Revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 877,782	$ 889,192
Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	847,038
Alternative Revenue Program Revenues	(8,739)
Other Revenues	536
Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	149,106
Alternative Revenue Program Revenues	(14,636)
Other Revenues	0
Gas sales revenue [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	814,688
Gas sales revenue [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	0
Gas sales revenue [Member] | Residential Customers [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	547,928
Gas sales revenue [Member] | Residential Customers [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	0
Gas sales revenue [Member] | Commercial Customers [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	218,938
Gas sales revenue [Member] | Commercial Customers [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	0
Gas sales revenue [Member] | Industrial Customers [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	34,537
Gas sales revenue [Member] | Industrial Customers [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	0
Gas sales revenue [Member] | Public Authority and Other Customers [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	13,285
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
Transportation revenue [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	25,400
Transportation revenue [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	147,424
Miscellaneous revenue [Member] | Distribution segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	6,950
Miscellaneous revenue [Member] | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customer	1,682
Operating Segments | Distribution segment
Disaggregation of Revenue [Line Items]
Total operating revenues	838,835	860,792
Operating Segments | Pipeline and storage segment
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 134,470	$ 126,463