Inventories - Narrative (Details) $ in Thousands	9 Months Ended
	Jun. 30, 2017 USD ($) agreement Bcf	Sep. 30, 2016 USD ($) Bcf	Jun. 30, 2016 USD ($) Bcf
Public Utilities, Inventory
Number of storage agreements	5
Number of storage agreements with Energy Services	4
Number of storage agreements with non-affiliates	1
Volume of gas storage inventories released under SCAAs with non-affiliates (in bcf) | Bcf	4.8	8.1	4.6
Carrying value of gas storage inventories released under SCAAs with non-affiliates | $	$ 14,146	$ 18,773	$ 8,390
Security deposit liability | $	$ 15,040	$ 19,100	$ 15,100
Minimum
Public Utilities, Inventory
Term of agreements (in years)	1 year
Maximum
Public Utilities, Inventory
Term of agreements (in years)	3 years