Segment Information - Schedule of Income Statements and Capital Expenditures By Segment (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Operating revenues	$ 701,549			$ 662,733			$ 3,507,243	$ 3,687,715
Purchased gas cost	(2,593)			44,550			960,571	1,449,010
Operation and maintenance expense	211,309			195,049			577,553	574,781
Depreciation and amortization expense	166,827			150,726			496,522	445,063
Taxes, other than income	105,739			103,155			302,235	305,784
Operating income	220,267			169,253			1,170,362	913,077
Other non-operating income	19,898			16,170			54,471	54,767
Interest charges	41,160			31,334			148,477	105,464
Income before income taxes	199,005			154,089			1,076,356	862,380
Income tax expense	33,441			16,282			167,477	95,042
Net income	165,564	$ 432,023	$ 311,292	137,807	$ 357,671	$ 271,860	908,879	767,338
Capital expenditures	713,611			668,137			2,129,137	2,083,486
Distribution
Segment Reporting Information [Line Items]
Operating revenues	632,446			615,259			3,325,459	3,554,389
Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	69,103			47,474			181,784	133,326
Eliminations
Segment Reporting Information [Line Items]
Operating revenues	(182,342)			(161,559)			(505,823)	(448,266)
Purchased gas cost	(182,084)			(161,304)			(505,069)	(447,545)
Operation and maintenance expense	(258)			(255)			(754)	(721)
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	(765)			(808)			(2,271)	(2,314)
Eliminations | Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	(181,577)			(160,751)			(503,552)	(445,952)
Operating Segments | Distribution
Segment Reporting Information [Line Items]
Operating revenues	633,211			616,067			3,327,730	3,556,703
Purchased gas cost	179,510			206,048			1,464,815	1,896,986
Operation and maintenance expense	153,140			138,351			435,711	426,173
Depreciation and amortization expense	122,912			107,809			363,981	319,783
Taxes, other than income	94,527			92,180			273,430	275,002
Operating income	83,122			71,679			789,793	638,759
Other non-operating income	11,452			6,695			26,650	20,934
Interest charges	22,550			16,146			93,915	60,405
Income before income taxes	72,024			62,228			722,528	599,288
Income tax expense	6,175			2,589			92,550	56,707
Net income	65,849			59,639			629,978	542,581
Capital expenditures	587,062			512,585			1,659,217	1,381,118
Operating Segments | Pipeline and Storage
Segment Reporting Information [Line Items]
Operating revenues	250,680			208,225			685,336	579,278
Purchased gas cost	(19)			(194)			825	(431)
Operation and maintenance expense	58,427			56,953			142,596	149,329
Depreciation and amortization expense	43,915			42,917			132,541	125,280
Taxes, other than income	11,212			10,975			28,805	30,782
Operating income	137,145			97,574			380,569	274,318
Other non-operating income	8,446			9,475			27,821	33,833
Interest charges	18,610			15,188			54,562	45,059
Income before income taxes	126,981			91,861			353,828	263,092
Income tax expense	27,266			13,693			74,927	38,335
Net income	99,715			78,168			278,901	224,757
Capital expenditures	$ 126,549			$ 155,552			$ 469,920	$ 702,368