Inventories (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Mar. 31, 2015 Bcf Storage_Agreement	Sep. 30, 2014 Bcf	Mar. 31, 2014 Bcf
Inventory Disclosure [Abstract]
Number of storage agreements	3
SCAA contract term (in years)	3 years
Volume of gas storage inventories released under SCAAs with non-affiliates (in cubic feet)	0.2	3.9	0.2
Carrying value of gas storage inventories released under SCAAs with non-affiliates	$ 0.7	$ 16.8	$ 0.8