Revenue and Accounts Receivable - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 4,201,662	$ 3,407,490	$ 2,821,137
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	4,031,936	3,238,753	2,624,251
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 169,726	168,737	196,886
Regulatory mechanism threshold (in percent)	75.00%
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 4,007,226	3,253,298	2,604,166
Alternative revenue program revenues	26,041	(13,303)	20,856
Other revenues	1,927	1,978	1,971
Total operating revenues	4,035,194	3,241,973	2,626,993
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	720,884	660,557	646,573
Alternative revenue program revenues	(27,224)	(23,210)	(37,234)
Other revenues	0	0	0
Total operating revenues	693,660	637,347	609,339
Operating Segments | Gas sales revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	3,883,901	3,134,505	2,485,646
Operating Segments | Gas sales revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0
Operating Segments | Gas sales revenues | Residential | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	2,472,461	2,129,704	1,704,444
Operating Segments | Gas sales revenues | Residential | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0
Operating Segments | Gas sales revenues | Commercial | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	1,120,322	841,145	650,396
Operating Segments | Gas sales revenues | Commercial | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0
Operating Segments | Gas sales revenues | Industrial | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	224,427	113,091	89,467
Operating Segments | Gas sales revenues | Industrial | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0
Operating Segments | Gas sales revenues | Public authority and other | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	66,691	50,565	41,339
Operating Segments | Gas sales revenues | Public authority and other | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	0	0	0
Operating Segments | Transportation revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	113,043	107,822	99,435
Operating Segments | Transportation revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	707,205	646,416	636,819
Operating Segments | Miscellaneous revenues | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	10,282	10,971	19,085
Operating Segments | Miscellaneous revenues | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 13,679	$ 14,141	$ 9,754