Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Mar. 31, 2012	Mar. 31, 2011	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 1,243,450	$ 1,581,497	$ 2,344,623	$ 2,714,759
Purchased gas cost	809,189	1,127,829	1,547,480	1,896,367
Gross profit	434,261	453,668	797,143	818,392
Operating expenses
Operation and maintenance	110,708	114,162	226,770	228,652
Depreciation and amortization	60,272	55,467	119,487	110,244
Taxes, other than income	54,919	53,558	98,117	93,726
Asset impairments	0	19,282	0	19,282
Total operating expenses	225,899	242,469	444,374	451,904
Operating income	208,362	211,199	352,769	366,488
Miscellaneous income (expense)	616	26,202	(1,259)	25,476
Interest charges	36,660	37,875	72,102	76,770
Income (loss) before income taxes	172,318	199,526	279,408	315,194
Income tax expense (benefit)	66,408	71,366	107,710	115,934
Income (loss) from continuing operations	105,910	128,160	171,698	199,260
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	3,201	4,049	5,920	6,946
Net income (loss)	109,111	132,209	177,618	206,206
Capital expenditures	156,729	123,501	311,123	246,663
ASSETS
Net property, plant and equipment	5,334,012		5,334,012		5,147,918
Current assets
Cash and cash equivalents	47,040	153,246	47,040	153,246	131,419	131,952
Total current assets	893,841		893,841		1,010,953
Total Assets	7,368,727		7,368,727		7,282,871
CAPITALIZATION AND LIABILITIES
Shareholders' equity	2,360,712		2,360,712		2,255,421
Long-term debt	1,956,213		1,956,213		2,206,117
Total capitalization	4,316,925		4,316,925		4,461,538
Current liabilities
Current maturities of long-term debt	250,131		250,131		2,434
Short-term debt	173,996		173,996		206,396
Total current liabilities	1,108,114		1,108,114		867,598
Deferred income taxes	1,062,488		1,062,488		960,093
Total Equity and Liabilities	7,368,727		7,368,727		7,282,871
Natural Gas Distribution Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	888,685	1,077,178	1,581,753	1,780,439
Intersegment revenues	323	236	547	437
Operating revenues	889,008	1,077,414	1,582,300	1,780,876
Purchased gas cost	508,206	698,410	910,413	1,110,936
Gross profit	380,802	379,004	671,887	669,940
Operating expenses
Operation and maintenance	89,443	92,266	182,857	181,495
Depreciation and amortization	51,755	48,555	102,586	96,449
Taxes, other than income	50,313	50,088	88,792	84,536
Asset impairments	0	0	0	0
Total operating expenses	191,511	190,909	374,235	362,480
Operating income	189,291	188,095	297,652	307,460
Miscellaneous income (expense)	733	20,156	(1,023)	19,458
Interest charges	28,833	29,605	56,688	59,302
Income (loss) before income taxes	161,191	178,646	239,941	267,616
Income tax expense (benefit)	62,890	64,085	93,735	98,634
Income (loss) from continuing operations	98,301	114,561	146,206	168,982
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	3,201	4,049	5,920	6,946
Net income (loss)	101,502	118,610	152,126	175,928
Capital expenditures	114,402	109,762	243,135	219,261
ASSETS
Net property, plant and equipment	4,382,291		4,382,291		4,248,198
Investment in subsidaries	674,594		674,594		670,993
Current assets
Cash and cash equivalents	40,140		40,140		24,646
Assets from risk management activities	502		502		843
Other current assets	632,486		632,486		655,716
Intercompany receivables	584,018		584,018		569,898
Total current assets	1,257,146		1,257,146		1,251,103
Intangible assets	0		0		0
Goodwill	572,908		572,908		572,908
Noncurrent assets from risk management activities	0		0		998
Deferred charges and other assets	366,329		366,329		353,960
