Segment Information (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Mar. 31, 2022 USD ($) state	Dec. 31, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2022 USD ($) state	Mar. 31, 2021 USD ($)	Sep. 30, 2021 USD ($)
Segment Reporting Information [Line Items]
Number of states with service areas | state	8				8
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 1,649,819		$ 1,319,073		$ 2,662,605	$ 2,233,553
Purchased gas cost	871,378		573,809		1,242,541	862,069
Operation and maintenance expense	163,352		156,375		322,462	295,018
Depreciation and amortization expense	133,374		118,636		261,230	233,921
Taxes, other than income	96,583		88,449		175,379	161,901
Operating income	385,132		381,804		660,993	680,644
Other non-operating income	5,213		2,834		13,915	8,906
Interest charges	28,928		26,096		48,779	48,106
Income before income taxes	361,417		358,542		626,129	641,444
Income tax expense	36,418		61,788		51,921	127,012
Net income	324,999	$ 249,209	296,754	$ 217,678	574,208	514,432
Capital expenditures	505,849		388,919		1,190,029	845,728
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	16,084,973				16,084,973		$ 15,063,970
Total assets	21,271,143				21,271,143		19,608,662
Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	1,609,667		1,281,826		2,581,303	2,157,713
Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	40,152		37,247		81,302	75,840
Eliminations
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(124,474)		(117,769)		(247,028)	(239,652)
Purchased gas cost	(124,159)		(117,451)		(246,384)	(239,019)
Operation and maintenance expense	(315)		(318)		(644)	(633)
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]
Property, plant and equipment, net	0				0		0
Total assets	(3,616,638)				(3,616,638)		(3,315,448)
Eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	879		848		1,665	1,611
Eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	123,595		116,921		245,363	238,041
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	1,610,546		1,282,674		2,582,968	2,159,324
Purchased gas cost	993,854		691,147		1,490,653	1,102,219
Operation and maintenance expense	121,541		123,990		244,825	232,792
Depreciation and amortization expense	96,612		85,667		189,409	168,537
Taxes, other than income	87,236		78,615		156,281	142,967
Operating income	311,303		303,255		501,800	512,809
Other non-operating income	549		(760)		2,465	75
Interest charges	15,157		14,017		23,705	24,729
Income before income taxes	296,695		288,478		480,560	488,155
Income tax expense	27,844		56,142		32,138	102,127
Net income	268,851		232,336		448,422	386,028
Capital expenditures	362,468		296,184		799,850	602,200
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	11,960,553				11,960,553		11,232,649
Total assets	20,515,446				20,515,446		18,847,266
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	163,747		154,168		326,665	313,881
Purchased gas cost	1,683		113		(1,728)	(1,131)
Operation and maintenance expense	42,126		32,703		78,281	62,859
Depreciation and amortization expense	36,762		32,969		71,821	65,384
Taxes, other than income	9,347		9,834		19,098	18,934
Operating income	73,829		78,549		159,193	167,835
Other non-operating income	4,664		3,594		11,450	8,831
Interest charges	13,771		12,079		25,074	23,377
Income before income taxes	64,722		70,064		145,569	153,289
Income tax expense	8,574		5,646		19,783	24,885
Net income	56,148		64,418		125,786	128,404
Capital expenditures	143,381		$ 92,735		390,179	$ 243,528
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	4,124,420				4,124,420		3,831,321
Total assets	$ 4,372,335				$ 4,372,335		$ 4,076,844