Revenue (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 2,821,137	$ 2,901,848	$ 3,115,546
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	2,624,251	2,742,824	3,000,404
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 196,886	159,024	115,142
Regulatory mechanism threshold	75.00%
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 2,604,166	2,756,387
Alternative revenue program revenues	20,856	(12,958)
Other revenues	1,971	2,032
Total operating revenues	2,626,993	2,745,461	3,003,047
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	646,573	631,868
Alternative revenue program revenues	(37,234)	(64,844)
Other revenues	0	0
Total operating revenues	609,339	567,024	$ 507,713
Operating Segments | Gas sales revenue [Member] | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	2,485,646	2,632,842
Operating Segments | Gas sales revenue [Member] | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Operating Segments | Gas sales revenue [Member] | Residential | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1,704,444	1,755,229
Operating Segments | Gas sales revenue [Member] | Residential | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Operating Segments | Gas sales revenue [Member] | Commercial | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	650,396	716,757
Operating Segments | Gas sales revenue [Member] | Commercial | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Operating Segments | Gas sales revenue [Member] | Industrial | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	89,467	118,060
Operating Segments | Gas sales revenue [Member] | Industrial | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Operating Segments | Gas sales revenue [Member] | Public authority and other | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	41,339	42,796
Operating Segments | Gas sales revenue [Member] | Public authority and other | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0	0
Operating Segments | Transportation revenue [Member] | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	99,435	97,495
Operating Segments | Transportation revenue [Member] | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	636,819	623,808
Operating Segments | Miscellaneous revenue [Member] | Distribution
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	19,085	26,050
Operating Segments | Miscellaneous revenue [Member] | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 9,754	$ 8,060