Revenue and Accounts Receivable - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 3,407,490	$ 2,821,137	$ 2,901,848
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	3,238,753	2,624,251	2,742,824
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 168,737	196,886	159,024
Regulatory mechanism threshold	75.00%
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 3,253,298	2,604,166	2,756,387
Alternative revenue program revenues	(13,303)	20,856	(12,958)
Other revenues	1,978	1,971	2,032
Total operating revenues	3,241,973	2,626,993	2,745,461
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	660,557	646,573	631,868
Alternative revenue program revenues	(23,210)	(37,234)	(64,844)
Other revenues	0	0	0
Total operating revenues	637,347	609,339	567,024
Operating Segments | Gas sales revenue | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	3,134,505	2,485,646	2,632,842
Operating Segments | Gas sales revenue | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Operating Segments | Gas sales revenue | Residential | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,129,704	1,704,444	1,755,229
Operating Segments | Gas sales revenue | Residential | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Operating Segments | Gas sales revenue | Commercial | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	841,145	650,396	716,757
Operating Segments | Gas sales revenue | Commercial | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Operating Segments | Gas sales revenue | Industrial | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	113,091	89,467	118,060
Operating Segments | Gas sales revenue | Industrial | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Operating Segments | Gas sales revenue | Public authority and other | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	50,565	41,339	42,796
Operating Segments | Gas sales revenue | Public authority and other | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0	0
Operating Segments | Transportation revenue | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	107,822	99,435	97,495
Operating Segments | Transportation revenue | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	646,416	636,819	623,808
Operating Segments | Miscellaneous revenue | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	10,971	19,085	26,050
Operating Segments | Miscellaneous revenue | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 14,141	$ 9,754	$ 8,060