Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 977,665	$ 1,094,645	$ 1,853,228	$ 1,972,427
Regulatory mechanism threshold (in percent)			75.00%
Distribution
Disaggregation of Revenue [Line Items]
Total operating revenues	932,296	1,057,192	$ 1,760,136	1,895,373
Pipeline and Storage
Disaggregation of Revenue [Line Items]
Total operating revenues	45,369	37,453	93,092	77,054
Operating Segments | Distribution
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	902,874	1,062,792	1,737,637	1,909,830
Alternative revenue program revenues	29,638	(5,397)	22,887	(14,136)
Other Revenues	493	494	985	1,030
Total operating revenues	933,005	1,057,889	1,761,509	1,896,724
Operating Segments | Distribution | Gas sales revenues:
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	867,384	1,027,746	1,668,721	1,842,434
Operating Segments | Distribution | Gas sales revenues: | Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	596,315	695,827	1,148,391	1,243,755
Operating Segments | Distribution | Gas sales revenues: | Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	230,779	278,945	442,093	497,883
Operating Segments | Distribution | Gas sales revenues: | Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	25,628	35,887	50,553	70,424
Operating Segments | Distribution | Gas sales revenues: | Public authority and other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	14,662	17,087	27,684	30,372
Operating Segments | Distribution | Transportation revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	28,504	27,682	55,144	53,082
Operating Segments | Distribution | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	6,986	7,364	13,772	14,314
Operating Segments | Pipeline and Storage
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	156,083	145,043	313,248	294,149
Alternative revenue program revenues	(9,846)	(9,393)	(18,835)	(24,029)
Other Revenues	0	0	0	0
Total operating revenues	146,237	135,650	294,413	270,120
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	154,748	142,270	306,758	289,694
Operating Segments | Pipeline and Storage | Miscellaneous revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 1,335	$ 2,773	$ 6,490	$ 4,455