CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Operating revenues
Total operating revenues									$ 3,115,546	$ 2,759,735	$ 2,454,648
Purchased gas cost
Operation and maintenance expense									599,595	546,798	538,592
Depreciation and amortization expense									361,083	319,448	290,791
Taxes, other than income									263,886	240,407	221,843
Operating income	$ 89,592	$ 122,993	$ 268,988	$ 241,561	$ 91,792	$ 140,664	$ 285,172	$ 209,918	723,134	727,546	657,230
Miscellaneous expense									(5,344)	(3,270)	(234)
Interest charges									106,646	120,182	114,812
Income from continuing operations before income taxes									611,144	604,094	542,184
Income tax expense									8,080	221,383	196,642
Income from continuing operations					35,853	70,808	162,012	114,038	603,064	382,711	345,542
Income from discontinued operations, net of tax ($0, $6,841 and $3,731)									0	10,994	4,562
Gain on sale of discontinued operations, net of tax ($0, $10,215 and $0)					0	0	2,716	0	0	2,716	0
Net Income	$ 38,747	$ 71,193	$ 178,992	$ 314,132	$ 35,853	$ 70,808	$ 164,728	$ 125,032	$ 603,064	$ 396,421	$ 350,104
Earnings Per Share, Basic and Diluted [Abstract]
Income per share from continuing operations (in dollars per share)					$ 0.34	$ 0.67	$ 1.52	$ 1.08	$ 5.43	$ 3.60	$ 3.33
Income per share from discontinued operations (in dollars per share)					0.00	0.00	0.03	0.11	0.00	0.13	0.05
Net income per share - basic and diluted (in dollars per share)	$ 0.35	$ 0.64	$ 1.60	$ 2.89	$ 0.34	$ 0.67	$ 1.55	$ 1.19	$ 5.43	$ 3.73	$ 3.38
Basic and diluted weighted average shares outstanding (in shares)									111,012	106,100	103,524
Natural Gas, US Regulated
Purchased gas cost
Total purchased gas cost	$ 43,085	$ 130,886	$ 626,960	$ 366,917	$ 72,561	$ 114,176	$ 427,494	$ 311,305	$ 1,167,848	$ 925,536	$ 746,192
Operating Segments | Distribution
Operating revenues
Total operating revenues									3,003,047	2,649,175	2,339,778
Purchased gas cost
Operation and maintenance expense									465,848	413,077	407,982
Depreciation and amortization expense									264,930	249,071	234,109
Taxes, other than income									231,566	211,929	197,227
Operating income									480,867	505,642	441,884
Miscellaneous expense									(1,849)	(1,695)	1,171
Interest charges									65,850	79,789	78,238
Income from continuing operations before income taxes									413,168	424,158	364,817
Income tax expense									(29,798)	155,789	130,987
Income from continuing operations										268,369	233,830
Income from discontinued operations, net of tax ($0, $6,841 and $3,731)										0	0
Gain on sale of discontinued operations, net of tax ($0, $10,215 and $0)										0
Net Income									442,966	268,369	233,830
Operating Segments | Distribution | Natural Gas, US Regulated
Purchased gas cost
Total purchased gas cost									1,559,836	1,269,456	1,058,576
Operating Segments | Pipeline and Storage
Operating revenues
Total operating revenues									507,713	457,030	427,196
Purchased gas cost
Operation and maintenance expense									134,995	133,765	130,610
Depreciation and amortization expense									96,153	70,377	56,682
Taxes, other than income									32,320	28,478	24,616
Operating income									242,267	221,904	215,346
Miscellaneous expense									(3,495)	(1,575)	(1,405)
Interest charges									40,796	40,393	36,574
Income from continuing operations before income taxes									197,976	179,936	177,367
Income tax expense									37,878	65,594	65,655
Income from continuing operations										114,342	111,712
Income from discontinued operations, net of tax ($0, $6,841 and $3,731)										0	0
Gain on sale of discontinued operations, net of tax ($0, $10,215 and $0)										0
Net Income									160,098	114,342	111,712
Operating Segments | Pipeline and Storage | Natural Gas, US Regulated
Purchased gas cost
Total purchased gas cost									1,978	2,506	(58)
Eliminations
Operating revenues
Total operating revenues									(395,214)	(346,470)	(312,326)
Purchased gas cost
Operation and maintenance expense									(1,248)	(44)	0
Depreciation and amortization expense									0	0	0
Taxes, other than income									0	0	0
Operating income									0	0	0
Miscellaneous expense									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
Income from discontinued operations, net of tax ($0, $6,841 and $3,731)										0	0
Gain on sale of discontinued operations, net of tax ($0, $10,215 and $0)										0
Net Income									0	0	0
Eliminations | Natural Gas, US Regulated
Purchased gas cost
Total purchased gas cost									$ (393,966)	$ (346,426)	$ (312,326)