Revenue and Accounts Receivable - Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Mar. 31, 2023	Mar. 31, 2022
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 1,540,973	$ 1,649,819	$ 3,024,982	$ 2,662,605
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 1,499,437	$ 1,609,667	$ 2,939,130	$ 2,581,303
Regulatory mechanism threshold (in percent)	75.00%	75.00%	75.00%	75.00%
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 41,536	$ 40,152	$ 85,852	$ 81,302
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,445,805	1,635,281	2,904,008	2,556,224
Alternative revenue program revenues	53,910	(25,246)	35,588	25,740
Other revenues	495	511	1,040	1,004
Total operating revenues	1,500,210	1,610,546	2,940,636	2,582,968
Operating Segments | Distribution | Gas sales revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,409,632	1,599,800	2,833,391	2,490,275
Operating Segments | Distribution | Gas sales revenues | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	943,090	1,090,702	1,896,141	1,666,543
Operating Segments | Distribution | Gas sales revenues | Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	398,812	428,330	787,479	679,091
Operating Segments | Distribution | Gas sales revenues | Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	45,044	54,372	104,259	103,053
Operating Segments | Distribution | Gas sales revenues | Public authority and other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	22,686	26,396	45,512	41,588
Operating Segments | Distribution | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	33,511	32,801	65,673	60,670
Operating Segments | Distribution | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,662	2,680	4,944	5,279
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	191,400	166,134	389,374	336,536
Alternative revenue program revenues	(6,976)	(2,387)	(18,321)	(9,871)
Other revenues	0	0	0	0
Total operating revenues	184,424	163,747	371,053	326,665
Operating Segments | Pipeline and Storage | Gas sales revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	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	0
Operating Segments | Pipeline and Storage | Gas sales revenues | Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	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	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	0
Operating Segments | Pipeline and Storage | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	190,248	163,850	385,500	327,709
Operating Segments | Pipeline and Storage | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 1,152	$ 2,284	$ 3,874	$ 8,827