Total Assets	7,253,268		7,253,268		7,098,160
CAPITALIZATION AND LIABILITIES
Shareholders' equity	2,360,712		2,360,712		2,255,421
Long-term debt	1,956,147		1,956,147		2,205,986
Total capitalization	4,316,859		4,316,859		4,461,407
Current liabilities
Current maturities of long-term debt	250,000		250,000		2,303
Short-term debt	371,996		371,996		387,691
Liabilities from risk management activities	47,281		47,281		11,916
Other current liabilities	507,354		507,354		474,783
Intercompany payables	0		0		0
Total current liabilities	1,176,631		1,176,631		876,693
Deferred income taxes	890,455		890,455		789,649
Noncurrent liabilities from risk management activities	1		1		67,862
Regulatory cost of removal obligation	414,001		414,001		428,947
Deferred credits and other liabilities	455,321		455,321		473,602
Total Equity and Liabilities	7,253,268		7,253,268		7,098,160
Regulated Transmission and Storage Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	20,430	21,597	39,870	42,830
Intersegment revenues	37,607	33,379	74,926	61,153
Operating revenues	58,037	54,976	114,796	103,983
Purchased gas cost	0	0	0	0
Gross profit	58,037	54,976	114,796	103,983
Operating expenses
Operation and maintenance	15,847	15,231	32,812	30,805
Depreciation and amortization	7,792	5,798	15,443	11,597
Taxes, other than income	3,915	4,113	7,699	7,666
Asset impairments	0	0	0	0
Total operating expenses	27,554	25,142	55,954	50,068
Operating income	30,483	29,834	58,842	53,915
Miscellaneous income (expense)	(56)	5,861	(336)	5,579
Interest charges	7,614	8,085	14,823	16,149
Income (loss) before income taxes	22,813	27,610	43,683	43,345
Income tax expense (benefit)	8,193	9,871	15,649	15,504
Income (loss) from continuing operations	14,620	17,739	28,034	27,841
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	0	0	0	0
Net income (loss)	14,620	17,739	28,034	27,841
Capital expenditures	38,871	11,818	62,991	24,557
ASSETS
Net property, plant and equipment	886,507		886,507		838,302
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	13,278		13,278		15,413
Intercompany receivables	0		0		0
Total current assets	13,278		13,278		15,413
Intangible assets	0		0		0
Goodwill	132,381		132,381		132,381
Noncurrent assets from risk management activities	0		0		0
Deferred charges and other assets	13,203		13,203		18,028
Total Assets	1,045,369		1,045,369		1,004,124
CAPITALIZATION AND LIABILITIES
Shareholders' equity	293,135		293,135		265,102
Long-term debt	0		0		0
Total capitalization	293,135		293,135		265,102
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	6,309		6,309		10,369
Intercompany payables	551,330		551,330		543,084
Total current liabilities	557,639		557,639		553,453
Deferred income taxes	188,936		188,936		173,351
Noncurrent liabilities from risk management activities	0		0		0
Regulatory cost of removal obligation	0		0		0
Deferred credits and other liabilities	5,659		5,659		12,218
Total Equity and Liabilities	1,045,369		1,045,369		1,004,124
Nonregulated Segment [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	334,335	482,722	723,000	891,490
Intersegment revenues	36,428	100,809	91,939	167,681
Operating revenues	370,763	583,531	814,939	1,059,171
Purchased gas cost	374,992	563,473	803,763	1,013,935
Gross profit	(4,229)	20,058	11,176	45,236
Operating expenses
Operation and maintenance	5,769	7,035	11,820	17,119
Depreciation and amortization	725	1,114	1,458	2,198
Taxes, other than income	691	(643)	1,626	1,524
Asset impairments	0	19,282	0	19,282
Total operating expenses	7,185	26,788	14,904	40,123
Operating income	(11,414)	(6,730)	(3,728)	5,113
