Inventories (Details) $ in Millions	9 Months Ended
	Jun. 30, 2015 USD ($) Storage_Agreement Bcf	Sep. 30, 2014 USD ($) Bcf	Jun. 30, 2014 USD ($) Bcf
Inventory Disclosure [Abstract]
Number of storage agreements | Storage_Agreement	3
SCAA contract term (in years)	3 years
Volume of gas storage inventories released under SCAAs with non-affiliates (in cubic feet) | Bcf	1.9	3.9	2.1
Carrying value of gas storage inventories released under SCAAs with non-affiliates	$ 4.5	$ 16.8	$ 8.9