Revenue and Accounts Receivable - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 1,342,585	$ 1,175,999
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	1,258,049	1,108,569
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	84,536	67,430
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	1,545,459	1,364,725
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,183,231	1,038,055
Alternative revenue program revenues	71,832	67,336
Other revenues	3,763	3,944
Total operating revenues	1,258,826	1,109,335
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	320,033	268,693
Alternative revenue program revenues	(33,400)	(13,303)
Other revenues	0	0
Total operating revenues	286,633	255,390
Operating Segments | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,138,915	998,306
Operating Segments | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Transportation revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	41,478	36,727
Operating Segments | Transportation revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	317,403	266,029
Operating Segments | Miscellaneous revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,838	3,022
Operating Segments | Miscellaneous revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,630	2,664
Operating Segments | Residential | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	780,412	693,050
Operating Segments | Residential | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Commercial | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	315,565	266,054
Operating Segments | Commercial | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Industrial | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	32,861	26,321
Operating Segments | Industrial | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0
Operating Segments | Public authority and other | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	10,077	12,881
Operating Segments | Public authority and other | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 0	$ 0