Miscellaneous income (expense)	567	306	603	596
Interest charges	839	306	1,091	1,476
Income (loss) before income taxes	(11,686)	(6,730)	(4,216)	4,233
Income tax expense (benefit)	(4,675)	(2,590)	(1,674)	1,796
Income (loss) from continuing operations	(7,011)	(4,140)	(2,542)	2,437
Discontinued Operation Income Loss From Discontinued Operation During Phase Out Period Net Of Tax	0	0	0	0
Net income (loss)	(7,011)	(4,140)	(2,542)	2,437
Capital expenditures	3,456	1,921	4,997	2,845
ASSETS
Net property, plant and equipment	65,214		65,214		61,418
Investment in subsidaries	(2,096)		(2,096)		(2,096)
Current assets
Cash and cash equivalents	6,900		6,900		106,773
Assets from risk management activities	2,877		2,877		17,501
Other current assets	399,389		399,389		386,215
Intercompany receivables	0		0		0
Total current assets	409,166		409,166		510,489
Intangible assets	185		185		207
Goodwill	34,711		34,711		34,711
Noncurrent assets from risk management activities	10,841		10,841		0
Deferred charges and other assets	10,316		10,316		10,807
Total Assets	528,337		528,337		615,536
CAPITALIZATION AND LIABILITIES
Shareholders' equity	381,459		381,459		405,891
Long-term debt	66		66		131
Total capitalization	381,525		381,525		406,022
Current liabilities
Current maturities of long-term debt	131		131		131
Short-term debt	0		0		0
Liabilities from risk management activities	5,296		5,296		3,537
Other current liabilities	119,382		119,382		170,926
Intercompany payables	32,688		32,688		26,814
Total current liabilities	157,497		157,497		201,408
Deferred income taxes	(16,903)		(16,903)		(2,907)
Noncurrent liabilities from risk management activities	5,300		5,300		10,227
Regulatory cost of removal obligation	0		0		0
Deferred credits and other liabilities	918		918		786
Total Equity and Liabilities	528,337		528,337		615,536
Intersegment Elimination [Member]
Segment Reporting Information Profit Loss [Abstract]
Revenues from external customers	0	0	0	0
Intersegment revenues	(74,358)	(134,424)	(167,412)	(229,271)
Operating revenues	(74,358)	(134,424)	(167,412)	(229,271)
Purchased gas cost	(74,009)	(134,054)	(166,696)	(228,504)
Gross profit	(349)	(370)	(716)	(767)
Operating expenses
Operation and maintenance	(351)	(370)	(719)	(767)
Depreciation and amortization	0	0	0	0
Taxes, other than income	0	0	0	0
Asset impairments	0	0	0	0
Total operating expenses	(351)	(370)	(719)	(767)
Operating income	2	0	3	0
Miscellaneous income (expense)	(628)	(121)	(503)	(157)
Interest charges	(626)	(121)	(500)	(157)
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	(672,498)		(672,498)		(668,897)
Current assets
Cash and cash equivalents	0		0		0
Assets from risk management activities	0		0		0
Other current assets	(201,731)		(201,731)		(196,154)
Intercompany receivables	(584,018)		(584,018)		(569,898)
Total current assets	(785,749)		(785,749)		(766,052)
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	(1,458,247)		(1,458,247)		(1,434,949)
CAPITALIZATION AND LIABILITIES
Shareholders' equity	(674,594)		(674,594)		(670,993)
Long-term debt	0		0		0
Total capitalization	(674,594)		(674,594)		(670,993)
Current liabilities
Current maturities of long-term debt	0		0		0
Short-term debt	(198,000)		(198,000)		(181,295)
Liabilities from risk management activities	0		0		0
Other current liabilities	(1,635)		(1,635)		(12,763)
Intercompany payables	(584,018)		(584,018)		(569,898)
Total current liabilities	(783,653)		(783,653)		(763,956)
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	$ (1,458,247)		$ (1,458,247)		$ (1,434,949)