Revenue and Accounts Receivable - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 4,165,187	$ 4,275,357	$ 4,201,662
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	3,912,134	4,096,661	4,031,936
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	253,053	178,696	169,726
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,847,167	4,051,778	4,007,226
Alternative revenue program revenues	52,401	43,139	26,041
Other revenues	15,573	4,773	1,927
Total operating revenues	3,915,141	4,099,690	4,035,194
Operating Segments | Distribution | Gas sales revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,700,731	3,920,314	3,883,901
Operating Segments | Distribution | Gas sales revenues | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,542,438	2,606,658	2,472,461
Operating Segments | Distribution | Gas sales revenues | Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,006,593	1,100,773	1,120,322
Operating Segments | Distribution | Gas sales revenues | Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	100,363	151,538	224,427
Operating Segments | Distribution | Gas sales revenues | Public authority and other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	51,337	61,345	66,691
Operating Segments | Distribution | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	134,600	121,420	113,043
Operating Segments | Distribution | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	11,836	10,044	10,282
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	998,687	824,148	720,884
Alternative revenue program revenues	(60,658)	(38,974)	(27,224)
Other revenues	0	0	0
Total operating revenues	938,029	785,174	693,660
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	982,795	811,968	707,205
Operating Segments | Pipeline and Storage | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 15,892	$ 12,180	$ 13,679