Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ 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 by Segment [Line Items]
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
Selected Quarterly Financial Information [Abstract]
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 income	75,853	117,607	250,210	187,725	83,944	106,605	250,080	170,720	631,395	611,349	501,879
Income (loss) from continuing operations									315,075	289,817	230,698
Income from discontinued operations, net of tax									0	0	7,202
Gain (loss) on sale of discontinued operations, net of tax									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
Regulated Distribution Segment [Member]
Operating Revenues by Segment [Line Items]
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
Selected Quarterly Financial Information [Abstract]
Gross profit									1,237,577	1,176,515	1,081,236
Operating income									420,149	384,568	343,093
Income (loss) from continuing operations											150,856
Income from discontinued operations, net of tax											7,202
Gain (loss) on sale of discontinued operations, net of tax											5,649
Net income									204,813	171,585	163,707
Regulated Pipeline Segment [Member]
Operating Revenues by Segment [Line Items]
Operating revenues	97,807	97,008	91,730	83,567	86,314	87,189	73,615	71,341	370,112	318,459	268,900
Selected Quarterly Financial Information [Abstract]
Gross profit									370,112	318,459	268,900
Operating income									181,267	172,819	139,853
Income (loss) from continuing operations											68,260
Income from discontinued operations, net of tax											0
Gain (loss) on sale of discontinued operations, net of tax											0
Net income									94,662	86,191	68,260
Nonregulated Segment [Member]
Operating Revenues by Segment [Line Items]
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
Selected Quarterly Financial Information [Abstract]
Gross profit									72,860	87,955	63,331
Operating income									29,979	53,962	18,927
Income (loss) from continuing operations											11,582
Income from discontinued operations, net of tax											0
Gain (loss) on sale of discontinued operations, net of tax											(355)
Net income									15,600	32,041	11,227
Intersegment Elimination [Member]
Operating Revenues by Segment [Line Items]
Operating revenues	$ (103,391)	$ (106,170)	$ (120,597)	$ (133,862)	$ (113,455)	$ (127,211)	$ (157,936)	$ (107,779)	(464,020)	(506,381)	(380,847)
Selected Quarterly Financial Information [Abstract]
Gross profit									(532)	(503)	(1,417)
Operating income									0	0	6
Income (loss) from continuing operations											0
Income from discontinued operations, net of tax											0
Gain (loss) on sale of discontinued operations, net of tax											0
Net income									$ 0	$ 0	$ 0