Segment Information (Details) $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2022 USD ($) state	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2022 USD ($) state	Jun. 30, 2021 USD ($)	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	$ 816,429			$ 605,553			$ 3,479,034	$ 2,839,106
Purchased gas cost	249,159			86,870			1,491,700	948,939
Operation and maintenance expense	182,325			184,470			504,787	479,488
Depreciation and amortization expense	134,231			119,348			395,461	353,269
Taxes, other than income	96,127			81,475			271,506	243,376
Operating income	154,587			133,390			815,580	814,034
Other non-operating income	13,263			5,887			27,178	14,793
Interest charges	26,190			20,962			74,969	69,068
Income before income taxes	141,660			118,315			767,789	759,759
Income tax expense	13,113			15,904			65,034	142,916
Net income	128,547	$ 324,999	$ 249,209	102,411	$ 296,754	$ 217,678	702,755	616,843
Capital expenditures	536,010			512,232			1,726,039	1,357,960
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	16,556,603						16,556,603		$ 15,063,970
Total assets	21,655,484						21,655,484		19,608,662
Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	772,497			557,931			3,353,800	2,715,644
Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	43,932			47,622			125,234	123,462
Eliminations
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(140,294)			(116,184)			(387,322)	(355,836)
Purchased gas cost	(140,053)			(115,871)			(386,437)	(354,890)
Operation and maintenance expense	(241)			(313)			(885)	(946)
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	(3,732,926)						(3,732,926)		(3,315,448)
Eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	814			819			2,479	2,430
Eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	139,480			115,365			384,843	353,406
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	773,311			558,750			3,356,279	2,718,074
Purchased gas cost	390,559			202,050			1,881,212	1,304,269
Operation and maintenance expense	133,654			130,454			378,479	363,246
Depreciation and amortization expense	97,106			86,099			286,515	254,636
Taxes, other than income	85,933			72,024			242,214	214,991
Operating income	66,059			68,123			567,859	580,932
Other non-operating income	6,708			1,060			9,173	1,135
Interest charges	12,341			8,540			36,046	33,269
Income before income taxes	60,426			60,643			540,986	548,798
Income tax expense	3,025			7,354			35,163	109,481
Net income	57,401			53,289			505,823	439,317
Capital expenditures	417,244			398,416			1,217,094	1,000,616
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	12,314,182						12,314,182		11,232,649
Total assets	20,891,285						20,891,285		18,847,266
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	183,412			162,987			510,077	476,868
Purchased gas cost	(1,347)			691			(3,075)	(440)
Operation and maintenance expense	48,912			54,329			127,193	117,188
Depreciation and amortization expense	37,125			33,249			108,946	98,633
Taxes, other than income	10,194			9,451			29,292	28,385
Operating income	88,528			65,267			247,721	233,102
Other non-operating income	6,555			4,827			18,005	13,658
Interest charges	13,849			12,422			38,923	35,799
Income before income taxes	81,234			57,672			226,803	210,961
Income tax expense	10,088			8,550			29,871	33,435
Net income	71,146			49,122			196,932	177,526
Capital expenditures	118,766			$ 113,816			508,945	$ 357,344
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	4,242,421						4,242,421		3,831,321
Total assets	$ 4,497,125						$ 4,497,125		$ 4,076,844