Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Operating Revenues by Segment [Line Items]
Operating revenues	$ 464,880	$ 526,501	$ 988,186	$ 780,168	$ 433,643	$ 456,212	$ 890,040	$ 674,753	$ 2,759,735	$ 2,454,648	$ 2,926,985
Selected Quarterly Financial Information [Abstract]
Purchased gas cost	72,561	114,176	427,494	311,305	63,927	64,583	377,356	240,326	925,536	746,192	1,295,675
Operating income	91,792	140,664	285,172	209,918	84,449	128,396	251,656	192,729	727,546	657,230	612,232
Income from Continuing Operations, Net of Tax	35,853	70,808	162,012	114,038	34,850	66,143	143,003	101,546	382,711	345,542	305,623
Net income	$ 35,853	$ 70,808	$ 164,728	$ 125,032	$ 34,240	$ 71,193	$ 141,810	$ 102,861	$ 396,421	$ 350,104	$ 315,075
Income (Loss) from Continuing Operations, Per Basic and Diluted Share	$ 0.34	$ 0.67	$ 1.52	$ 1.08	$ 0.33	$ 0.64	$ 1.39	$ 0.99	$ 3.60	$ 3.33	$ 3.00
Earnings Per Share, Basic and Diluted	$ 0.34	$ 0.67	$ 1.55	$ 1.19	$ 0.33	$ 0.69	$ 1.38	$ 1.00	$ 3.73	$ 3.38	$ 3.09
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income from discontinued operations, net of tax, before gain on disposal									$ 10,994	$ 4,562	$ 9,452
Gain on sale from discontinued operations, net of tax									$ 2,716	$ 0	$ 0
Income per share from discontinued operations									$ 0.13	$ 0.05	$ 0.09
Natural Gas Marketing Segment [Member]
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Operating revenues					$ 244,876	$ 176,704	$ 242,253	$ 231,468
Purchased gas cost					238,448	160,889	236,993	221,999
Operating income					184	8,768	(1,640)	3,476
Income from discontinued operations, net of tax, before gain on disposal	$ 0	$ 0	$ 0	$ 10,994	$ (610)	$ 5,050	$ (1,193)	$ 1,315
Gain on sale from discontinued operations, net of tax	$ 0	$ 0	$ 2,716	$ 0
Income per share from discontinued operations	$ 0.00	$ 0.00	$ 0.03	$ 0.11	$ 0.00	$ 0.05	$ (0.01)	$ 0.01
Intersegment Elimination [Member]
Operating Revenues by Segment [Line Items]
Operating revenues	$ (90,861)	$ (84,842)	$ (86,327)	$ (84,440)	$ (82,432)	$ (82,548)	$ (74,240)	$ (73,106)	$ (346,470)	$ (312,326)	$ (279,334)
Selected Quarterly Financial Information [Abstract]
Purchased gas cost									(346,426)	(312,326)	(279,334)
Operating income									0	0	0
Income from Continuing Operations, Net of Tax									0	0	0
Net income									0	0	0
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income from discontinued operations, net of tax, before gain on disposal									0	0	0
Gain on sale from discontinued operations, net of tax									0
Distribution Segment [Member]
Operating Revenues by Segment [Line Items]
Operating revenues	437,918	494,060	962,541	754,656	403,303	424,905	862,127	649,443	2,649,175	2,339,778	2,821,362
Selected Quarterly Financial Information [Abstract]
Purchased gas cost									1,269,456	1,058,576	1,574,447
Operating income									505,642	441,884	422,692
Income from Continuing Operations, Net of Tax									268,369	233,830	205,820
Net income									268,369	233,830	205,820
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income from discontinued operations, net of tax, before gain on disposal									0	0	0
Gain on sale from discontinued operations, net of tax									0
Pipeline and Storage Segment [Member]
Operating Revenues by Segment [Line Items]
Operating revenues	$ 117,823	$ 117,283	$ 111,972	$ 109,952	$ 112,772	$ 113,855	$ 102,153	$ 98,416	457,030	427,196	384,957
Selected Quarterly Financial Information [Abstract]
Purchased gas cost									2,506	(58)	562
Operating income									221,904	215,346	189,540
Income from Continuing Operations, Net of Tax									114,342	111,712	99,803
Net income									114,342	111,712	99,803
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income from discontinued operations, net of tax, before gain on disposal									0	$ 0	$ 0
Gain on sale from discontinued operations, net of tax									$ 0