Segment Information - Income Statements and Capital Expenditures By Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Operating revenues	$ 4,275,357	$ 4,201,662	$ 3,407,490
Purchased gas cost	1,452,173	1,682,656	1,032,717
Operation and maintenance expense	764,906	710,161	679,019
Depreciation and amortization expense	604,327	535,655	477,977
Taxes, other than income	386,804	352,208	312,779
Operating income	1,067,147	920,982	904,998
Other non-operating income (expense)	69,775	33,737	(2,145)
Interest charges	137,281	102,811	83,554
Income before income taxes	999,641	851,908	819,299
Income tax expense	113,779	77,510	153,736
Net income	885,862	774,398	665,563
Capital expenditures	2,805,973	2,444,420	1,969,540
Distribution
Segment Reporting Information [Line Items]
Operating revenues	4,096,661	4,031,936	3,238,753
Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	178,696	169,726	168,737
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(609,507)	(527,192)	(471,830)
Purchased gas cost	(608,527)	(526,063)	(470,560)
Operation and maintenance expense	(980)	(1,129)	(1,270)
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	3,029	3,258	3,220
Eliminations | Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	606,478	523,934	468,610
Operating Segments | Distribution
Segment Reporting Information [Line Items]
Operating revenues	4,099,690	4,035,194	3,241,973
Purchased gas cost	2,061,920	2,210,302	1,501,695
Operation and maintenance expense	565,179	518,443	501,209
Depreciation and amortization expense	434,721	387,858	345,481
Taxes, other than income	345,244	314,046	275,074
Operating income	692,626	604,545	618,514
Other non-operating income (expense)	24,988	6,946	(20,694)
Interest charges	77,185	49,921	36,629
Income before income taxes	640,429	561,570	561,191
Income tax expense	60,032	39,593	115,329
Net income	580,397	521,977	445,862
Capital expenditures	1,927,125	1,675,798	1,454,195
Operating Segments | Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	785,174	693,660	637,347
Purchased gas cost	(1,220)	(1,583)	1,582
Operation and maintenance expense	200,707	192,847	179,080
Depreciation and amortization expense	169,606	147,797	132,496
Taxes, other than income	41,560	38,162	37,705
Operating income	374,521	316,437	286,484
Other non-operating income (expense)	44,787	26,791	18,549
Interest charges	60,096	52,890	46,925
Income before income taxes	359,212	290,338	258,108
Income tax expense	53,747	37,917	38,407
Net income	305,465	252,421	219,701
Capital expenditures	$ 878,848	$ 768,622	$ 515,345