Inventories - Additional Information (Details) $ in Thousands	12 Months Ended
	Sep. 30, 2017 USD ($) storage_agreement agreement Bcf	Sep. 30, 2016 USD ($) 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	9.1	8.1
Carrying value of gas storage inventories released under SCAAs with non-affiliates | $	$ 26,064	$ 18,773
Security deposit liability | $	$ 15,040	$ 19,100
Minimum
Public Utilities, Inventory
Storage agreement, term (in years)	1 year
Maximum
Public Utilities, Inventory
Storage agreement, term (in years)	3 years