Segment Information - Schedule of Income Statements and Capital Expenditures By Segment (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Operating revenues	$ 1,950,502		$ 1,647,227		$ 3,126,501	$ 2,805,694
Purchased gas cost	779,233		624,295		1,013,281	963,164
Operation and maintenance expense	233,296		199,899		440,340	366,244
Depreciation and amortization expense	182,750		165,087		363,283	329,695
Taxes, other than income	126,284		106,956		221,178	196,496
Operating income	628,939		550,990		1,088,419	950,095
Other non-operating income (expense)	24,172		16,687		48,806	34,573
Interest charges	50,014		55,442		102,939	107,317
Income before income taxes	603,097		512,235		1,034,286	877,351
Income tax expense	117,521		80,212		196,852	134,036
Net income	485,576	$ 351,858	432,023	$ 311,292	837,434	743,315
Capital expenditures	839,666		645,876		1,730,857	1,415,526
Distribution
Segment Reporting Information [Line Items]
Operating revenues	1,881,742		1,588,394		2,990,311	2,693,013
Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	68,760		58,833		136,190	112,681
Operating Segments | Distribution
Segment Reporting Information [Line Items]
Operating revenues	1,882,528		1,589,181		2,991,863	2,694,519
Purchased gas cost	969,037		788,643		1,391,607	1,285,305
Operation and maintenance expense	181,887		154,956		336,401	282,571
Depreciation and amortization expense	134,546		121,384		268,173	241,069
Taxes, other than income	113,340		98,008		195,916	178,903
Operating income	483,718		426,190		799,766	706,671
Other non-operating income (expense)	13,444		9,359		23,528	15,198
Interest charges	30,087		36,784		64,336	71,365
Income before income taxes	467,075		398,765		758,958	650,504
Income tax expense	86,432		56,073		138,102	86,375
Net income	380,643		342,692		620,856	564,129
Capital expenditures	594,853		532,997		1,220,502	1,072,155
Operating Segments | Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	258,999		223,487		514,389	434,656
Purchased gas cost	968		840		910	844
Operation and maintenance expense	51,662		45,196		104,454	84,169
Depreciation and amortization expense	48,204		43,703		95,110	88,626
Taxes, other than income	12,944		8,948		25,262	17,593
Operating income	145,221		124,800		288,653	243,424
Other non-operating income (expense)	10,728		7,328		25,278	19,375
Interest charges	19,927		18,658		38,603	35,952
Income before income taxes	136,022		113,470		275,328	226,847
Income tax expense	31,089		24,139		58,750	47,661
Net income	104,933		89,331		216,578	179,186
Capital expenditures	244,813		112,879		510,355	343,371
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(191,025)		(165,441)		(379,751)	(323,481)
Purchased gas cost	(190,772)		(165,188)		(379,236)	(322,985)
Operation and maintenance expense	(253)		(253)		(515)	(496)
Depreciation and amortization expense	0		0		0	0
Taxes, other than income	0		0		0	0
Operating income	0		0		0	0
Other non-operating income (expense)	0		0		0	0
Interest charges	0		0		0	0
Income before income taxes	0		0		0	0
Income tax expense	0		0		0	0
Net income	0		0		0	0
Capital expenditures	0		0		0	0
Eliminations | Distribution
Segment Reporting Information [Line Items]
Operating revenues	(786)		(787)		(1,552)	(1,506)
Eliminations | Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	$ (190,239)		$ (164,654)		$ (378,199)	$ (321,975)