		827,962
Intangible assets	142		142		164
Goodwill	740,683		740,683		740,683
Noncurrent assets from risk management activities	40,830		40,830		2,283
Deferred charges and other assets	459,382		459,382		448,979
Total Assets	7,953,730		7,953,730		7,495,675
CAPITALIZATION AND LIABILITIES
Shareholders' equity	2,543,470		2,543,470		2,359,243
Long-term debt	2,455,514		2,455,514		1,956,305
Total capitalization	4,998,984		4,998,984		4,315,548
Current liabilities
Current maturities of long-term debt	0		0		131
Short-term debt	232,998		232,998		570,929
Liabilities from risk management activities	59		59		85,381
Other current liabilities	693,709		693,709		619,513
Intercompany payables	0		0		0
Total current liabilities	926,766		926,766		1,275,954
Deferred income taxes	1,168,140		1,168,140		1,015,083
Noncurrent liabilities from risk management activities	0		0		9,206
Regulatory cost of removal obligation	366,854		366,854		381,164
Deferred credits and other liabilities	492,986		492,986		498,720
Total Equity and Liabilities	7,953,730		7,953,730		7,495,675
Number Of Operating Segments			3
Natural Gas Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	904,181	870,744	1,569,730	1,546,633
Intersegment revenues	995	323	2,233	547
Total operating revenues	905,176	871,067	1,571,963	1,547,180
Purchased gas cost	558,170	498,739	945,326	891,257
Gross profit	347,006	372,328	626,637	655,923
Operating expenses
Operation and maintenance	89,344	88,035	173,080	180,031
Depreciation and amortization	47,631	50,903	97,691	100,885
Taxes, other than income	49,592	50,029	86,343	88,221
Total operating expenses	186,567	188,967	357,114	369,137
Operating income	160,439	183,361	269,523	286,786
Miscellaneous income (expense)	2,591	623	2,460	(1,274)
Interest charges	25,664	28,816	49,227	56,955
Income (loss) before income taxes	137,366	155,168	222,756	228,557
Income tax expense (benefit)	51,176	60,693	83,473	89,581
Income (loss) from continuing operations	86,190	94,475	139,283	138,976
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	4,085	7,027	7,202	13,150
Net income (loss)	90,275	101,502	146,485	152,126
Capital expenditures	131,465	114,402	277,336	243,135
ASSETS
Net property, plant and equipment	4,585,510		4,585,510		4,432,017
Investment in subsidaries	806,009		806,009		747,496
Current assets
Cash and cash equivalents	27,463		27,463		12,787
Assets from risk management activities	3,376		3,376		6,934
Other current assets	659,313		659,313		546,187
Intercompany receivables	666,407		666,407		636,557
Total current assets	1,356,559		1,356,559		1,202,465
Intangible assets	0		0		0
Goodwill	573,550		573,550		573,550
Noncurrent assets from risk management activities	36,582		36,582		2,283
Deferred charges and other assets	432,774		432,774		417,893
Total Assets	7,790,984		7,790,984		7,375,704
CAPITALIZATION AND LIABILITIES
Shareholders' equity	2,543,470		2,543,470		2,359,243
Long-term debt	2,455,514		2,455,514		1,956,305
Total capitalization	4,998,984		4,998,984		4,315,548
Current liabilities
Current maturities of long-term debt	0		0		0
Short-term debt	491,498		491,498		782,719
Liabilities from risk management activities	59		59		85,366
Other current liabilities	546,766		546,766		526,089
Intercompany payables	0		0		0
Total current liabilities	1,038,323		1,038,323		1,394,174
Deferred income taxes	896,660		896,660		789,288
Noncurrent liabilities from risk management activities	0		0		0
Regulatory cost of removal obligation	366,854		366,854		381,164
Deferred credits and other liabilities	490,163		490,163		495,530
Total Equity and Liabilities	7,790,984		7,790,984		7,375,704
Regulated Transmission and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	19,655	20,430	38,354	39,870
Intersegment revenues	42,193	37,607	84,175	74,926
Total operating revenues	61,848	58,037	122,529	114,796
Purchased gas cost	0	0	0	0
Gross profit	61,848	58,037	122,529	114,796
Operating expenses
Operation and maintenance	15,390	15,847	31,710	32,812
Depreciation and amortization	8,690	7,792	17,080	15,443
Taxes, other than income	4,277	3,915	8,226	7,699
Total operating expenses	28,357	27,554	57,016	55,954
Operating income	33,491	30,483	65,513	58,842
Miscellaneous income (expense)	(99)	(56)	(226)	(336)
Interest charges	7,857	7,614	14,728	14,823
Income (loss) before income taxes	25,535	22,813	50,559	43,683
Income tax expense (benefit)	9,005	8,193	17,924	15,649
Income (loss) from continuing operations	16,530	14,620	32,635	28,034
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	0	0	0	0
Net income (loss)	16,530	14,620	32,635	28,034
Capital expenditures	67,208	38,871	111,039	62,991
ASSETS
Net property, plant and equipment	1,064,816		1,064,816		979,443
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	28,205		28,205		11,788
Intercompany receivables	0		0		0
