Inventories - Additional Information (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2018 USD ($) storage_agreement agreement Bcf	Sep. 30, 2017 USD ($) Bcf
Public Utilities, Inventory
Number of storage agreements | storage_agreement	5
Number of storage agreements with Energy Services | agreement	4
Number of storage agreements with non-affiliates | agreement	1
Volume of gas storage inventories released under SCAAs with non-affiliates (in bcf) | Bcf	9.0	9.1
Carrying value of gas storage inventories released under SCAAs with non-affiliates | $	$ 23,136	$ 26,064
Security deposit liability | $	$ 13,840	$ 15,040
Minimum
Public Utilities, Inventory
Storage agreement, term (in years)	1 year
Maximum
Public Utilities, Inventory
Storage agreement, term (in years)	3 years