Segment Information (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2023 USD ($) state	Dec. 31, 2022 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Mar. 31, 2023 USD ($) state	Mar. 31, 2022 USD ($)	Sep. 30, 2022 USD ($)
Segment Reporting Information [Line Items]
Number of states with service areas | state	8				8
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 1,540,973		$ 1,649,819		$ 3,024,982	$ 2,662,605
Purchased gas cost	666,211		871,378		1,404,460	1,242,541
Operation and maintenance expense	194,716		163,352		379,732	322,462
Depreciation and amortization expense	148,317		133,374		294,337	261,230
Taxes, other than income	109,091		96,583		202,629	175,379
Operating income	422,638		385,132		743,824	660,993
Other non-operating income	17,406		5,213		38,597	13,915
Interest charges	37,370		28,928		74,130	48,779
Income before income taxes	402,674		361,417		708,291	626,129
Income tax expense	45,003		36,418		78,760	51,921
Net income	357,671	$ 271,860	324,999	$ 249,209	629,531	574,208
Capital expenditures	619,689		505,849		1,415,349	1,190,029
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	18,445,886				18,445,886		$ 17,240,239
Total assets	21,311,861				21,311,861		22,192,989
Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	1,499,437		1,609,667		2,939,130	2,581,303
Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	41,536		40,152		85,852	81,302
Eliminations
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(143,661)		(124,474)		(286,707)	(247,028)
Purchased gas cost	(143,433)		(124,159)		(286,241)	(246,384)
Operation and maintenance expense	(228)		(315)		(466)	(644)
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
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	0				0		0
Total assets	(4,480,439)				(4,480,439)		(4,028,803)
Eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	773		879		1,506	1,665
Eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	142,888		123,595		285,201	245,363
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	1,500,210		1,610,546		2,940,636	2,582,968
Purchased gas cost	809,023		993,854		1,690,938	1,490,653
Operation and maintenance expense	151,353		121,541		287,822	244,825
Depreciation and amortization expense	106,310		96,612		211,974	189,409
Taxes, other than income	98,200		87,236		182,822	156,281
Operating income	335,324		311,303		567,080	501,800
Other non-operating income	7,465		549		14,239	2,465
Interest charges	21,420		15,157		44,259	23,705
Income before income taxes	321,369		296,695		537,060	480,560
Income tax expense	32,895		27,844		54,118	32,138
Net income	288,474		268,851		482,942	448,422
Capital expenditures	424,989		362,468		868,533	799,850
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	13,495,410				13,495,410		12,723,532
Total assets	20,565,390				20,565,390		21,424,586
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	184,424		163,747		371,053	326,665
Purchased gas cost	621		1,683		(237)	(1,728)
Operation and maintenance expense	43,591		42,126		92,376	78,281
Depreciation and amortization expense	42,007		36,762		82,363	71,821
Taxes, other than income	10,891		9,347		19,807	19,098
Operating income	87,314		73,829		176,744	159,193
Other non-operating income	9,941		4,664		24,358	11,450
Interest charges	15,950		13,771		29,871	25,074
Income before income taxes	81,305		64,722		171,231	145,569
Income tax expense	12,108		8,574		24,642	19,783
Net income	69,197		56,148		146,589	125,786
Capital expenditures	194,700		$ 143,381		546,816	$ 390,179
Segment Reporting Information, Balance Sheet [Abstract]
Net property, plant and equipment	4,950,476				4,950,476		4,516,707
Total assets	$ 5,226,910				$ 5,226,910		$ 4,797,206