Segment Information (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2019 USD ($) state	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)
Segment Reporting Information [Line Items]
Number of states with service areas | state	8
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ (875,563)	$ (877,782)
Purchased gas cost	296,868	342,165
Operation and maintenance expense	152,245	138,600
Depreciation and amortization expense	105,062	96,065
Taxes, other than income	68,607	64,488
Operating income	252,781	236,464
Other non-operating income (expense)	4,887	(7,723)
Interest charges	27,229	27,849
Income before income taxes	230,439	200,892
Income tax (benefit) expense	51,766	43,246
Net income	178,673	157,646
Capital expenditures	529,186	416,404
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	12,250,423		$ 11,787,669
Total assets	14,388,125		13,367,619
Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(827,840)	(838,181)
Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(47,723)	(39,601)
Operating Segments | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(828,504)	(838,835)
Purchased gas cost	397,558	437,732
Operation and maintenance expense	114,352	105,767
Depreciation and amortization expense	76,074	69,709
Taxes, other than income	60,243	56,190
Operating income	180,277	169,437
Other non-operating income (expense)	1,954	(6,477)
Interest charges	16,362	18,210
Income before income taxes	165,869	144,750
Income tax (benefit) expense	36,112	30,365
Net income	129,757	114,385
Capital expenditures	404,247	302,545
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	9,083,765		8,737,590
Total assets	13,599,293		12,579,741
Operating Segments | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	(148,176)	(134,470)
Purchased gas cost	99	(358)
Operation and maintenance expense	38,221	33,147
Depreciation and amortization expense	28,988	26,356
Taxes, other than income	8,364	8,298
Operating income	72,504	67,027
Other non-operating income (expense)	2,933	(1,246)
Interest charges	10,867	9,639
Income before income taxes	64,570	56,142
Income tax (benefit) expense	15,654	12,881
Net income	48,916	43,261
Capital expenditures	124,939	113,859
Segment Reporting Information, Balance Sheet [Abstract]
Property, plant and equipment, net	3,166,658		3,050,079
Total assets	3,389,655		3,279,323
Eliminations
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	101,117	95,523
Purchased gas cost	(100,789)	(95,209)
Operation and maintenance expense	(328)	(314)
Depreciation and amortization expense	0	0
Taxes, other than income	0	0
Operating income	0	0
Other non-operating income (expense)	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	(2,600,823)		$ (2,491,445)
Eliminations | Distribution
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	664	(654)
Eliminations | Pipeline and Storage
Segment Reporting Information Profit Loss [Abstract]
Operating revenues	$ 100,453	$ (94,869)