Revenue and Accounts Receivable - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 1,647,227	$ 1,540,973	$ 2,805,694	$ 3,024,982
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	1,588,394	1,499,437	2,693,013	2,939,130
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	58,833	41,536	112,681	85,852
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,562,867	1,445,805	2,647,027	2,904,008
Alternative revenue program revenues	22,315	53,910	39,716	35,588
Other revenues	3,999	495	7,776	1,040
Total operating revenues	1,589,181	1,500,210	2,694,519	2,940,636
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	225,321	191,400	443,668	389,374
Alternative revenue program revenues	(1,834)	(6,976)	(9,012)	(18,321)
Other revenues	0	0	0	0
Total operating revenues	223,487	184,424	434,656	371,053
Operating Segments | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,521,536	1,409,632	2,569,286	2,833,391
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	37,607	33,511	71,374	65,673
Operating Segments | Transportation revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	223,159	190,248	438,464	385,500
Operating Segments | Miscellaneous revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,724	2,662	6,367	4,944
Operating Segments | Miscellaneous revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,162	1,152	5,204	3,874
Operating Segments | Residential | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,065,296	943,090	1,792,978	1,896,141
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	404,701	398,812	681,954	787,479
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	30,419	45,044	58,650	104,259
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	21,120	22,686	35,704	45,512
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