Segment Information (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2021 USD ($) state	Dec. 31, 2020 USD ($)	Sep. 30, 2021 USD ($)
Segment Reporting Information [Line Items]
Number of states with service areas | state	8
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 1,012,786	$ 914,480
Purchased gas cost	371,163	288,260
Operation and maintenance expense	159,110	138,643
Depreciation and amortization expense	127,856	115,285
Taxes, other than income	78,796	73,452
Operating income	275,861	298,840
Other non-operating income	8,702	6,072
Interest charges	19,851	22,010
Income before income taxes	264,712	282,902
Income tax expense	15,503	65,224
Net income	249,209	217,678
Capital expenditures	684,180	456,809
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	15,726,791		$ 15,063,970
Total assets	20,545,784		19,608,662
Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	971,636	875,887
Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	41,150	38,593
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	972,422	876,650
Purchased gas cost	496,799	411,072
Operation and maintenance expense	123,284	108,802
Depreciation and amortization expense	92,797	82,870
Taxes, other than income	69,045	64,352
Operating income	190,497	209,554
Other non-operating income	1,916	835
Interest charges	8,548	10,712
Income before income taxes	183,865	199,677
Income tax expense	4,294	45,985
Net income	179,571	153,692
Capital expenditures	437,382	306,016
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	11,676,614		11,232,649
Total assets	19,778,943		18,847,266
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	162,918	159,713
Purchased gas cost	(3,411)	(1,244)
Operation and maintenance expense	36,155	30,156
Depreciation and amortization expense	35,059	32,415
Taxes, other than income	9,751	9,100
Operating income	85,364	89,286
Other non-operating income	6,786	5,237
Interest charges	11,303	11,298
Income before income taxes	80,847	83,225
Income tax expense	11,209	19,239
Net income	69,638	63,986
Capital expenditures	246,798	150,793
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	4,050,177		3,831,321
Total assets	4,309,629		4,076,844
Eliminations
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(122,554)	(121,883)
Purchased gas cost	(122,225)	(121,568)
Operation and maintenance expense	(329)	(315)
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 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	(3,542,788)		$ (3,315,448)
Eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	786	763
Eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 121,768	$ 121,120