Segment Information - Income Statements and Capital Expenditures By Segment (Details) - USD ($) $ 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
Segment Reporting Information [Line Items]
Total operating revenues									$ 3,115,546	$ 2,759,735	$ 2,454,648
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 (benefit)									(5,344)	(3,270)	(234)
Interest charges									106,646	120,182	114,812
Income before income taxes									611,144	604,094	542,184
Income tax (benefit) expense									8,080	221,383	196,642
Net Income	38,747	71,193	178,992	314,132	35,853	70,808	164,728	125,032	603,064	396,421	350,104
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	10,994	4,562
Gain on sale of discontinued operations, net of tax					0	0	2,716	0	0	2,716	0
Capital expenditures									1,467,591	1,137,089	1,086,950
Natural Gas, US Regulated
Segment Reporting Information [Line Items]
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
Reportable Subsegments
Segment Reporting Information [Line Items]
Total operating revenues									3,115,546	2,759,735	2,454,648
Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									0	0	0
Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
Total operating revenues									3,000,404	2,647,813	2,338,404
Distribution | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									2,643	1,362	1,374
Pipeline and Storage | Reportable Subsegments
Segment Reporting Information [Line Items]
Total operating revenues									115,142	111,922	116,244
Pipeline and Storage | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									392,571	345,108	310,952
Natural Gas Marketing | Reportable Subsegments
Segment Reporting Information [Line Items]
Total operating revenues										0	0
Natural Gas Marketing | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues										0	0
Operating Segments | Distribution
Segment Reporting Information [Line Items]
Total operating revenues									3,003,047	2,649,175	2,339,778
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 (benefit)									(1,849)	(1,695)	1,171
Interest charges									65,850	79,789	78,238
Income before income taxes									413,168	424,158	364,817
Income tax (benefit) expense									(29,798)	155,789	130,987
Net Income									442,966	268,369	233,830
Income from continuing operations										268,369	233,830
Income from discontinued operations, net of tax										0	0
Gain on sale of discontinued operations, net of tax										0
Capital expenditures									1,025,800	849,950	740,246
Operating Segments | Distribution | Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Total purchased gas cost									1,559,836	1,269,456	1,058,576
Operating Segments | Pipeline and Storage
Segment Reporting Information [Line Items]
Total operating revenues									507,713	457,030	427,196
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 (benefit)									(3,495)	(1,575)	(1,405)
Interest charges									40,796	40,393	36,574
Income before income taxes									197,976	179,936	177,367
Income tax (benefit) expense									37,878	65,594	65,655
Net Income									160,098	114,342	111,712
Income from continuing operations										114,342	111,712
Income from discontinued operations, net of tax										0	0
Gain on sale of discontinued operations, net of tax										0
Capital expenditures									441,791	287,139	346,383
Operating Segments | Pipeline and Storage | Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Total purchased gas cost									1,978	2,506	(58)
Operating Segments | Natural Gas Marketing
Segment Reporting Information [Line Items]
Total operating revenues										0	0
Operation and maintenance expense										0	0
Depreciation and amortization expense										0	0
Taxes, other than income										0	0
Operating income										0	0
Miscellaneous expense (benefit)										0	0
Interest charges										0	0
Income before income taxes										0	0
Income tax (benefit) expense										0	0
Net Income										13,710	4,562
Income from continuing operations										0	0
Income from discontinued operations, net of tax										10,994	4,562
Gain on sale of discontinued operations, net of tax										2,716
Capital expenditures										0	321
Operating Segments | Natural Gas Marketing | Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Total purchased gas cost										0	0
Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									(395,214)	(346,470)	(312,326)
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 (benefit)									0	0	0
Interest charges									0	0	0
Income before income taxes									0	0	0
Income tax (benefit) expense									0	0	0
Net Income									0	0	0
Income from continuing operations										0	0
Income from discontinued operations, net of tax										0	0
Gain on sale of discontinued operations, net of tax										0
Capital expenditures									0	0	0
Eliminations | Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Total purchased gas cost									(393,966)	(346,426)	(312,326)
Eliminations | Reportable Subsegments
Segment Reporting Information [Line Items]
Total operating revenues									0	0	0
Eliminations | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									$ (395,214)	$ (346,470)	$ (312,326)