CONSOLIDATED STATEMENTS OF INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2011
Operating revenues
Operating revenues	$ 685,171	$ 857,935	$ 1,308,996	$ 1,034,155	$ 552,566	$ 576,414	$ 1,225,509	$ 1,083,994	$ 3,886,257	$ 3,438,483	$ 4,286,435
Purchased gas cost
Purchased gas cost									2,474,207	2,114,744	2,985,615
Gross profit	300,440	316,497	432,751	362,362	249,389	293,171	425,787	355,392	1,412,050	1,323,739	1,300,820
Operating expenses
Operation and maintenance									488,020	453,613	442,965
Depreciation and amortization									235,079	237,525	223,832
Taxes, other than income									187,072	181,073	177,767
Asset impairments									0	5,288	30,270
Total operating expenses									910,171	877,499	874,834
Operating income	50,383	86,396	210,178	154,922	22,791	81,546	202,432	139,471	501,879	446,240	425,986
Miscellaneous income (expense), net									(197)	(14,644)	21,184
Interest charges									128,385	141,174	150,763
Income from continuing operations before income taxes									373,297	290,422	296,407
Income tax expense									142,599	98,226	106,819
Income from continuing operations	7,536	33,474	112,340	77,348	(286)	28,014	102,084	62,384	230,698	192,196	189,588
Income from discontinued operations, net of tax	0	0	4,085	3,117	1,904	3,118	7,027	6,123	7,202	18,172	18,013
Gain on sale of discontinued operations, net of tax	0	5,294	0	0	6,349	0	0	0	5,294	6,349	0
Net income	7,536	38,768	116,425	80,465	7,967	31,132	109,111	68,507	243,194	216,717	207,601
Income per share from continuing operations - basic (usd per share)	$ 0.08	$ 0.37	$ 1.24	$ 0.85	$ 0.00	$ 0.31	$ 1.12	$ 0.68	$ 2.54	$ 2.12	$ 2.08
Income per share from discontinued operations - basic (usd per share)	$ 0.00	$ 0.06	$ 0.04	$ 0.04	$ 0.09	$ 0.03	$ 0.08	$ 0.07	$ 0.14	$ 0.27	$ 0.20
Net income per share — basic (usd per share)	$ 0.08	$ 0.43	$ 1.28	$ 0.89	$ 0.09	$ 0.34	$ 1.20	$ 0.75	$ 2.68	$ 2.39	$ 2.28
Income per share from continuing operations - diluted (usd per share)	$ 0.08	$ 0.36	$ 1.23	$ 0.85	$ 0.00	$ 0.31	$ 1.12	$ 0.68	$ 2.50	$ 2.10	$ 2.07
Income per share from discontinued operations - diluted (usd per share)	$ 0.00	$ 0.06	$ 0.04	$ 0.03	$ 0.09	$ 0.03	$ 0.08	$ 0.07	$ 0.14	$ 0.27	$ 0.20
Net income per share — diluted (usd per share)	$ 0.08	$ 0.42	$ 1.27	$ 0.88	$ 0.09	$ 0.34	$ 1.20	$ 0.75	$ 2.64	$ 2.37	$ 2.27
Weighted average shares outstanding:
Basic (shares)									90,533	90,150	90,201
Diluted (shares)									91,711	91,172	90,652
Natural Gas Distribution Segment [Member]
Operating revenues
Operating revenues	360,386	467,144	905,176	666,787	282,516	315,634	871,067	676,113	2,399,493	2,145,330	2,470,664
Purchased gas cost
Purchased gas cost									1,318,257	1,122,587	1,452,721
Gross profit									1,081,236	1,022,743	1,017,943
Operating expenses
Operation and maintenance									375,188	353,879	341,758
Depreciation and amortization									195,581	202,026	193,642
Taxes, other than income									167,374	162,377	160,455
Asset impairments										0	0
Total operating expenses									738,143	718,282	695,855
Operating income									343,093	304,461	322,088
Miscellaneous income (expense), net									2,535	(12,657)	16,242
Interest charges									98,296	110,642	115,740
Income from continuing operations before income taxes									247,332	181,162	222,590
Income tax expense									96,476	57,314	77,885
Income from continuing operations									150,856	123,848	144,705
Income from discontinued operations, net of tax									7,202	18,172	18,013
Gain on sale of discontinued operations, net of tax									5,649	6,349
Net income									163,707	148,369	162,718
Regulated Transmission and Storage Segment [Member]
Operating revenues
Operating revenues	72,330	74,041	61,848	60,681	65,482	67,073	58,037	56,759	268,900	247,351	219,373
Purchased gas cost
Purchased gas cost									0	0	0
Gross profit									268,900	247,351	219,373
Operating expenses
Operation and maintenance									76,686	71,521	70,401
Depreciation and amortization									35,302	31,438	25,997
Taxes, other than income									17,059	15,568	14,700
Asset impairments										0	0
Total operating expenses									129,047	118,527	111,098
Operating income									139,853	128,824	108,275
Miscellaneous income (expense), net									(2,285)	(1,051)	4,715
Interest charges									30,678	29,414	31,432
Income from continuing operations before income taxes									106,890	98,359	81,558
Income tax expense									38,630	35,300	29,143
Income from continuing operations									68,260	63,059	52,415
Income from discontinued operations, net of tax									0	0	0
Gain on sale of discontinued operations, net of tax									0	0
Net income									68,260	63,059	52,415
Nonregulated Segment [Member]
Operating revenues
Operating revenues	348,061	421,808	428,948	399,894	280,114	256,250	370,763	444,176	1,598,711	1,351,303	2,024,893
Purchased gas cost
Purchased gas cost									1,535,380	1,296,179	1,959,893
Gross profit									63,331	55,124	65,000
Operating expenses
Operation and maintenance									37,569	29,697	32,308
Depreciation and amortization									4,196	4,061	4,193
Taxes, other than income									2,639	3,128	2,612
Asset impairments										5,288	30,270
Total operating expenses									44,404	42,174	69,383
Operating income									18,927	12,950	(4,383)
Miscellaneous income (expense), net									2,316	1,035	657
Interest charges									2,168	3,084	4,015
Income from continuing operations before income taxes									19,075	10,901	(7,741)
Income tax expense									7,493	5,612	(209)
Income from continuing operations									11,582	5,289	(7,532)
Income from discontinued operations, net of tax									0	0	0
Gain on sale of discontinued operations, net of tax									(355)	0
Net income									11,227	5,289	(7,532)
Intersegment Elimination [Member]
Operating revenues
Operating revenues	(95,606)	(105,058)	(86,976)	(93,207)	(75,546)	(62,543)	(74,358)	(93,054)	(380,847)	(305,501)	(428,495)
Purchased gas cost
Purchased gas cost									(379,430)	(304,022)	(426,999)
Gross profit									(1,417)	(1,479)	(1,496)
Operating expenses
Operation and maintenance									(1,423)	(1,484)	(1,502)
Depreciation and amortization									0	0	0
Taxes, other than income									0	0	0
Asset impairments										0	0
Total operating expenses									(1,423)	(1,484)	(1,502)
Operating income									6	5	6
Miscellaneous income (expense), net									(2,763)	(1,971)	(430)
Interest charges									(2,757)	(1,966)	(424)
Income from continuing operations before income taxes									0	0	0
Income tax expense									0	0	0
Income from continuing operations									0	0	0
Income from discontinued operations, net of tax									0	0	0
Gain on sale of discontinued operations, net of tax									0	0
Net income									$ 0	$ 0	$ 0