Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Operating Revenues by Segment [Line Items]
Operating revenues	$ 678,519	$ 632,916	$ 1,132,293	$ 906,221	$ 656,902	$ 686,401	$ 1,540,068	$ 1,258,765	$ 3,349,949	$ 4,142,136	$ 4,940,916
Selected Quarterly Financial Information [Abstract]
Gross profit	376,011	407,311	517,811	443,763	354,321	381,673	520,738	423,285	1,744,896	1,680,017	1,582,426
Operating income	84,633	137,164	250,016	196,205	75,853	117,607	250,210	187,725	668,018	631,395	611,349
Net income	$ 34,240	$ 71,193	$ 141,810	$ 102,861	$ 23,515	$ 56,281	$ 137,684	$ 97,595	$ 350,104	$ 315,075	$ 289,817
Income per share - basic (usd per share)									$ 3.38	$ 3.09	$ 2.96
Net income per share — basic (usd per share)	$ 0.33	$ 0.69	$ 1.38	$ 1.00	$ 0.23	$ 0.55	$ 1.35	$ 0.96	3.38	3.09	2.96
Net income per share — diluted (usd per share)	$ 0.33	$ 0.69	$ 1.38	$ 1.00	$ 0.23	$ 0.55	$ 1.35	$ 0.96	$ 3.38	$ 3.09	$ 2.96
Regulated Distribution Segment [Member]
Operating Revenues by Segment [Line Items]
Operating revenues	$ 389,353	$ 414,226	$ 849,685	$ 638,602	$ 369,656	$ 416,794	$ 1,130,613	$ 846,772	$ 2,291,866	$ 2,763,835	$ 3,061,546
Selected Quarterly Financial Information [Abstract]
Gross profit									1,272,805	1,237,577	1,176,515
Operating income									439,584	420,149	384,568
Net income									232,370	204,813	171,585
Regulated Pipeline Segment [Member]
Operating Revenues by Segment [Line Items]
Operating revenues	109,204	109,249	95,703	94,677	97,807	97,008	91,730	83,567	408,833	370,112	318,459
Selected Quarterly Financial Information [Abstract]
Gross profit									408,833	370,112	318,459
Operating income									199,434	181,267	172,819
Net income									101,689	94,662	86,191
Nonregulated Segment [Member]
Operating Revenues by Segment [Line Items]
Operating revenues	291,889	214,555	287,395	272,524	292,830	278,769	438,322	462,288	1,066,363	1,472,209	2,067,292
Selected Quarterly Financial Information [Abstract]
Gross profit									63,790	72,860	87,955
Operating income									29,000	29,979	53,962
Net income									16,045	15,600	32,041
Intersegment Elimination [Member]
Operating Revenues by Segment [Line Items]
Operating revenues	$ (111,927)	$ (105,114)	$ (100,490)	$ (99,582)	$ (103,391)	$ (106,170)	$ (120,597)	$ (133,862)	(417,113)	(464,020)	(506,381)
Selected Quarterly Financial Information [Abstract]
Gross profit									(532)	(532)	(503)
Operating income									0	0	0
Net income									$ 0	$ 0	$ 0