Revenue and Accounts Receivable - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 838,774	$ 701,549	$ 3,965,275	$ 3,507,243
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	766,380	632,446	3,756,691	3,325,459
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	72,394	69,103	208,584	181,784
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	762,378	617,256	3,699,612	3,264,283
Alternative revenue program revenues	1,438	11,997	48,657	51,713
Other revenues	3,316	3,958	10,726	11,734
Total operating revenues	767,132	633,211	3,758,995	3,327,730
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	288,759	268,371	828,225	712,039
Alternative revenue program revenues	(16,371)	(17,691)	(41,448)	(26,703)
Other revenues	0	0	0	0
Total operating revenues	272,388	250,680	786,777	685,336
Operating Segments | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	723,144	583,802	3,573,055	3,153,088
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	35,908	30,773	115,987	102,147
Operating Segments | Transportation revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	284,103	260,779	816,646	699,243
Operating Segments | Miscellaneous revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,326	2,681	10,570	9,048
Operating Segments | Miscellaneous revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	4,656	7,592	11,579	12,796
Operating Segments | Residential | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	473,478	384,957	2,475,219	2,177,935
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	213,377	169,508	959,539	851,462
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	27,369	21,160	92,575	79,810
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	8,920	8,177	45,722	43,881
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