Revenue and Accounts Receivable - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 4,275,357	$ 4,201,662	$ 3,407,490
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	4,096,661	4,031,936	3,238,753
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 178,696	$ 169,726	$ 168,737
Regulatory mechanism threshold (in percent)	75.00%	75.00%	75.00%
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 4,051,778	$ 4,007,226	$ 3,253,298
Alternative revenue program revenues	43,139	26,041	(13,303)
Other revenues	4,773	1,927	1,978
Total operating revenues	4,099,690	4,035,194	3,241,973
Operating Segments | Distribution | Gas sales revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,920,314	3,883,901	3,134,505
Operating Segments | Distribution | Gas sales revenues | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,606,658	2,472,461	2,129,704
Operating Segments | Distribution | Gas sales revenues | Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,100,773	1,120,322	841,145
Operating Segments | Distribution | Gas sales revenues | Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	151,538	224,427	113,091
Operating Segments | Distribution | Gas sales revenues | Public authority and other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	61,345	66,691	50,565
Operating Segments | Distribution | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	121,420	113,043	107,822
Operating Segments | Distribution | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	10,044	10,282	10,971
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	824,148	720,884	660,557
Alternative revenue program revenues	(38,974)	(27,224)	(23,210)
Other revenues	0	0	0
Total operating revenues	785,174	693,660	637,347
Operating Segments | Pipeline and Storage | Gas sales revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0
Operating Segments | Pipeline and Storage | Gas sales revenues | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0
Operating Segments | Pipeline and Storage | Gas sales revenues | Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0
Operating Segments | Pipeline and Storage | Gas sales revenues | Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0
Operating Segments | Pipeline and Storage | Gas sales revenues | Public authority and other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0
Operating Segments | Pipeline and Storage | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	811,968	707,205	646,416
Operating Segments | Pipeline and Storage | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 12,180	$ 13,679	$ 14,141