Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, 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
Discontinued Operations Quarterly Information [Abstract]
Discontinued Operations Operating Revenues									$ 114,703	$ 141,227	$ 128,630
Discontinued Operations Gross Profit									51,801	57,690	50,805
Disposal Group Including Discontinued Operation Operating Income Loss									27,627	30,328	25,603
Explanatory Disclosure		Operating revenues for natural gas distribution, gross profit and operating income are shown net of discontinued operations from our Georgia operations of $9.4 million, $5.6 million and $3.2 million, which were not previously reported as discontinued operations.	Operating revenues for natural gas distribution, gross profit and operating income are shown net of discontinued operations from our Georgia operations of $17.9 million, $8.5 million and $5.9 million, which were not previously reported as discontinued operations.	Operating revenues for natural gas distribution, gross profit and operating income are shown net of discontinued operations from our Georgia operations of $17.2 million, $7.5 million and $4.9 million, which were not previously reported as discontinued operations.	Operating revenues for natural gas distribution, gross profit and operating income are shown net of discontinued operations from our Georgia operations of $9.6 million, $4.9 million and $2.1 million, which were not previously reported as discontinued operations.	Operating revenues for natural gas distribution, gross profit and operating income are shown net of discontinued operations from our Georgia operations of $10.4 million, $5.2 million and $2.7 million, which were not previously reported as discontinued operations.	Operating revenues for natural gas distribution, gross profit and operating income are shown net of discontinued operations from our Georgia operations of $25.1 million, $9.2 million and $6.6 million, which were not previously reported as discontinued operations.	Operating revenues for natural gas distribution, gross profit and operating income are shown net of discontinued operations from our Georgia operations of $16.0 million, $7.1 million and $4.5 million, which were not previously reported as discontinued operations.
Operating Revenues by Segment [Line Items]
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
Selected Quarterly Financial Information Abstract
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 income	22,791	81,546	202,432	139,471	39,195	31,394	204,624	150,773	446,240	425,986	463,833
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
Income (loss) from continuing operations per share - Basic	$ 0.00	$ 0.31	$ 1.12	$ 0.68	$ 0.00	$ (0.04)	$ 1.36	$ 0.75	$ 2.12	$ 2.08	$ 2.05
Income (loss) from discontinued operations per share - Basic	$ 0.09	$ 0.03	$ 0.08	$ 0.07	$ 0.02	$ 0.03	$ 0.09	$ 0.06	$ 0.27	$ 0.20	$ 0.17
Basic net income (loss) per share	$ 0.09	$ 0.34	$ 1.20	$ 0.75	$ 0.02	$ (0.01)	$ 1.45	$ 0.81	$ 2.39	$ 2.28	$ 2.22
Income Loss From Continuing Operations Per Diluted Share	$ 0.00	$ 0.31	$ 1.12	$ 0.68	$ 0.00	$ (0.04)	$ 1.36	$ 0.75	$ 2.10	$ 2.07	$ 2.03
Income (loss) from discontinued operations per share - Diluted	$ 0.09	$ 0.03	$ 0.08	$ 0.07	$ 0.02	$ 0.03	$ 0.09	$ 0.06	$ 0.27	$ 0.20	$ 0.17
Diluted net income (loss) per share	$ 0.09	$ 0.34	$ 1.20	$ 0.75	$ 0.02	$ (0.01)	$ 1.45	$ 0.81	$ 2.37	$ 2.27	$ 2.20
Natural Gas Distribution Segment [Member]
Operating Revenues by Segment [Line Items]
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
Selected Quarterly Financial Information Abstract
Gross profit									1,022,743	1,017,943	998,642
Operating income									304,461	322,088	296,851
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
Regulated Transmission and Storage Segment [Member]
Operating Revenues by Segment [Line Items]
Operating revenues	65,482	67,073	58,037	56,759	61,820	53,570	54,976	49,007	247,351	219,373	203,013
Selected Quarterly Financial Information Abstract
Gross profit									247,351	219,373	203,013
Operating income									128,824	108,275	97,038
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
Nonregulated Segment [Member]
Operating Revenues by Segment [Line Items]
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
Selected Quarterly Financial Information Abstract
Gross profit									55,124	65,000	114,091
Operating income									12,950	(4,383)	69,944
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
Intersegment Elimination [Member]
Operating Revenues by Segment [Line Items]
Operating revenues	(75,546)	(62,543)	(74,358)	(93,054)	(90,953)	(108,271)	(134,424)	(94,847)	(305,501)	(428,495)	(472,474)
Selected Quarterly Financial Information Abstract
Gross profit									(1,479)	(1,496)	(1,610)
Operating income									5	6	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