Inventories - Narrative (Details) $ in Thousands	6 Months Ended
	Mar. 31, 2017 USD ($) agreement Bcf	Sep. 30, 2016 USD ($) Bcf	Mar. 31, 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	0.8	8.1	1.1
Carrying value of gas storage inventories released under SCAAs with non-affiliates | $	$ 1,964	$ 18,773	$ 2,593
Other Current Liabilities
Public Utilities, Inventory
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