Segment Information - Schedule of Income Statements and Capital Expenditures By Segment (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting Information [Line Items]
Operating revenues	$ 838,774			$ 701,549			$ 3,965,275	$ 3,507,243
Purchased gas cost	53,840			(2,593)			1,067,121	960,571
Operation and maintenance expense	222,100			211,309			662,440	577,553
Depreciation and amortization expense	185,786			166,827			549,069	496,522
Taxes, other than income	124,981			105,739			346,159	302,235
Operating income	252,067			220,267			1,340,486	1,170,362
Other non-operating income	20,100			19,898			68,906	54,471
Interest charges	41,537			41,160			144,476	148,477
Income before income taxes	230,630			199,005			1,264,916	1,076,356
Income tax expense	44,201			33,441			241,053	167,477
Net income	186,429	$ 485,576	$ 351,858	165,564	$ 432,023	$ 311,292	1,023,863	908,879
Capital expenditures	866,930			713,611			2,597,787	2,129,137
Distribution
Segment Reporting Information [Line Items]
Operating revenues	766,380			632,446			3,756,691	3,325,459
Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	72,394			69,103			208,584	181,784
Operating Segments | Distribution
Segment Reporting Information [Line Items]
Operating revenues	767,132			633,211			3,758,995	3,327,730
Purchased gas cost	255,883			179,510			1,647,490	1,464,815
Operation and maintenance expense	166,744			153,140			503,145	435,711
Depreciation and amortization expense	137,106			122,912			405,279	363,981
Taxes, other than income	111,933			94,527			307,849	273,430
Operating income	95,466			83,122			895,232	789,793
Other non-operating income	7,017			11,452			30,545	26,650
Interest charges	22,271			22,550			86,607	93,915
Income before income taxes	80,212			72,024			839,170	722,528
Income tax expense	9,731			6,175			147,833	92,550
Net income	70,481			65,849			691,337	629,978
Capital expenditures	669,452			587,062			1,889,954	1,659,217
Operating Segments | Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	272,388			250,680			786,777	685,336
Purchased gas cost	(1,548)			(19)			(638)	825
Operation and maintenance expense	55,607			58,427			160,061	142,596
Depreciation and amortization expense	48,680			43,915			143,790	132,541
Taxes, other than income	13,048			11,212			38,310	28,805
Operating income	156,601			137,145			445,254	380,569
Other non-operating income	13,083			8,446			38,361	27,821
Interest charges	19,266			18,610			57,869	54,562
Income before income taxes	150,418			126,981			425,746	353,828
Income tax expense	34,470			27,266			93,220	74,927
Net income	115,948			99,715			332,526	278,901
Capital expenditures	197,478			126,549			707,833	469,920
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(200,746)			(182,342)			(580,497)	(505,823)
Purchased gas cost	(200,495)			(182,084)			(579,731)	(505,069)
Operation and maintenance expense	(251)			(258)			(766)	(754)
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 Reporting Information [Line Items]
Operating revenues	(752)			(765)			(2,304)	(2,271)
Eliminations | Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	$ (199,994)			$ (181,577)			$ (578,193)	$ (503,552)