Segment Information - Income Statements and Capital Expenditures By Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Operating revenues	$ 4,201,662	$ 3,407,490	$ 2,821,137
Purchased gas cost	1,682,656	1,032,717	658,854
Operation and maintenance expense	710,161	679,019	629,601
Depreciation and amortization expense	535,655	477,977	429,828
Taxes, other than income	352,208	312,779	278,755
Operating income	920,982	904,998	824,099
Other non-operating income (expense)	33,737	(2,145)	7,171
Interest charges	102,811	83,554	84,474
Income before income taxes	851,908	819,299	746,796
Income tax expense	77,510	153,736	145,353
Net income	774,398	665,563	601,443
Capital expenditures	2,444,420	1,969,540	1,935,676
Distribution
Segment Reporting Information [Line Items]
Operating revenues	4,031,936	3,238,753	2,624,251
Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	169,726	168,737	196,886
Operating Segments | Distribution
Segment Reporting Information [Line Items]
Operating revenues	4,035,194	3,241,973	2,626,993
Purchased gas cost	2,210,302	1,501,695	1,071,227
Operation and maintenance expense	518,443	501,209	472,760
Depreciation and amortization expense	387,858	345,481	309,582
Taxes, other than income	314,046	275,074	245,181
Operating income	604,545	618,514	528,243
Other non-operating income (expense)	6,946	(20,694)	(1,265)
Interest charges	49,921	36,629	39,634
Income before income taxes	561,570	561,191	487,344
Income tax expense	39,593	115,329	91,680
Net income	521,977	445,862	395,664
Capital expenditures	1,675,798	1,454,195	1,466,631
Operating Segments | Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	693,660	637,347	609,339
Purchased gas cost	(1,583)	1,582	1,548
Operation and maintenance expense	192,847	179,080	158,115
Depreciation and amortization expense	147,797	132,496	120,246
Taxes, other than income	38,162	37,705	33,574
Operating income	316,437	286,484	295,856
Other non-operating income (expense)	26,791	18,549	8,436
Interest charges	52,890	46,925	44,840
Income before income taxes	290,338	258,108	259,452
Income tax expense	37,917	38,407	53,673
Net income	252,421	219,701	205,779
Capital expenditures	768,622	515,345	469,045
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(527,192)	(471,830)	(415,195)
Purchased gas cost	(526,063)	(470,560)	(413,921)
Operation and maintenance expense	(1,129)	(1,270)	(1,274)
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]
Operating revenues	3,258	3,220	2,742
Eliminations | Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	$ 523,934	$ 468,610	$ 412,453