Inventories - Narrative (Details) $ in Thousands	9 Months Ended
	Jun. 30, 2019 USD ($) agreement Bcf	Sep. 30, 2018 USD ($) Bcf	Jun. 30, 2018 USD ($) Bcf
Public Utilities, Inventory
Number of storage agreements | agreement	4
Number of storage agreements with Energy Services | agreement	4
Volume of gas storage inventories released under SCAAs with non-affiliates (in bcf) | Bcf	3.3	9.0	4.7
Carrying value of gas storage inventories released under SCAAs with non-affiliates | $	$ 8,234	$ 23,136	$ 11,944
Security deposit liability | $	$ 7,640	$ 13,840	$ 13,840
Maximum
Public Utilities, Inventory
Term of agreements (in years)	3 years