CONSOLIDATED STATEMENTS OF INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
Operating revenues
Operating revenues	$ 778,728	$ 942,718	$ 1,964,322	$ 1,255,148	$ 674,374	$ 857,935	$ 1,308,996	$ 1,034,155	$ 4,940,916	$ 3,875,460	$ 3,436,162
Purchased gas cost
Purchased gas cost									3,358,490	2,463,410	2,112,423
Gross profit	337,659	359,533	496,277	388,957	300,440	316,497	432,751	362,362	1,582,426	1,412,050	1,323,739
Operating expenses
Operation and maintenance									505,154	488,020	453,613
Depreciation and amortization									253,987	235,079	237,525
Taxes, other than income									211,936	187,072	181,073
Asset impairments									0	0	5,288
Total operating expenses									971,077	910,171	877,499
Operating income	83,944	106,605	250,080	170,720	50,383	86,396	210,178	154,922	611,349	501,879	446,240
Miscellaneous income (expense), net									(5,235)	(197)	(14,644)
Interest charges									129,295	128,385	141,174
Income from continuing operations before income taxes									476,819	373,297	290,422
Income tax expense									187,002	142,599	98,226
Income from continuing operations					7,536	33,474	112,340	77,348	289,817	230,698	192,196
Income from discontinued operations, net of tax					0	0	4,085	3,117	0	7,202	18,172
Gain on sale of discontinued operations, net of tax					0	5,294	0	0	0	5,294	6,349
Net income	23,713	45,721	133,367	87,016	7,536	38,768	116,425	80,465	289,817	243,194	216,717
Income per share from continuing operations - basic (usd per share)					$ 0.08	$ 0.37	$ 1.24	$ 0.85	$ 2.96	$ 2.54	$ 2.12
Income per share from discontinued operations - basic (usd per share)					$ 0.00	$ 0.06	$ 0.04	$ 0.04	$ 0.00	$ 0.14	$ 0.27
Net income per share — basic (usd per share)	$ 0.23	$ 0.45	$ 1.38	$ 0.94	$ 0.08	$ 0.43	$ 1.28	$ 0.89	$ 2.96	$ 2.68	$ 2.39
Income per share from continuing operations - diluted (usd per share)					$ 0.08	$ 0.36	$ 1.23	$ 0.85	$ 2.96	$ 2.50	$ 2.10
Income per share from discontinued operations - diluted (usd per share)					$ 0.00	$ 0.06	$ 0.04	$ 0.03	$ 0.00	$ 0.14	$ 0.27
Net income per share — diluted (usd per share)	$ 0.23	$ 0.45	$ 1.38	$ 0.94	$ 0.08	$ 0.42	$ 1.27	$ 0.88	$ 2.96	$ 2.64	$ 2.37
Weighted average shares outstanding:
Basic (shares)									97,606	90,533	90,150
Diluted (shares)									97,608	91,711	91,172
Regulated Distribution Segment [Member]
Operating revenues
Operating revenues	409,014	517,707	1,290,960	843,865	360,386	467,144	905,176	666,787	3,061,546	2,399,493	2,145,330
Purchased gas cost
Purchased gas cost									1,885,031	1,318,257	1,122,587
Gross profit									1,176,515	1,081,236	1,022,743
Operating expenses
Operation and maintenance									387,228	375,188	353,879
Depreciation and amortization									208,376	195,581	202,026
Taxes, other than income									196,343	167,374	162,377
Asset impairments											0
Total operating expenses									791,947	738,143	718,282
Operating income									384,568	343,093	304,461
Miscellaneous income (expense), net									(381)	2,535	(12,657)
Interest charges									94,918	98,296	110,642
Income from continuing operations before income taxes									289,269	247,332	181,162
Income tax expense									117,684	96,476	57,314
Income from continuing operations										150,856	123,848
Income from discontinued operations, net of tax										7,202	18,172
Gain on sale of discontinued operations, net of tax										5,649	6,349
Net income									171,585	163,707	148,369
Regulated Pipeline Segment [Member]
Operating revenues
Operating revenues	86,314	87,189	73,615	71,341	72,330	74,041	61,848	60,681	318,459	268,900	247,351
Purchased gas cost
Purchased gas cost									0	0	0
Gross profit									318,459	268,900	247,351
Operating expenses
Operation and maintenance									91,466	76,686	71,521
Depreciation and amortization									41,031	35,302	31,438
Taxes, other than income									13,143	17,059	15,568
Asset impairments											0
Total operating expenses									145,640	129,047	118,527
Operating income									172,819	139,853	128,824
Miscellaneous income (expense), net									(3,181)	(2,285)	(1,051)
Interest charges									36,280	30,678	29,414
Income from continuing operations before income taxes									133,358	106,890	98,359
Income tax expense									47,167	38,630	35,300
Income from continuing operations										68,260	63,059
Income from discontinued operations, net of tax										0	0
Gain on sale of discontinued operations, net of tax										0
Net income									86,191	68,260	63,059
Nonregulated Segment [Member]
Operating revenues
Operating revenues	396,855	465,033	757,683	447,721	337,264	421,808	428,948	399,894	2,067,292	1,587,914	1,348,982
Purchased gas cost
Purchased gas cost									1,979,337	1,524,583	1,293,858
Gross profit									87,955	63,331	55,124
Operating expenses
Operation and maintenance									26,963	37,569	29,697
Depreciation and amortization									4,580	4,196	4,061
Taxes, other than income									2,450	2,639	3,128
Asset impairments											5,288
Total operating expenses									33,993	44,404	42,174
Operating income									53,962	18,927	12,950
Miscellaneous income (expense), net									2,216	2,316	1,035
Interest charges									1,986	2,168	3,084
Income from continuing operations before income taxes									54,192	19,075	10,901
Income tax expense									22,151	7,493	5,612
Income from continuing operations										11,582	5,289
Income from discontinued operations, net of tax										0	0
Gain on sale of discontinued operations, net of tax										(355)
Net income									32,041	11,227	5,289
Intersegment Elimination [Member]
Operating revenues
Operating revenues	(113,455)	(127,211)	(157,936)	(107,779)	(95,606)	(105,058)	(86,976)	(93,207)	(506,381)	(380,847)	(305,501)
Purchased gas cost
Purchased gas cost									(505,878)	(379,430)	(304,022)
Gross profit									(503)	(1,417)	(1,479)
Operating expenses
Operation and maintenance									(503)	(1,423)	(1,484)
Depreciation and amortization									0	0	0
Taxes, other than income									0	0	0
Asset impairments											0
Total operating expenses									(503)	(1,423)	(1,484)
Operating income									0	6	5
Miscellaneous income (expense), net									(3,889)	(2,763)	(1,971)
Interest charges									(3,889)	(2,757)	(1,966)
Income from continuing operations before income taxes									0	0	0
Income tax expense									0	0	0
Income from continuing operations										0	0
Income from discontinued operations, net of tax										0	0
Gain on sale of discontinued operations, net of tax										0
Net income									$ 0	$ 0	$ 0