Revenue and Accounts Receivable - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 701,549	$ 662,733	$ 3,507,243	$ 3,687,715
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	632,446	615,259	3,325,459	3,554,389
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	69,103	47,474	181,784	133,326
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	617,256	608,818	3,264,283	3,512,826
Alternative revenue program revenues	11,997	6,772	51,713	42,360
Other revenues	3,958	477	11,734	1,517
Total operating revenues	633,211	616,067	3,327,730	3,556,703
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	268,371	216,719	712,039	606,093
Alternative revenue program revenues	(17,691)	(8,494)	(26,703)	(26,815)
Other revenues	0	0	0	0
Total operating revenues	250,680	208,225	685,336	579,278
Operating Segments | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	583,802	578,124	3,153,088	3,411,515
Operating Segments | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Transportation revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	30,773	27,988	102,147	93,661
Operating Segments | Transportation revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	260,779	212,322	699,243	597,822
Operating Segments | Miscellaneous revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,681	2,706	9,048	7,650
Operating Segments | Miscellaneous revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7,592	4,397	12,796	8,271
Operating Segments | Residential | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	384,957	380,099	2,177,935	2,276,240
Operating Segments | Residential | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Commercial | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	169,508	165,930	851,462	953,409
Operating Segments | Commercial | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Industrial | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	21,160	23,533	79,810	127,792
Operating Segments | Industrial | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0	0
Operating Segments | Public authority and other | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	8,177	8,562	43,881	54,074
Operating Segments | Public authority and other | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 0	$ 0	$ 0	$ 0