Inventories - Narrative (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2019 USD ($) agreement Bcf	Sep. 30, 2018 USD ($) Bcf	Mar. 31, 2018 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	0.8	9.0	0.9
Carrying value of gas storage inventories released under SCAAs with non-affiliates | $	$ 2,075	$ 23,136	$ 2,532
Security deposit liability | $	$ 11,840	$ 13,840	$ 13,840
Maximum
Public Utilities, Inventory
Term of agreements (in years)	3 years