Segment Information - Income Statements and Capital Expenditures By Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Total operating revenues									$ 2,901,848	$ 3,115,546	$ 2,759,735
Operation and maintenance expense									630,308	594,795	538,716
Depreciation and amortization expense									391,456	361,083	319,448
Taxes, other than income									275,189	263,886	240,407
Operating income	$ 89,715	$ 122,202	$ 297,677	$ 236,464	$ 90,629	$ 124,320	$ 270,902	$ 242,083	746,058	727,934	735,628
Miscellaneous expense (benefit)									7,404	(10,144)	(11,352)
Interest charges									103,153	106,646	120,182
Income from continuing operations before income taxes									650,309	611,144	604,094
Income tax (benefit) expense									138,903	8,080	221,383
Net Income	58,406	80,466	214,888	157,646	38,747	71,193	178,992	314,132	511,406	603,064	396,421
Income from continuing operations									511,406	603,064	382,711
Income from discontinued operations, net of tax									0	0	10,994
Gain on sale of discontinued operations, net of tax									0	0	2,716
Capital expenditures									1,693,477	1,467,591	1,137,089
Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Total purchased gas cost	$ 13,670	$ 31,326	$ 471,676	$ 342,165	$ 43,085	$ 130,886	$ 626,960	$ 366,917	858,837	1,167,848	925,536
Reportable Subsegments
Segment Reporting Information [Line Items]
Total operating revenues									2,901,848	3,115,546	2,759,735
Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									0	0	0
Distribution | Reportable Subsegments
Segment Reporting Information [Line Items]
Total operating revenues									2,742,824	3,000,404	2,647,813
Distribution | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									2,637	2,643	1,362
Pipeline and Storage | Reportable Subsegments
Segment Reporting Information [Line Items]
Total operating revenues									159,024	115,142	111,922
Pipeline and Storage | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									408,000	392,571	345,108
Natural Gas Marketing | Reportable Subsegments
Segment Reporting Information [Line Items]
Total operating revenues											0
Natural Gas Marketing | Intersubsegment Eliminations
Segment Reporting Information [Line Items]
Total operating revenues											0
Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									(410,637)
Operating Segments | Distribution
Segment Reporting Information [Line Items]
Total operating revenues									2,745,461	3,003,047	2,649,175
Operation and maintenance expense									480,222	461,048	404,995
Depreciation and amortization expense									283,697	264,930	249,071
Taxes, other than income									242,179	231,566	211,929
Operating income									470,772	485,667	513,724
Miscellaneous expense (benefit)									6,241	(6,649)	(9,777)
Interest charges									60,031	65,850	79,789
Income from continuing operations before income taxes									416,982	413,168	424,158
Income tax (benefit) expense									88,168	(29,798)	155,789
Net Income									328,814	442,966	268,369
Income from continuing operations											268,369
Income from discontinued operations, net of tax											0
Gain on sale of discontinued operations, net of tax											0
Capital expenditures									1,274,613	1,025,800	849,950
Operating Segments | Distribution | Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Total purchased gas cost									1,268,591	1,559,836	1,269,456
Operating Segments | Pipeline and Storage
Segment Reporting Information [Line Items]
Total operating revenues									567,024	507,713	457,030
Operation and maintenance expense									151,329	134,995	133,765
Depreciation and amortization expense									107,759	96,153	70,377
Taxes, other than income									33,010	32,320	28,478
Operating income									275,286	242,267	221,904
Miscellaneous expense (benefit)									1,163	(3,495)	(1,575)
Interest charges									43,122	40,796	40,393
Income from continuing operations before income taxes									233,327	197,976	179,936
Income tax (benefit) expense									50,735	37,878	65,594
Net Income									182,592	160,098	114,342
Income from continuing operations											114,342
Income from discontinued operations, net of tax											0
Gain on sale of discontinued operations, net of tax											0
Capital expenditures									418,864	441,791	287,139
Operating Segments | Pipeline and Storage | Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Total purchased gas cost									(360)	1,978	2,506
Operating Segments | Natural Gas Marketing
Segment Reporting Information [Line Items]
Total operating revenues											0
Operation and maintenance expense											0
Depreciation and amortization expense											0
Taxes, other than income											0
Operating income											0
Miscellaneous expense (benefit)											0
Interest charges											0
Income from continuing operations before income taxes											0
Income tax (benefit) expense											0
Net Income											13,710
Income from continuing operations											0
Income from discontinued operations, net of tax											10,994
Gain on sale of discontinued operations, net of tax											2,716
Capital expenditures											0
Operating Segments | Natural Gas Marketing | Natural Gas, US Regulated
Segment Reporting Information [Line Items]
Total purchased gas cost											0
Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									(410,637)	(395,214)	(346,470)
Operation and maintenance expense									(1,243)	(1,248)	(44)
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 from continuing operations before income taxes									0	0	0
Income tax (benefit) expense									0	0	0
Net Income									0	0	0
Income from continuing operations											0
Income from discontinued operations, net of tax											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									(409,394)	(393,966)	(346,426)
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									$ (410,637)	$ (395,214)	$ (346,470)