Segment Information (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2020 USD ($) state	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)
Segment Reporting Information [Line Items]
Number of states with service areas | state	8
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 914,480	$ 875,563
Purchased gas cost	288,260	296,868
Operation and maintenance expense	138,643	152,245
Depreciation and amortization expense	115,285	105,062
Taxes, other than income	73,452	68,607
Operating income	298,840	252,781
Other non-operating income	6,072	4,887
Interest charges	22,010	27,229
Income before income taxes	282,902	230,439
Income tax (benefit) expense	65,224	51,766
Net income	217,678	178,673
Capital expenditures	456,809	529,186
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	13,762,143		$ 13,355,347
Total assets	16,475,879		15,359,032
Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	875,887	827,840
Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	38,593	47,723
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	876,650	828,504
Purchased gas cost	411,072	397,558
Operation and maintenance expense	108,802	114,352
Depreciation and amortization expense	82,870	76,074
Taxes, other than income	64,352	60,243
Operating income	209,554	180,277
Other non-operating income	835	1,954
Interest charges	10,712	16,362
Income before income taxes	199,677	165,869
Income tax (benefit) expense	45,985	36,112
Net income	153,692	129,757
Capital expenditures	306,016	404,247
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	10,235,658		9,944,978
Total assets	15,694,179		14,578,176
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	159,713	148,176
Purchased gas cost	(1,244)	99
Operation and maintenance expense	30,156	38,221
Depreciation and amortization expense	32,415	28,988
Taxes, other than income	9,100	8,364
Operating income	89,286	72,504
Other non-operating income	5,237	2,933
Interest charges	11,298	10,867
Income before income taxes	83,225	64,570
Income tax (benefit) expense	19,239	15,654
Net income	63,986	48,916
Capital expenditures	150,793	124,939
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	3,526,485		3,410,369
Total assets	3,757,875		3,647,907
Eliminations
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(121,883)	(101,117)
Purchased gas cost	(121,568)	(100,789)
Operation and maintenance expense	(315)	(328)
Depreciation and amortization expense	0	0
Taxes, other than income	0	0
Operating income	0	0
Other non-operating income	0	0
Interest charges	0	0
Income before income taxes	0	0
Income tax (benefit) expense	0	0
Net income	0	0
Capital expenditures	0	0
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	0		0
Total assets	(2,976,175)		$ (2,867,051)
Eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	763	664
Eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 121,120	$ 100,453