Segment Information (Details) $ in Thousands	3 Months Ended						9 Months Ended
	Jun. 30, 2021 USD ($) state	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2021 USD ($) state	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of states with service areas | state	8						8
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 605,553			$ 492,995			$ 2,839,106	$ 2,346,223
Purchased gas cost	86,870			26,072			948,939	640,823
Operation and maintenance expense	184,470			149,460			479,488	449,529
Depreciation and amortization expense	119,348			107,104			353,269	318,082
Taxes, other than income	81,475			71,324			243,376	214,535
Operating income	133,390			139,035			814,034	723,254
Other non-operating income	5,887			7,235			14,793	9,133
Interest charges	20,962			19,580			69,068	68,980
Income before income taxes	118,315			126,690			759,759	663,407
Income tax expense	15,904			8,899			142,916	127,297
Net income	102,411	$ 296,754	$ 217,678	117,791	$ 239,646	$ 178,673	616,843	536,110
Capital expenditures	512,232			410,936			1,357,960	1,405,673
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	14,477,749						14,477,749		$ 13,355,347
Total assets	19,315,435						19,315,435		15,359,032
Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	557,931			434,650			2,715,644	2,194,786
Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	47,622			58,345			123,462	151,437
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	558,750			435,308			2,718,074	2,196,817
Purchased gas cost	202,050			126,093			1,304,269	942,586
Operation and maintenance expense	130,454			107,537			363,246	337,740
Depreciation and amortization expense	86,099			77,187			254,636	229,526
Taxes, other than income	72,024			61,980			214,991	190,636
Operating income	68,123			62,511			580,932	496,329
Other non-operating income	1,060			5,167			1,135	1,930
Interest charges	8,540			7,969			33,269	35,128
Income before income taxes	60,643			59,709			548,798	463,131
Income tax expense	7,354			810			109,481	87,411
Net income	53,289			58,899			439,317	375,720
Capital expenditures	398,416			342,385			1,000,616	1,119,945
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	10,802,062						10,802,062		9,944,978
Total assets	18,563,345						18,563,345		14,578,176
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	162,987			158,008			476,868	452,421
Purchased gas cost	691			(11)			(440)	290
Operation and maintenance expense	54,329			42,234			117,188	112,751
Depreciation and amortization expense	33,249			29,917			98,633	88,556
Taxes, other than income	9,451			9,344			28,385	23,899
Operating income	65,267			76,524			233,102	226,925
Other non-operating income	4,827			2,068			13,658	7,203
Interest charges	12,422			11,611			35,799	33,852
Income before income taxes	57,672			66,981			210,961	200,276
Income tax expense	8,550			8,089			33,435	39,886
Net income	49,122			58,892			177,526	160,390
Capital expenditures	113,816			68,551			357,344	285,728
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	3,675,687						3,675,687		3,410,369
Total assets	3,906,816						3,906,816		3,647,907
Eliminations
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(116,184)			(100,321)			(355,836)	(303,015)
Purchased gas cost	(115,871)			(100,010)			(354,890)	(302,053)
Operation and maintenance expense	(313)			(311)			(946)	(962)
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	(3,154,726)						(3,154,726)		$ (2,867,051)
Eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	819			658			2,430	2,031
Eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 115,365			$ 99,663			$ 353,406	$ 300,984