Segment Information - Income Statements and Capital Expenditures By Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Total operating revenues	$ 3,407,490	$ 2,821,137	$ 2,901,848
Total purchased gas cost	1,032,717	658,854	858,837
Operation and maintenance expense	679,019	629,601	630,308
Depreciation and amortization expense	477,977	429,828	391,456
Taxes, other than income	312,779	278,755	275,189
Operating income	904,998	824,099	746,058
Other non-operating income (expense)	(2,145)	7,171	7,404
Interest charges	83,554	84,474	103,153
Income before income taxes	819,299	746,796	650,309
Income tax expense	153,736	145,353	138,903
Net Income	665,563	601,443	511,406
Capital expenditures	1,969,540	1,935,676	1,693,477
Distribution
Segment Reporting Information [Line Items]
Total operating revenues	3,238,753	2,624,251	2,742,824
Pipeline and Storage
Segment Reporting Information [Line Items]
Total operating revenues	168,737	196,886	159,024
Operating Segments | Distribution
Segment Reporting Information [Line Items]
Total operating revenues	3,241,973	2,626,993	2,745,461
Total purchased gas cost	1,501,695	1,071,227	1,268,591
Operation and maintenance expense	501,209	472,760	480,222
Depreciation and amortization expense	345,481	309,582	283,697
Taxes, other than income	275,074	245,181	242,179
Operating income	618,514	528,243	470,772
Other non-operating income (expense)	(20,694)	(1,265)	6,241
Interest charges	36,629	39,634	60,031
Income before income taxes	561,191	487,344	416,982
Income tax expense	115,329	91,680	88,168
Net Income	445,862	395,664	328,814
Capital expenditures	1,454,195	1,466,631	1,274,613
Operating Segments | Pipeline and Storage
Segment Reporting Information [Line Items]
Total operating revenues	637,347	609,339	567,024
Total purchased gas cost	1,582	1,548	(360)
Operation and maintenance expense	179,080	158,115	151,329
Depreciation and amortization expense	132,496	120,246	107,759
Taxes, other than income	37,705	33,574	33,010
Operating income	286,484	295,856	275,286
Other non-operating income (expense)	18,549	8,436	1,163
Interest charges	46,925	44,840	43,122
Income before income taxes	258,108	259,452	233,327
Income tax expense	38,407	53,673	50,735
Net Income	219,701	205,779	182,592
Capital expenditures	515,345	469,045	418,864
Eliminations
Segment Reporting Information [Line Items]
Total operating revenues	(471,830)	(415,195)	(410,637)
Total purchased gas cost	(470,560)	(413,921)	(409,394)
Operation and maintenance expense	(1,270)	(1,274)	(1,243)
Depreciation and amortization expense	0	0	0
Taxes, other than income	0	0	0
Operating income	0	0	0
Other non-operating income (expense)	0	0	0
Interest charges	0	0	0
Income before income taxes	0	0	0
Income tax expense	0	0	0
Net Income	0	0	0
Capital expenditures	0	0	0
Eliminations | Distribution
Segment Reporting Information [Line Items]
Total operating revenues	3,220	2,742	2,637
Eliminations | Pipeline and Storage
Segment Reporting Information [Line Items]
Total operating revenues	$ 468,610	$ 412,453	$ 408,000