Segment Information - Income Statements and Capital Expenditures By Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total operating revenues									$ 2,821,137	$ 2,901,848	$ 3,115,546
Total purchased gas cost									658,854	858,837	1,167,848
Operation and maintenance expense									629,601	630,308	594,795
Depreciation and amortization expense									429,828	391,456	361,083
Taxes, other than income									278,755	275,189	263,886
Operating income	$ 100,845	$ 139,035	$ 331,438	$ 252,781	$ 89,715	$ 122,202	$ 297,677	$ 236,464	824,099	746,058	727,934
Miscellaneous expense (benefit)									7,171	7,404	(10,144)
Interest charges									84,474	103,153	106,646
Income from continuing operations before income taxes									746,796	650,309	611,144
Income tax (benefit) expense									145,353	138,903	8,080
Net Income									601,443	511,406	603,064
Capital expenditures									1,935,676	1,693,477	1,467,591
Distribution
Segment Reporting Information [Line Items]
Total operating revenues									2,624,251	2,742,824	3,000,404
Pipeline and Storage
Segment Reporting Information [Line Items]
Total operating revenues									196,886	159,024	115,142
Operating Segments | Distribution
Segment Reporting Information [Line Items]
Total operating revenues									2,626,993	2,745,461	3,003,047
Total purchased gas cost									1,071,227	1,268,591	1,559,836
Operation and maintenance expense									472,760	480,222	461,048
Depreciation and amortization expense									309,582	283,697	264,930
Taxes, other than income									245,181	242,179	231,566
Operating income									528,243	470,772	485,667
Miscellaneous expense (benefit)									(1,265)	6,241	(6,649)
Interest charges									39,634	60,031	65,850
Income from continuing operations before income taxes									487,344	416,982	413,168
Income tax (benefit) expense									91,680	88,168	(29,798)
Net Income									395,664	328,814	442,966
Capital expenditures									1,466,631	1,274,613	1,025,800
Operating Segments | Pipeline and Storage
Segment Reporting Information [Line Items]
Total operating revenues									609,339	567,024	507,713
Total purchased gas cost									1,548	(360)	1,978
Operation and maintenance expense									158,115	151,329	134,995
Depreciation and amortization expense									120,246	107,759	96,153
Taxes, other than income									33,574	33,010	32,320
Operating income									295,856	275,286	242,267
Miscellaneous expense (benefit)									8,436	1,163	(3,495)
Interest charges									44,840	43,122	40,796
Income from continuing operations before income taxes									259,452	233,327	197,976
Income tax (benefit) expense									53,673	50,735	37,878
Net Income									205,779	182,592	160,098
Capital expenditures									469,045	418,864	441,791
Eliminations
Segment Reporting Information [Line Items]
Total operating revenues									(415,195)	(410,637)	(395,214)
Total purchased gas cost									(413,921)	(409,394)	(393,966)
Operation and maintenance expense									(1,274)	(1,243)	(1,248)
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
Capital expenditures									0	0	0
Eliminations | Distribution
Segment Reporting Information [Line Items]
Total operating revenues									2,742	2,637	2,643
Eliminations | Pipeline and Storage
Segment Reporting Information [Line Items]
Total operating revenues									$ 412,453	$ 408,000	$ 392,571