CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ 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
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
Purchased gas cost
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
Operation and maintenance expense									546,798	538,592	516,406
Depreciation and amortization expense									319,448	290,791	272,408
Taxes, other than income									240,407	221,843	230,264
Operating income	91,792	140,664	285,172	209,918	84,449	128,396	251,656	192,729	727,546	657,230	612,232
Miscellaneous income (expense), net									(3,270)	(234)	(819)
Interest charges									120,182	114,812	116,241
Income from continuing operations before income taxes									604,094	542,184	495,172
Income tax expense									221,383	196,642	189,549
Income from continuing operations	35,853	70,808	162,012	114,038	34,850	66,143	143,003	101,546	382,711	345,542	305,623
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 from continuing operations	$ 35,853	$ 70,808	$ 164,728	$ 125,032	$ 34,240	$ 71,193	$ 141,810	$ 102,861	$ 396,421	$ 350,104	$ 315,075
Income per share from continuing operations	$ 0.34	$ 0.67	$ 1.52	$ 1.08	$ 0.33	$ 0.64	$ 1.39	$ 0.99	$ 3.60	$ 3.33	$ 3.00
Income per share from discontinued operations									0.13	0.05	0.09
Net income 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
Weighted Average Number of Shares Outstanding, Basic and Diluted									106,100	103,524	101,892
Distribution Segment [Member]
Operating revenues
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
Purchased gas cost
Purchased gas cost									1,269,456	1,058,576	1,574,447
Operation and maintenance expense									413,077	407,982	393,504
Depreciation and amortization expense									249,071	234,109	224,094
Taxes, other than income									211,929	197,227	206,625
Operating income									505,642	441,884	422,692
Miscellaneous income (expense), net									(1,695)	1,171	284
Interest charges									79,789	78,238	83,087
Income from continuing operations before income taxes									424,158	364,817	339,889
Income tax expense									155,789	130,987	134,069
Income from continuing operations									268,369	233,830	205,820
Income from discontinued operations, net of tax, before gain on disposal									0	0	0
Gain on sale from discontinued operations, net of tax									0
Income from continuing operations									268,369	233,830	205,820
Pipeline and Storage Segment [Member]
Operating revenues
Operating revenues	117,823	117,283	111,972	109,952	112,772	113,855	102,153	98,416	457,030	427,196	384,957
Purchased gas cost
Purchased gas cost									2,506	(58)	562
Operation and maintenance expense									133,765	130,610	122,902
Depreciation and amortization expense									70,377	56,682	48,314
Taxes, other than income									28,478	24,616	23,639
Operating income									221,904	215,346	189,540
Miscellaneous income (expense), net									(1,575)	(1,405)	(1,103)
Interest charges									40,393	36,574	33,154
Income from continuing operations before income taxes									179,936	177,367	155,283
Income tax expense									65,594	65,655	55,480
Income from continuing operations									114,342	111,712	99,803
Income from discontinued operations, net of tax, before gain on disposal									0	0	0
Gain on sale from discontinued operations, net of tax									0
Income from continuing operations									114,342	111,712	99,803
Intersegment Elimination [Member]
Operating revenues
Operating revenues	$ (90,861)	$ (84,842)	$ (86,327)	$ (84,440)	$ (82,432)	$ (82,548)	$ (74,240)	$ (73,106)	(346,470)	(312,326)	(279,334)
Purchased gas cost
Purchased gas cost									(346,426)	(312,326)	(279,334)
Operation and maintenance expense									(44)	0	0
Depreciation and amortization expense									0	0	0
Taxes, other than income									0	0	0
Operating income									0	0	0
Miscellaneous income (expense), net									0	0	0
Interest charges									0	0	0
Income from continuing operations before income taxes									0	0	0
Income tax expense									0	0	0
Income from continuing operations									0	0	0
Income from discontinued operations, net of tax, before gain on disposal									0	0	0
Gain on sale from discontinued operations, net of tax									0
Income from continuing operations									$ 0	$ 0	$ 0