Segment Information - Schedule of Income Statements and Capital Expenditures By Segment (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Operating revenues	$ 1,647,227		$ 1,540,973		$ 2,805,694	$ 3,024,982
Purchased gas cost	624,295		666,211		963,164	1,404,460
Operation and maintenance expense	199,899		194,716		366,244	379,732
Depreciation and amortization expense	165,087		148,317		329,695	294,337
Taxes, other than income	106,956		109,091		196,496	202,629
Operating income	550,990		422,638		950,095	743,824
Other non-operating income	16,687		17,406		34,573	38,597
Interest charges	55,442		37,370		107,317	74,130
Income before income taxes	512,235		402,674		877,351	708,291
Income tax expense	80,212		45,003		134,036	78,760
Net income	432,023	$ 311,292	357,671	$ 271,860	743,315	629,531
Capital expenditures	645,876		619,689		1,415,526	1,415,349
Distribution segment
Segment Reporting Information [Line Items]
Operating revenues	1,588,394		1,499,437		2,693,013	2,939,130
Pipeline and storage segment
Segment Reporting Information [Line Items]
Operating revenues	58,833		41,536		112,681	85,852
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(165,441)		(143,661)		(323,481)	(286,707)
Purchased gas cost	(165,188)		(143,433)		(322,985)	(286,241)
Operation and maintenance expense	(253)		(228)		(496)	(466)
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	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
Segment Reporting Information [Line Items]
Operating revenues	787		773		1,506	1,506
Eliminations | Pipeline and storage segment
Segment Reporting Information [Line Items]
Operating revenues	164,654		142,888		321,975	285,201
Operating Segments | Distribution segment
Segment Reporting Information [Line Items]
Operating revenues	1,589,181		1,500,210		2,694,519	2,940,636
Purchased gas cost	788,643		809,023		1,285,305	1,690,938
Operation and maintenance expense	154,956		151,353		282,571	287,822
Depreciation and amortization expense	121,384		106,310		241,069	211,974
Taxes, other than income	98,008		98,200		178,903	182,822
Operating income	426,190		335,324		706,671	567,080
Other non-operating income	9,359		7,465		15,198	14,239
Interest charges	36,784		21,420		71,365	44,259
Income before income taxes	398,765		321,369		650,504	537,060
Income tax expense	56,073		32,895		86,375	54,118
Net income	342,692		288,474		564,129	482,942
Capital expenditures	532,997		424,989		1,072,155	868,533
Operating Segments | Pipeline and storage segment
Segment Reporting Information [Line Items]
Operating revenues	223,487		184,424		434,656	371,053
Purchased gas cost	840		621		844	(237)
Operation and maintenance expense	45,196		43,591		84,169	92,376
Depreciation and amortization expense	43,703		42,007		88,626	82,363
Taxes, other than income	8,948		10,891		17,593	19,807
Operating income	124,800		87,314		243,424	176,744
Other non-operating income	7,328		9,941		19,375	24,358
Interest charges	18,658		15,950		35,952	29,871
Income before income taxes	113,470		81,305		226,847	171,231
Income tax expense	24,139		12,108		47,661	24,642
Net income	89,331		69,197		179,186	146,589
Capital expenditures	$ 112,879		$ 194,700		$ 343,371	$ 546,816