Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 1,094,645	$ 1,219,409	$ 1,972,427	$ 2,108,601
Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	1,062,792		1,909,830
Alternative Revenue Program Revenues	(5,397)		(14,136)
Other Revenues	494		1,030
Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	145,043		294,149
Alternative Revenue Program Revenues	(9,393)		(24,029)
Other Revenues	0		0
Gas sales revenue [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	1,027,746		1,842,434
Gas sales revenue [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	0		0
Gas sales revenue [Member] | Residential Customers [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	695,827		1,243,755
Gas sales revenue [Member] | Residential Customers [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	0		0
Gas sales revenue [Member] | Commercial Customers [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	278,945		497,883
Gas sales revenue [Member] | Commercial Customers [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	0		0
Gas sales revenue [Member] | Industrial Customers [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	35,887		70,424
Gas sales revenue [Member] | Industrial Customers [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	0		0
Gas sales revenue [Member] | Public Authority and Other Customers [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	17,087		30,372
Gas sales revenue [Member] | Public Authority and Other Customers [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	0		0
Transportation revenue [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	27,682		53,082
Transportation revenue [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	142,270		289,694
Miscellaneous revenue [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	7,364		14,314
Miscellaneous revenue [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	2,773		4,455
Operating Segments | Distribution segment
Disaggregation of Revenue [Line Items]
Total operating revenues	1,057,889	1,199,291	1,896,724	2,060,083
Operating Segments | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 135,650	$ 120,955	$ 270,120	$ 247,418