CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
Operating revenues
Operating revenues	$ 656,902	$ 686,401	$ 1,540,068	$ 1,258,765	$ 789,034	$ 943,170	$ 1,964,854	$ 1,243,858	$ 4,142,136	$ 4,940,916	$ 3,875,460
Purchased gas cost
Purchased gas cost									2,462,119	3,358,490	2,463,410
Gross profit	354,321	381,673	520,738	423,285	337,659	359,533	496,277	388,957	1,680,017	1,582,426	1,412,050
Operating expenses
Operation and maintenance									541,868	505,154	488,020
Depreciation and amortization									274,796	253,987	235,079
Taxes, other than income									231,958	211,936	187,072
Total operating expenses									1,048,622	971,077	910,171
Operating income	75,853	117,607	250,210	187,725	83,944	106,605	250,080	170,720	631,395	611,349	501,879
Miscellaneous income (expense), net									(4,389)	(5,235)	(197)
Interest charges									116,241	129,295	128,385
Income from continuing operations before income taxes									510,765	476,819	373,297
Income tax expense									195,690	187,002	142,599
Income from continuing operations									315,075	289,817	230,698
Income from discontinued operations, net of tax ($0, $0 and $3,986)									0	0	7,202
Gain on sale of discontinued operations, net of tax ($0, $0 and $2,909)									0	0	5,294
Net income	$ 23,515	$ 56,281	$ 137,684	$ 97,595	$ 23,713	$ 45,721	$ 133,367	$ 87,016	$ 315,075	$ 289,817	$ 243,194
Income per share from continuing operations - basic (usd per share)									$ 3.09	$ 2.96	$ 2.54
Income per share from discontinued operations - basic (usd per share)									0.00	0.00	0.14
Net income per share — basic (usd per share)	$ 0.23	$ 0.55	$ 1.35	$ 0.96	$ 0.23	$ 0.45	$ 1.38	$ 0.95	3.09	2.96	2.68
Income per share from continuing operations - diluted (usd per share)									3.09	2.96	2.50
Income per share from discontinued operations - diluted (usd per share)									0.00	0.00	0.14
Net income per share — diluted (usd per share)	$ 0.23	$ 0.55	$ 1.35	$ 0.96	$ 0.23	$ 0.45	$ 1.38	$ 0.95	$ 3.09	$ 2.96	$ 2.64
Weighted average shares outstanding:
Basic (shares)									101,892	97,606	90,533
Diluted (shares)									101,892	97,608	91,711
Intersegment Elimination [Member]
Operating revenues
Operating revenues									$ (464,020)	$ (506,381)	$ (380,847)
Purchased gas cost
Purchased gas cost									(463,488)	(505,878)	(379,430)
Regulated Distribution Segment [Member]
Operating revenues
Operating revenues	$ 369,656	$ 416,794	$ 1,130,613	$ 846,772	$ 409,014	$ 517,707	$ 1,290,960	$ 843,865	2,763,835	3,061,546	2,399,493
Purchased gas cost
Purchased gas cost									1,526,258	1,885,031	1,318,257
Gross profit									1,237,577	1,176,515	1,081,236
Operating expenses
Operation and maintenance									388,486	387,228	375,188
Depreciation and amortization									223,048	208,376	195,581
Taxes, other than income									205,894	196,343	167,374
Total operating expenses									817,428	791,947	738,143
Operating income									420,149	384,568	343,093
Miscellaneous income (expense), net									(377)	(381)	2,535
Interest charges									84,132	94,918	98,296
Income from continuing operations before income taxes									335,640	289,269	247,332
Income tax expense									130,827	117,684	96,476
Income from continuing operations											150,856
Income from discontinued operations, net of tax ($0, $0 and $3,986)											7,202
Gain on sale of discontinued operations, net of tax ($0, $0 and $2,909)											5,649
Net income									204,813	171,585	163,707
Regulated Pipeline Segment [Member]
Operating revenues
Operating revenues	97,807	97,008	91,730	83,567	86,314	87,189	73,615	71,341	370,112	318,459	268,900
Purchased gas cost
Purchased gas cost									0	0	0
Gross profit									370,112	318,459	268,900
Operating expenses
Operation and maintenance									118,866	91,466	76,686
Depreciation and amortization									47,236	41,031	35,302
Taxes, other than income									22,743	13,143	17,059
Total operating expenses									188,845	145,640	129,047
Operating income									181,267	172,819	139,853
Miscellaneous income (expense), net									(1,243)	(3,181)	(2,285)
Interest charges									33,151	36,280	30,678
Income from continuing operations before income taxes									146,873	133,358	106,890
Income tax expense									52,211	47,167	38,630
Income from continuing operations											68,260
Income from discontinued operations, net of tax ($0, $0 and $3,986)											0
Gain on sale of discontinued operations, net of tax ($0, $0 and $2,909)											0
Net income									94,662	86,191	68,260
Nonregulated Segment [Member]
Operating revenues
Operating revenues	$ 292,830	$ 278,769	$ 438,322	$ 462,288	$ 407,161	$ 465,485	$ 758,215	$ 436,431	1,472,209	2,067,292	1,587,914
Purchased gas cost
Purchased gas cost									1,399,349	1,979,337	1,524,583
Gross profit									72,860	87,955	63,331
Operating expenses
Operation and maintenance									35,048	26,963	37,569
Depreciation and amortization									4,512	4,580	4,196
Taxes, other than income									3,321	2,450	2,639
Total operating expenses									42,881	33,993	44,404
Operating income									29,979	53,962	18,927
Miscellaneous income (expense), net									(760)	2,216	2,316
Interest charges									967	1,986	2,168
Income from continuing operations before income taxes									28,252	54,192	19,075
Income tax expense									12,652	22,151	7,493
Income from continuing operations											11,582
Income from discontinued operations, net of tax ($0, $0 and $3,986)											0
Gain on sale of discontinued operations, net of tax ($0, $0 and $2,909)											(355)
Net income									$ 15,600	$ 32,041	$ 11,227