Total current assets	28,205		28,205		11,788
Intangible assets	0		0		0
Goodwill	132,422		132,422		132,422
Noncurrent assets from risk management activities	0		0		0
Deferred charges and other assets	17,766		17,766		24,353
Total Assets	1,243,209		1,243,209		1,148,006
CAPITALIZATION AND LIABILITIES
Shareholders' equity	360,796		360,796		328,161
Long-term debt	0		0		0
Total capitalization	360,796		360,796		328,161
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	7,677		7,677		12,478
Intercompany payables	612,744		612,744		584,578
Total current liabilities	620,421		620,421		597,056
Deferred income taxes	260,798		260,798		220,647
Noncurrent liabilities from risk management activities	0		0		0
Regulatory cost of removal obligation	0		0		0
Deferred credits and other liabilities	1,194		1,194		2,142
Total Equity and Liabilities	1,243,209		1,243,209		1,148,006
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	385,160	334,335	735,067	723,000
Intersegment revenues	43,788	36,428	93,775	91,939
Total operating revenues	428,948	370,763	828,842	814,939
Purchased gas cost	404,641	374,992	782,076	803,763
Gross profit	24,307	(4,229)	46,766	11,176
Operating expenses
Operation and maintenance	6,763	5,769	13,645	11,820
Depreciation and amortization	859	725	1,988	1,458
Taxes, other than income	438	691	1,072	1,626
Total operating expenses	8,060	7,185	16,705	14,904
Operating income	16,247	(11,414)	30,061	(3,728)
Miscellaneous income (expense)	(91)	567	1,576	603
Interest charges	498	839	1,295	1,091
Income (loss) before income taxes	15,658	(11,686)	30,342	(4,216)
Income tax expense (benefit)	6,038	(4,675)	12,572	(1,674)
Income (loss) from continuing operations	9,620	(7,011)	17,770	(2,542)
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	0	0	0	0
Net income (loss)	9,620	(7,011)	17,770	(2,542)
Capital expenditures	417	3,456	742	4,997
ASSETS
Net property, plant and equipment	59,947		59,947		64,144
Investment in subsidaries	(2,096)		(2,096)		(2,096)
Current assets
Cash and cash equivalents	38,084		38,084		51,452
Assets from risk management activities	3,704		3,704		17,773
Other current assets	520,574		520,574		404,097
Intercompany receivables	0		0		0
Total current assets	562,362		562,362		473,322
Intangible assets	142		142		164
Goodwill	34,711		34,711		34,711
Noncurrent assets from risk management activities	4,248		4,248		0
Deferred charges and other assets	8,842		8,842		6,733
Total Assets	668,156		668,156		576,978
CAPITALIZATION AND LIABILITIES
Shareholders' equity	445,213		445,213		419,335
Long-term debt	0		0		0
Total capitalization	445,213		445,213		419,335
Current liabilities
Current maturities of long-term debt	0		0		131
Short-term debt	0		0		0
Liabilities from risk management activities	0		0		15
Other current liabilities	156,969		156,969		90,116
Intercompany payables	53,663		53,663		51,979
Total current liabilities	210,632		210,632		142,241
Deferred income taxes	10,682		10,682		5,148
Noncurrent liabilities from risk management activities	0		0		9,206
Regulatory cost of removal obligation	0		0		0
Deferred credits and other liabilities	1,629		1,629		1,048
Total Equity and Liabilities	668,156		668,156		576,978
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	0	0	0	0
Intersegment revenues	(86,976)	(74,358)	(180,183)	(167,412)
Total operating revenues	(86,976)	(74,358)	(180,183)	(167,412)
Purchased gas cost	(86,566)	(74,009)	(179,364)	(166,696)
Gross profit	(410)	(349)	(819)	(716)
Operating expenses
Operation and maintenance	(411)	(351)	(822)	(719)
Depreciation and amortization	0	0	0	0
Taxes, other than income	0	0	0	0
Total operating expenses	(411)	(351)	(822)	(719)
Operating income	1	2	3	3
Miscellaneous income (expense)	(689)	(628)	(1,400)	(503)
Interest charges	(688)	(626)	(1,397)	(500)
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	(803,913)		(803,913)		(745,400)
Current assets
Cash and cash equivalents	0		0		0
Assets from risk management activities	0		0		0
Other current assets	(278,299)		(278,299)		(223,056)
Intercompany receivables	(666,407)		(666,407)		(636,557)
Total current assets	(944,706)		(944,706)		(859,613)
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	(1,748,619)		(1,748,619)		(1,605,013)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(806,009)		(806,009)		(747,496)
Long-term debt	0		0		0
Total capitalization	(806,009)		(806,009)		(747,496)
Current liabilities
Current maturities of long-term debt	0		0		0
Short-term debt	(258,500)		(258,500)		(211,790)
Liabilities from risk management activities	0		0		0
Other current liabilities	(17,703)		(17,703)		(9,170)
Intercompany payables	(666,407)		(666,407)		(636,557)
Total current liabilities	(942,610)		(942,610)		(857,517)
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	$ (1,748,619)		$ (1,748,619)		$ (1,605,013)