Inventories (Details Textual) (USD $) In Millions, unless otherwise specified	Jun. 30, 2011 Billion-cubic-feet	Sep. 30, 2010 Billion-cubic-feet	Jun. 30, 2010 Billion-cubic-feet
Inventories (Textuals) [Abstract]
Volume of gas storage inventories released under SCAAs with non-affiliates (In Cubic Feet)	2,000,000,000	8,000,000,000	4,200,000,000
Carrying value of gas storage inventories released under SCAAs with non-affiliates	$ 9.6	$ 41.9	$ 23.2