Inventories (Details) (USD $) In Millions, unless otherwise specified	0 Months Ended
	Dec. 31, 2014	Sep. 30, 2014 ft3	Dec. 31, 2013 ft3
Inventory
Number of storage agreements	4
Volume of gas storage inventories released under SCAAs with non-affiliates (in cubic feet)	3,400,000,000	3,900,000,000	3,100,000,000
Carrying value of gas storage inventories released under SCAAs with non-affiliates	14.4	$ 16.8	$ 12.3
Minimum
Inventory
SCAA contract term (in years)	1 year
Maximum
Inventory
SCAA contract term (in years)	3 years