Revenue and Accounts Receivable - Disaggregation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Mar. 31, 2022	Mar. 31, 2021
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 1,649,819	$ 1,319,073	$ 2,662,605	$ 2,233,553
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	1,609,667	1,281,826	$ 2,581,303	2,157,713
Regulatory mechanism threshold (in percent)			75.00%
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	40,152	37,247	$ 81,302	75,840
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,635,281	1,305,493	2,556,224	2,174,207
Alternative revenue program revenues	(25,246)	(23,313)	25,740	(15,872)
Other revenues	511	494	1,004	989
Total operating revenues	1,610,546	1,282,674	2,582,968	2,159,324
Operating Segments | Distribution | Gas sales revenues:
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,599,800	1,271,195	2,490,275	2,109,746
Operating Segments | Distribution | Gas sales revenues: | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,090,702	893,720	1,666,543	1,485,554
Operating Segments | Distribution | Gas sales revenues: | Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	428,330	326,182	679,091	535,129
Operating Segments | Distribution | Gas sales revenues: | Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	54,372	31,066	103,053	55,774
Operating Segments | Distribution | Gas sales revenues: | Public authority and other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	26,396	20,227	41,588	33,289
Operating Segments | Distribution | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	32,801	30,973	60,670	58,740
Operating Segments | Distribution | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,680	3,325	5,279	5,721
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	166,134	155,443	336,536	325,352
Alternative revenue program revenues	(2,387)	(1,275)	(9,871)	(11,471)
Other revenues	0	0	0	0
Total operating revenues	163,747	154,168	326,665	313,881
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	163,850	151,565	327,709	316,326
Operating Segments | Pipeline and Storage | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 2,284	$ 3,878	$ 8,827	$ 9,026